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The electronic structure of LiNiO$_2$, a promising Li-ion battery cathode material, has remained a challenge to understand due to its highly covalent yet correlated nature. Here we elucidate the electronic structure in LiNiO$_2$ and the related compound NaNiO$_2$ using x-ray absorption spectra (XAS) and quantum many-body calculations. Notably, we use inverse partial fluorescence yield to correctly measure the Ni $L$-edge XAS, which is inaccurate using conventional methods. We show that the XAS are indicative of a strong Jahn-Teller effect in NaNiO$_2$ and a bond disproportionated state in LiNiO$_2$, supporting a theory of a high-entropy, glassy disproportionated state that stabilizes charging cycles in LiNiO$_2$., Comment: 5 pages, 3 figures

We present a detailed magnetic study of the triangular antiferromagnet ErMgGaO$_4$. A point charge calculation under the single ion approximation reveals a crystal field ground state doublet with a strong Ising-like behavior of the Er$^{3+}$ moment along the c axis. Magnetic susceptibility and specific heat measurements indicate no presence of magnetic transitions above 0.5~K and no evidence of residual entropy as temperature approaching zero. Zero field (ZF) $\mu$SR measurements shows no sign of static uniform or random field and longitudinal field (LF) $\mu$SR measurements exhibit persistent spin fluctuations down to our lowest temperature of 25~mK. Our results provide evidence of a quantum spin liquid state in the triangular antiferromagnet ErMgGaO$_4$.

Rare earth garnets are an exciting playground for studying the exotic magnetic properties of the frustrated hyperkagome lattice. Here we present a comprehensive study of the single ion and collective magnetic properties of the garnet Er$_3$Ga$_5$O$_{12}$. Using inelastic neutron scattering, we find a crystal field ground state doublet for Er$^{3+}$ with strong Ising anisotropy along local [100] axes. Magnetic susceptibility and heat capacity measurements provide evidence for long-range magnetic ordering with $T_N$~$=$~0.8~K, and no evidence for residual entropy is found when cooling through the ordering transition. Neutron powder diffraction reveals that the ground state spin configuration corresponds to the six-sublattice, Ising antiferromagnetic state ($\Gamma_3$) common to many of the rare earth garnets. However, we also found that $\mu$SR appears to be insensitive to the ordering transition in this material, in which a low-temperature relaxation plateau was observed with no evidence of spontaneous muon precession. The combined muon and neutron results may be indicative of a dynamical ground state with a relatively long correlation time. Despite this potential complication, our work indicates that Er$_3$Ga$_5$O$_{12}$ is an excellent model system for studying the complex metamagnetism expected for a multi-axis antiferromagnet.

The three central phenomena of cuprate superconductors are linked by a common doping $p^{\star}$, where the enigmatic pseudogap phase ends, around which the superconducting phase forms a dome, and at which the resistivity exhibits an anomalous linear dependence on temperature as $T \to 0$. However, the fundamental nature of $p^{\star}$ remains unclear, in particular whether it marks a true quantum phase transition. We have measured the specific heat $C$ of the cuprates Eu-LSCO and Nd-LSCO at low temperature in magnetic fields large enough to suppress superconductivity, over a wide doping range across $p^{\star}$. As a function of doping, we find that the electronic term $C_{\rm el}$ is strongly peaked at $p^{\star}$, where it exhibits a $-T$log$T$ dependence as $T \to 0$. These are the classic signatures of a quantum critical point, as observed in heavy-fermion and iron-based superconductors where their antiferromagnetic phase ends. We conclude that the pseudogap phase of cuprates ends at a quantum critical point, whose associated fluctuations are most likely involved in the $d$-wave pairing and the anomalous scattering., Comment: Includes Supplementary Information

We have carried out small angle neutron scattering measurements on single crystals of two members of the family of cubic rare-earth titanate pyrochlores that display ferromagnetic Curie-Weiss susceptibilities,Yb$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$. Ho$_2$Ti$_2$O$_7$ is established as displaying a prototypical classical dipolar spin ice ground state, while Yb$_2$Ti$_2$O$_7$ has been purported as a candidate for a quantum spin ice ground state. While both materials have been well studied with neutron scattering techniques, neither has been previously explored in single crystal form with small angle neutron scattering (SANS). Our results for Yb$_2$Ti$_2$O$_7$ show distinct SANS features below its $\Theta_{CW}$$\sim$ 0.50 K, with rods of diffuse scattering extending along $\langle 111 \rangle$ directions in reciprocal space, off-rod scattering which peaks in temperature near $\Theta_{CW}$, and quasi-Bragg scattering at very small angles which correlates well with T$_C$ $\sim$ 0.26 K. The quasi-Bragg scattering corresponds to finite extent ferromagnetic domains $\sim$ 140 \AA$~$ across, at the lowest temperatures. We interpret the $\langle 111\rangle$ rods of diffuse scattering as arising from domain boundaries between the finite-extent ferromagnetic domains. In contrast the SANS signal in Ho$_2$Ti$_2$O$_7$ is isotropic within the (HHL) plane around $\textbf{Q}$=0. However the strength of this overall SANS signal has a temperature dependence resembling that of the magnetic heat capacity, with a peak near 3 K. Below the break between the field-cooled and the zero-field cooled susceptibility in Ho$_2$Ti$_2$O$_7$ at $\sim$ 0.60 K, the SANS signal is very low, approaching zero., Comment: 10 pages, 9 figures

Neutron diffraction and muon spin relaxation ($\mu$SR) studies are presented for the newly characterized polymorph of NiNb$_2$O$_6$ ($\beta$-NiNb$_2$O$_6$) with space group P4$_2$/n and $\mu$SR data only for the previously known columbite structure polymorph with space group Pbcn. The magnetic structure of the P4$_2$/n form was determined from neutron diffraction using both powder and single crystal data. Powder neutron diffraction determined an ordering wave vector $\vec{k}$ = ($\frac{1}{2},\frac{1}{2},\frac{1}{2}$). Single crystal data confirmed the same $\vec{k}$-vector and showed that the correct magnetic structure consists of antiferromagnetically-coupled chains running along the a or b-axes in adjacent Ni$^{2+}$ layers perpendicular to the c-axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related tri-rutile phases, NiSb$_2$O$_6$ and NiTa$_2$O$_6$. $\mu$SR data finds a transition temperature of $T_N \sim$ 15 K for this system, while the columbite polymorph exhibits a lower $T_N =$ 5.7(3) K. Our $\mu$SR measurements also allowed us to estimate the critical exponent of the order parameter $\beta$ for each polymorph. We found $\beta =$ 0.25(3) and 0.16(2) for the $\beta$ and columbite polymorphs respectively. The single crystal neutron scattering data gives a value for the critical exponent $\beta =$~0.28(3) for $\beta$-NiNb$_2$O$_6$, in agreement with the $\mu$SR value. While both systems have $\beta$ values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to well-described by $S = 1$ spin chains, the interchain interactions in the $\beta$-polymorph are likely much larger., Comment: 10 pages, 12 figures

A quantum spin liquid is a novel state of matter characterized by quantum entanglement and the absence of any broken symmetry. In condensed matter, the frustrated rare-earth pyrochlore magnets Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$, so-called spin ices, exhibit a classical spin liquid state with fractionalized thermal excitations (magnetic monopoles). Evidence for a quantum spin ice, in which the magnetic monopoles become long range entangled and an emergent quantum electrodynamics arises, seems within reach. The magnetic properties of the quantum spin ice candidate Yb$_2$Ti$_2$O$_7$ have eluded a global understanding and even the presence or absence of static magnetic order at low temperatures is controversial. Here we show that sensitivity to pressure is the missing key to the low temperature behaviour of Yb$_2$Ti$_2$O$_7$. By combining neutron diffraction and muon spin relaxation on a stoichiometric sample under pressure, we evidence a magnetic transition from a disordered, non-magnetic, ground state to a splayed ferromagnetic ground state.

The frustrated magnet SrDy$_2$O$_4$ exhibits a field-induced phase with a magnetization plateau at $1/3$ of the saturation value for magnetic fields applied along the $b$-axis. We report here a neutron scattering study of the nature and symmetry of the magnetic order in this field-induced phase. Below $T\approx 0.5$ K, there are strong hysteretic effects, and the order is short or long ranged for zero-field and field cooling, respectively. We find that the long-range ordered magnetic structure within the zig-zag chains is identical to that expected for the one-dimensional axial next-nearest neighbour Ising (ANNNI) model in longitudinal field. The long-range ordered structure in field contrasts with the short-range order found at zero field, and is probably reached through enhanced quantum fluctuations with increasing fields., Comment: 8 pages, 9 figures

Magnetic frustration and low dimensionality can prevent long range magnetic order and lead to exotic correlated ground states. SrDy$_2$O$_4$ consists of magnetic Dy$^{3+}$ ions forming magnetically frustrated zig-zag chains along the c-axis and shows no long range order to temperatures as low as $T=60$ mK. We carried out neutron scattering and AC magnetic susceptibility measurements using powder and single crystals of SrDy$_2$O$_4$. Diffuse neutron scattering indicates strong one-dimensional (1D) magnetic correlations along the chain direction that can be qualitatively accounted for by the axial next-nearest neighbour Ising (ANNNI) model with nearest-neighbor and next-nearest-neighbor exchange $J_1=0.3$ meV and $J_2=0.2$ meV, respectively. Three-dimensional (3D) correlations become important below $T^*\approx0.7$ K. At $T=60$ mK, the short range correlations are characterized by a putative propagation vector $\textbf{k}_{1/2}=(0,\frac{1}{2},\frac{1}{2})$. We argue that the absence of long range order arises from the presence of slowly decaying 1D domain walls that are trapped due to 3D correlations. This stabilizes a low-temperature phase without long range magnetic order, but with well-ordered chain segments separated by slowly-moving domain walls., Comment: 13 pages, 14 figures, 2 tables

The XY pyrochlore antiferromagnet Er$_2$Ti$_2$O$_7$ exhibits a rare case of $Z_6$ discrete symmetry breaking in its $\psi_2$ magnetic ground state. Despite being well-studied theoretically, systems with high discrete symmetry breakings are uncommon in nature and, thus, Er$_2$Ti$_2$O$_7$ provides an experimental playground for the study of broken $Z_n$ symmetry, for $n>2$. A recent theoretical work examined the effect of a magnetic field on a pyrochlore lattice with broken $Z_6$ symmetry and applied it to Er$_2$Ti$_2$O$_7$. This study predicted multiple domain transitions depending on the crystallographic orientation of the magnetic field, inducing rich and controllable magnetothermodynamic behavior. In this work, we present neutron scattering measurements on Er$_2$Ti$_2$O$_7$ with a magnetic field applied along the [001] and [111] directions, and provide the first experimental observation of these exotic domain transitions. In a [001] field, we observe a $\psi_2$ to $\psi_3$ transition at a critical field of 0.18$\pm$0.05T. We are thus able to extend the concept of the spin-flop transition, which has long been observed in Ising systems, to higher discrete $Z_n$ symmetries. In a [111] field, we observe a series of domain-based phase transitions for fields of 0.15$\pm$0.03T and 0.40$\pm$0.03T. We show that these field-induced transitions are consistent with the emergence of two-fold, three-fold and possibly six-fold Zeeman terms. Considering all the possible $\psi_2$ and $\psi_3$ domains, these Zeeman terms can be mapped onto an analog clock - exemplifying a literal clock anisotropy. Lastly, our quantitative analysis of the [001] domain transition in Er$_2$Ti$_2$O$_7$ is consistent with order-by-disorder as the dominant ground state selection mechanism., Comment: 10 pages, 7 figures, Accepted for publication as regular article in Phys. Rev. B

Below $T_N = 1.1$K, the XY pyrochlore Er$_2$Ti$_2$O$_7$ orders into a $k=0$ non-collinear, antiferromagnetic structure referred to as the $\psi_2$ state. The magnetic order in Er$_2$Ti$_2$O$_7$ is known to obey conventional three dimensional (3D) percolation in the presence of magnetic dilution, and in that sense is robust to disorder. Recently, however, two theoretical studies have predicted that the $\psi_2$ structure should be unstable to the formation of a related $\psi_3$ magnetic structure in the presence of magnetic vacancies. To investigate these theories, we have carried out systematic elastic and inelastic neutron scattering studies of three single crystals of Er$_{2-x}$Y$_x$Ti$_2$O$_7$ with $x=0$ (pure), 0.2 (10$\%$-Y) and 0.4 (20$\%$-Y), where magnetic Er$^{3+}$ is substituted by non-magnetic Y$^{3+}$. We find that the $\psi_2$ ground state of pure Er$_2$Ti$_2$O$_7$ is significantly affected by magnetic dilution. The characteristic domain selection associated with the $\psi_2$ state, and the corresponding energy gap separating $\psi_2$ from $\psi_3$, vanish for Y$^{3+}$ substitutions between 10$\%$-Y and 20$\%$-Y, far removed from the 3D percolation threshold of $\sim$60$\%$-Y. The resulting ground state for Er$_2$Ti$_2$O$_7$ with magnetic dilutions from 20$\%$-Y up to the percolation threshold is naturally interpreted as a frozen mosaic of $\psi_2$ and $\psi_3$ domains., Comment: 8 pages,7 figures, 1 table

Low temperature ultrasound velocity measurements are presented on the frustrated spin system SrDy2O4 that allow us to define high resolution phase diagrams with the magnetic field applied along all three principal axes. For H||b, a region of field-induced long range order is delimited by a dome of first-order phase transitions. An unusual magnetization process is observed with significant irreversibility at very low temperatures when passing between the low-field spin liquid phase and the long range ordered phase which we attribute to large energy barriers. For H||c, the system appears to remain effectively one-dimensional, exhibiting two transitions as a function of magnetic field, but no finite-temperature long range order.

The ground state of the quantum spin ice candidate magnet Yb2Ti2O7 is known to be sensitive to weak disorder at the 1 percent level which occurs in single crystals grown from the melt. Powders produced by solid state synthesis tend to be stoichiometric and display large and sharp heat capacity anomalies at relatively high temperatures, with Tc about 0.26 K. We have carried out neutron elastic and inelastic measurements on well characterized and equilibrated stoichiometric powder samples of Yb2Ti2O7 which show resolution-limited Bragg peaks to appear at low temperatures, but whose onset correlates with temperatures much higher than Tc. The corresponding magnetic structure is best described as an ice-like splayed ferromagnet. The spin dynamics in Yb2Ti2O7 are shown to be gapless on an energy scale smaller than 0.09 meV at all temperatures, and organized into a continuum of scattering with vestiges of highly overdamped ferromagnetic spin waves present. These excitations differ greatly from conventional spin waves predicted for Yb2Ti2O7 s mean field ordered state, but appear robust to weak disorder as they are largely consistent with those displayed by non stoichiometric crushed single crystals and single crystals, as well as by powder samples of Yb2Ti2O7 s sister quantum magnet Yb2Sn2O7., Comment: 8 pages, 6 figures, 1 table, to be appeared in Phys. Rev. B

The application of a magnetic field transverse to the easy axis, Ising direction in the quasi-two-dimensional Kagome staircase magnet, Co3V2O8, induces three quantum phase transitions at low temperatures, ultimately producing a novel high field polarized state, with two distinct sublattices. New time-of-flight neutron scattering techniques, accompanied by large angular access, high magnetic field infrastructure allow the mapping of a sequence of ferromagnetic and incommensurate phases and their accompanying spin excitations. At least one of the transitions to incommensurate phases at \mu 0Hc1~6.25 T and \mu 0Hc2~7 T is discontinuous, while the final quantum critical point at \mu 0Hc3~13 T is continuous., Comment: 5 pages manuscript, 3 pages supplemental material

Recent low temperature heat capacity (C$_P$) measurements on polycrystalline samples of the pyrochlore antiferromagnet Tb$_{2+x}$Ti$_{2-x}$O$_{7+\delta}$ have shown a strong sensitivity to the precise Tb concentration $x$, with a large anomaly exhibited for $x \sim 0.005$ at $T_C \sim 0.5$ K and no such anomaly and corresponding phase transition for $x \le 0$. We have grown single crystal samples of Tb$_{2+x}$Ti$_{2-x}$O$_{7+\delta}$, with approximate composition $x=-0.001, +0.0042$, and $+0.0147$, where the $x=0.0042$ single crystal exhibits a large C$_P$ anomaly at $T_C$=0.45 K, but neither the $x=-0.001$ nor the $x=+0.0147$ single crystals display any such anomaly. We present new time-of-flight neutron scattering measurements on the $x=-0.001$ and the $x=+0.0147$ samples which show strong $\left(\frac{1}{2},\frac{1}{2},\frac{1}{2}\right)$ quasi-Bragg peaks at low temperatures characteristic of short range antiferromagnetic spin ice (AFSI) order at zero magnetic field but only under field-cooled conditions, as was previously observed in our $x = 0.0042$ single crystal. These results show that the strong $\left(\frac{1}{2},\frac{1}{2},\frac{1}{2}\right)$ quasi-Bragg peaks and gapped AFSI state at low temperatures under field cooled conditions are robust features of Tb$_2$Ti$_2$O$_7$, and are not correlated with the presence or absence of the C$_P$ anomaly and phase transition at low temperatures. Further, these results show that the ordered state giving rise to the C$_P$ anomaly is confined to $0 \leq x \leq 0.01$ for Tb$_{2+x}$Ti$_{2-x}$O$_{7+\delta}$, and is not obviously connected with conventional order of magnetic dipole degrees of freedom., Comment: 7 pages, 3 figures

We present time-of-fight neutron-scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO) with 0 $\leq$ x $\leq$ 0.095 and $La_{2-x}Sr_{x}CuO_{4}$ (LSCO) with x = 0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high temperature cuprate superconductivity, ranging from insulating, three dimensional (3D) commensurate long range antiferromagnetic order, for x $\leq$ 0.02, to two dimensional (2D) incommensurate antiferromagnetism co-existing with superconductivity for x $\geq$ 0.05. Previous work on lightly doped LBCO with x = 0.035 showed a clear resonant enhancement of the inelastic scattering coincident with the low energy crossings of the highly dispersive spin excitations and quasi-2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore we show that the low temperature, low energy magnetic spectral weight is substantially larger for samples with non-superconducting ground states relative to any of the samples with superconducting ground states. Spin gaps, suppression of low energy magnetic spectral weight as a function of decreasing temperature, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO., Comment: 9 pages, 7 figures

Time-of-flight neutron spectroscopy has been used to determine the crystalline electric field (CEF) Hamiltonian, eigenvalues and eigenvectors appropriate to the $J$ = 7/2 Yb$^{3+}$ ion in the candidate quantum spin ice pyrochlore magnet $\rm Yb_2Ti_2O_7$. The precise ground state (GS) of this exotic, geometrically-frustrated magnet is known to be sensitive to weak disorder associated with the growth of single crystals from the melt. Such materials display weak "stuffing" wherein a small proportion, $\approx$ 2\%, of the non-magnetic Ti$^{4+}$ sites are occupied by excess Yb$^{3+}$. We have carried out neutron spectroscopic measurements on a stoichiometric powder sample of Yb$_2$Ti$_2$O$_7$, as well as a crushed single crystal with weak stuffing and an approximate composition of Yb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ with $x$ = 0.046. All samples display three CEF transitions out of the GS, and the GS doublet itself is identified as primarily composed of m$_J$ = $\pm$1/2, as expected. However,"stuffing" at low temperatures in Yb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ induces a similar finite CEF lifetime as is induced in stoichiometric Yb$_2$Ti$_2$O$_7$ by elevated temperature. We conclude that an extended strain field exists about each local "stuffed" site, which produces a distribution of random CEF environments in the lightly stuffed Yb$_{2+x}$Ti$_{2-x}$O$_{7+y}$, in addition to producing a small fraction of Yb-ions in defective environments with grossly different CEF eigenvalues and eigenvectors., Comment: 11 pages, 9 figures, 4 tables

We present time-of-flight inelastic neutron scattering measurements of $La_{1.965}Ba_{0.035}CuO_{4}$ (LBCO), a lightly doped member of the high temperature superconducting La-based cuprate family. By using time-of-flight neutron instrumentation coupled with single crystal sample rotation we obtain a four-dimensional data set (three {\bf Q} and one energy) that is both comprehensive and spans a large region of reciprocal space. Our measurements identify rich structure in the energy dependence of the highly dispersive spin excitations, which are centered at equivalent ($\frac{1}{2},\frac{1}{2}, L$) wave-vectors. These structures correlate strongly with several crossings of the spin excitations with the lightly dispersive phonons found in this system. These effects are significant and account for on the order of 25$\%$ of the total inelastic scattering for energies between $\sim$5 and 40 meV at low $|{\bf Q}|$. Interestingly, this scattering also presents little or no $L$-dependence. As the phonons and dispersive spin excitations centered at equivalent ($\frac{1}{2},\frac{1}{2}, L$) wave-vectors are common to all members of La-based 214 copper oxides, we conclude such strong quasi-two dimensional scattering enhancements are likely to occur in all such 214 families of materials, including those concentrations corresponding to superconducting ground states. Such a phenomenon appears to be a fundamental characteristic of these materials and is potentially related to superconducting pairing., Comment: 10 pages, 9 figures in main manuscript, 4 pages, 5 figures in supplemental materials

Magnetic excitations in the isostructural spin-dimer systems Sr3Cr2O8 and Ba3Cr2O8 are probed by means of high-field electron spin resonance at sub-terahertz frequencies. Three types of magnetic modes were observed. One mode is gapless and corresponds to transitions within excited states, while two other sets of modes are gapped and correspond to transitions from the ground to the first excited states. The selection rules of the gapped modes are analyzed in terms of a dynamical Dzyaloshinskii-Moriya interaction, suggesting the presence of phonon-assisted effects in the low-temperature spin dynamics of Sr3Cr2O8 and Ba3Cr2O8, Comment: 6 pages, 4 figures, all comments are welcome and appreciated

The transverse field Ising chain (TFIC) model is ideally suited for testing the fundamental ideas of quantum phase transitions, because its well-known $T=0$ ground state can be extrapolated to finite temperatures. Nonetheless, the lack of appropriate model materials hindered the past effort to test the theoretical predictions. Here we map the evolution of quantum fluctuations in the TFIC based on Nuclear Magnetic Resonance (NMR) measurements of CoNb$_2$O$_6$, and demonstrate the finite temperature effects on quantum criticality for the first time. From the temperature dependence of the $^{93}$Nb longitudinal relaxation rate $1/T_1$, we identify the renormalized classical, quantum critical, and quantum disordered scaling regimes in the temperature ($T$) vs. transverse magnetic field ($h_{\perp}$) phase diagram. Precisely at the critical field $h_{\perp}^{c}=5.25 \pm 0.15$ T, we observe a power-law behavior, $1/T_{1} \sim T^{-3/4}$, as predicted by quantum critical scaling. Our parameter-free comparison between the data and theory reveals that quantum fluctuations persist up to as high as $T \sim 0.4 J$, where the intra-chain exchange interaction $J$ is the only energy scale of the problem., Comment: Final version to appear in PRX

The recent determination of a robust spin Hamiltonian for the anti-ferromagnetic XY pyrochlore Er2Ti2O7 reveals a most convincing case of the "order by quantum disorder" (ObQD) mechanism for ground state selection. This mechanism relies on quantum fluctuations to remove an accidental symmetry of the magnetic ground state, and selects a particular ordered spin structure below T_N=1.2K. The removal of the continuous degeneracy results in an energy gap in the spectrum of spin wave excitations, long wavelength pseudo-Goldstone modes. We have measured the ObQD spin wave gap at a zone center in Er2Ti2O7, using low incident energy neutrons and the time-of-flight inelastic scattering method. We report a gap of Delta =0.053 +/- 0.006 meV, which is consistent with upper bounds placed on it from heat capacity measurements and roughly consistent with theoretical estimate of ~ 0.02 meV, further validating the spin Hamiltonian that led to that prediction. The gap is observed to vary with square of the order parameter, and goes to zero for T ~ T_N., Comment: 5 pages, 3 figures

We present a parametric study of the diffuse magnetic scattering at (1/2,1/2,1/2) positions in reciprocal space, ascribed to a frozen antiferromagnetic spin ice state in single crystalline Tb2Ti2O7. Our high-resolution neutron scattering measurements show that the elastic (-0.02 meV < E < 0.02 meV) (1/2,1/2,1/2) scattering develops strongly below ~275 mK, and correlates with the opening of a spin gap of ~0.06-0.08 meV over most of the Brillouin zone. The concomitant low-lying magnetic spin excitations are weakly dispersive and appear to soften near the (1/2,1/2,1/2) wave vector at 80 mK. The nature of the transition at 275 mK has many characteristics of spin glass behavior, consistent with ac-susceptibility measurements. The application of a magnetic field of 0.075 T applied along the [1-10] direction destroys the (1/2,1/2,1/2) elastic scattering, revealing the fragility of this short-range ordered ground state. We construct a refined H-T phase diagram for Tb2Ti2O7 and [1-10] fields which incorporates this frozen spin ice regime and the antiferromagnetic long-range order previously known to be induced in relatively large fields. Specific heat measurements on the same crystal reveal a sharp anomaly at Tc~450 mK and no indication of a transition near ~275 mK. We conclude that the higher temperature specific heat peak is not related to the magnetic ordering but is likely a signal of other, nonmagnetic, correlations., Comment: 9 pages, 7 figures

Single-crystal diffuse scattering data have been collected at room temperature on synthetic titanite using both neutrons and high-energy X-rays. A simple ball-and-springs model reproduces the observed diffuse scattering well, confirming its origin to be primarily due to thermal motion of the atoms. Ab initio phonons are calculated using density-functional perturbation theory and are shown to reproduce the experimental diffuse scattering. The observed X-ray and neutron scattering patterns are consistent with a summation of mode frequencies and displacement eigenvectors associated with the entire phonon spectrum, rather than with a simple, short-range static displacement. A band gap is observed between 600 and 700 cm-1 with only two modes crossing this region, both associated with antiferroelectric Ti-O motion along a. One of these modes (of Bu symmetry), displays a large LO-TO mode-splitting (562-701.4 cm-1) and has a dominant component coming from Ti-O bond stretching and, thus, the mode-splitting is related to the polarizability of the Ti-O bonds along the chain direction. Similar mode-splitting is observed in piezo- and ferroelectric materials. The calculated phonon dispersion model may be of use to others in the future to understand the phase transition at higher temperatures, as well as in the interpretation of measured phonon dispersion curves., Comment: 19 pages, 8 figures

The geometrically frustrated double perovskite Ba2YRuO6 has magnetic 4d3 Ru5+ ions decorating an undistorted face-centered cubic (FCC) lattice. This material has been previously reported to exhibit commensurate long-range antiferromagnetic order below T_N = 36K, a factor f = 15 times lower than its Curie-Weiss temperature Theta_CW = -522 K, and purported short-range order to T* = 47K. We report new time-of-flight neutron spectroscopy of Ba2YRuO6 which shows the development of a 5 meV spin gap in the vicinity of the [100] magnetic ordering wavevector below T_N = 36K, with the transition to long-range order occurring at T* = 47K. We also report spin waves extending to 14 meV, a surprisingly small bandwidth in light of the large Theta_CW. We compare the spin gap and bandwidth to relevant neutron studies of the isostructural 4d1 material Ba2YMoO6,and discuss the results in the framework of relatively strong spin-orbit coupling expected in 4d magnetic systems., Comment: 6 pages, 7 figures, Accepted to Physical Review B

We present neutron scattering measurements on single crystals of lightly doped $La_{2-x}Ba_{x}CuO_{4}$, with $0 \leq x \leq? 0.035$. These reveal the evolution of the magnetism in this prototypical doped Mott insulator from a three dimensional (3D) commensurate (C) antiferromagnetic ground state, which orders at a relatively high TN, to a two dimensional (2D) incommensurate (IC) ground state with finite ranged static correlations, which appear below a relatively low effective TN. At low temperatures, the 2D IC magnetism co-exists with the 3D C magnetism for doping concentrations as low as ? 0.0125. We find no signal of a 3D C magnetic ground state by x $\sim$? 0.025, consistent with the upper limit of x $\sim$? 0.02 observed in the sister family of doped Mott insulators, $La_{2-x}Sr_{x}CuO_{4}$. The 2D IC ground states observed for $0.0125 \leq x \leq 0.035$ are diagonal, and are rotated by 45 degrees within the orthorhombic basal plane compared with those previously reported for samples with superconducting ground states: $La_{2-x}Ba_{x}CuO_{4}$, with $0.05 \leq? x \leq? 0.095. We construct a phase diagram based solely on magnetic order parameter measurements, which displays much of the complexity of standard high temperature superconductivity phase diagrams discussed in the literature. Analysis of high energy-resolution inelastic neutron scattering at moderately low temperatures shows a progressive depletion of the very low energy dynamic magnetic susceptibility as x increases from 0.0125 to 0.035. This low energy, dynamic susceptibility falls off? with increasing temperature on a scale much higher than the effective 2D IC TN appropriate to these materials. Appreciable dynamic 2D IC magnetic fluctuations inhabit much of the "pseudogap" regime of the phase diagram., Comment: 12 pages, 10 figures

Diluted magnetic semiconductors (DMS) have received much attention due to its potential applications to spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since 1990s. The simultaneous spin and charge doping via hetero-valence (Ga3+,Mn2+) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Previously we synthesized a new diluted ferromagnetic semiconductor of bulk Li(Zn,Mn)As with Tc up to 50K, where isovalent (Zn,Mn) spin doping was separated from charge control via Li concentrations. Here we report the synthesis of a new diluted ferromagnetic semiconductor (Ba1-xKx)(Zn1-yMny)2As2, isostructural to iron 122 system, where holes are doped via (Ba2+, K1+), while spins via (Zn2+,Mn2+) substitutions. Bulk samples with x=0.1-0.3 and y=0.05-0.15 exhibit ferromagnetic order with TC up to 180K, comparable to that of record high Tc for Ga(MnAs), significantly enhanced than Li(Zn,Mn)As. Moreover the (Ba,K)(Zn,Mn)2As2 shares the same 122 crystal structure with semiconducting BaZn2As2, antiferromagnetic BaMn2As2, and superconducting (Ba,K)Fe2As2, which makes them promising to the development of multilayer functional devices., Comment: 12 pages, 3 Figures, 3 tables

We report low temperature specific heat and muon spin relaxation/rotation ($\mu$SR) measurements on both polycrystalline and single crystal samples of the pyrochlore magnet Yb$_2$Ti$_2$O$_7$. This system is believed to possess a spin Hamiltonian supporting a Quantum Spin Ice (QSI) ground state and to display sample variation in its low temperature heat capacity. Our two samples exhibit extremes of this sample variation, yet our $\mu$SR measurements indicate a similar disordered low temperature state down to 16 mK in both. We report little temperature dependence to the spin relaxation and no evidence for ferromagnetic order, in contrast to recent reports by Chang \emph{et al.} (Nat. Comm. {\bf 3}, 992 (2012)). Transverse field (TF) $\mu$SR measurements show changes in the temperature dependence of the muon Knight shift which coincide with heat capacity anomalies. We are therefore led to propose that Yb$_2$Ti$_2$O$_7$ enters a hidden order ground state below $T_c\sim265$ mK where the nature of the ordered state is unknown but distinct from simple long range order., Comment: Accepted for publication in Phys. Rev. B. 5 pages, 4 figures

We present high-resolution single crystal time-of-flight neutron scattering measurements on the candidate quantum spin liquid pyrochlore Tb2Ti2O7 at low temperature and in a magnetic field. At ~70 mK and in zero field, Tb2Ti2O7 reveals diffuse magnetic elastic scattering at (1/2,1/2,1/2) positions in reciprocal space, consistent with short-range correlated regions based on a two-in, two-out spin ice configuration on a doubled conventional unit cell. This elastic scattering is separated from very low-energy magnetic inelastic scattering by an energy gap of ~0.06-0.08 meV. The elastic signal disappears under the application of small magnetic fields and upon elevating temperature. Pinch-point-like elastic diffuse scattering is observed near (1,1,1) and (0,0,2) in zero field at ~70 mK, in agreement with Fennell et al. (Ref. 1), supporting the quantum spin ice interpretation of Tb2Ti2O7., Comment: 7 pages, 6 figures. Replaced previous version with now published version with corrected figure 5

Recent neutron scattering and specific heat studies on the pyrochlore Yb2Ti2O7 have revealed variations in its magnetic behavior below 265mK. In the best samples, a sharp anomaly in the specific heat is observed at T=265mK. Other samples, especially single crystals, have broad features in the specific heat which vary in sharpness and temperature depending on the sample, indicating that the magnetic ground state may be qualitatively different in such samples. We performed detailed comparisons of the chemical structure of a pulverised single crystal of Yb2Ti2O7, grown by the floating zone technique, to a sintered powder sample of Yb2Ti2O7. Rietveld refinements of neutron powder diffraction data on these samples reveal that the crushed single crystal is best described as a "stuffed" pyrochlore, Yb2(Ti(2-x)Yb(x))O7-(x/2) with x = 0.046(4), despite perfectly stoichiometric starting material. Substituting magnetic Yb3+ on the non-magnetic Ti4+ sublattice would introduce random exchange bonds and local lattice deformations. These are expected to be the mechanism leading to of the variation of the delicate magnetic ground state of Yb2Ti2O7. Determination of the cubic cell length, a, could be useful as a method for characterizing the stoichiometry of non- pulverised single crystals at room temperature., Comment: 11 pages, 7 figures. Some additional discussion. Now published in Phys. Rev. B

We have explored the spin liquid state in Tb2Ti2O7 with vibrating coil magnetometry down to 0.04 K under magnetic fields up to 5 T. We observe magnetic history dependence below $T < 0.2$K reminiscent of the classical spin ice systems Ho2Ti2O7 and Dy2Ti2O7. The magnetic phase diagram inferred from the magnetization is essentially isotropic, without evidence of magnetization plateaux as anticipated for so-called quantum spin ice, predicted theoretically for [111] when quantum fluctuations renormalize the interactions. Instead, the magnetization for $T \ll T*$ agrees semi-quantitatively with the predictions of "all-in/all-out" (AIAO) antiferromagnetism. Taken together this suggests that the spin liquid state in Tb2Ti2O7 is akin to an incipient AIAO-antiferromagnet., Comment: accepted in Physical Review Letters

We respond to the comment of Bramwell et al (arXiv:1111.4168v1) to our original publication (S. R. Dunsiger et al, Phys. Rev. Lett. 107, 207207 (2011)), detailing muon spin rotation measurements of the Spin Ice compound Dy2Ti2O7., Comment: 2 pages; 1 figure; response to arXiv:1111.4168v1

The Shastry-Sutherland model, which consists of a set of spin 1/2 dimers on a 2-dimensional square lattice, is simple and soluble, but captures a central theme of condensed matter physics by sitting precariously on the quantum edge between isolated, gapped excitations and collective, ordered ground states. We compress the model Shastry-Sutherland material, SrCu2(BO3)2, in a diamond anvil cell at cryogenic temperatures to continuously tune the coupling energies and induce changes in state. High-resolution x-ray measurements exploit what emerges as a remarkably strong spin-lattice coupling to both monitor the magnetic behavior and the absence or presence of structural discontinuities. In the low-pressure spin-singlet regime, the onset of magnetism results in an expansion of the lattice with decreasing temperature, which permits a determination of the pressure dependent energy gap and the almost isotropic spin-lattice coupling energies. The singlet-triplet gap energy is suppressed continuously with increasing pressure, vanishing completely by 2 GPa. This continuous quantum phase transition is followed by a structural distortion at higher pressure., Comment: 16 pages, 4 figures. Accepted for publication in PNAS

Theory predicts the low-temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (muSR) experiment [S T Bramwell et al, Nature 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation - the so-called second Wien effect. We demonstrate via a new series of muSR experiments in Dy_2Ti_2O_7 that such an effect is not observable in a TF muSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations., Comment: 6 pages; 4 figures; published in Phys Rev. Lett. 107, 207207 (2011); Supplementary material and body of paper combined together in 1 file

The pyrochlore material Yb2Ti2O7 displays unexpected quasi-two-dimensional (2D) magnetic correlations within a cubic lattice environment at low temperatures, before entering an exotic disordered ground state below T=265mK. We report neutron scattering measurements of the thermal evolution of the 2D spin correlations in space and time. Short range three dimensional (3D) spin correlations develop below 400 mK, accompanied by a suppression in the quasi-elastic (QE) scattering below ~ 0.2 meV. These show a slowly fluctuating ground state with spins correlated over short distances within a kagome-triangular-kagome (KTK) stack along [111], which evolves to isolated kagome spin-stars at higher temperatures. Furthermore, low-temperature specific heat results indicate a sample dependence to the putative transition temperature that is bounded by 265mK, which we discuss in the context of recent mean field theoretical analysis., Comment: 5 pages, 6 figures

The geometrically frustrated double perovskite Ba2YMoO6 is characterized by quantum s=1/2 spins at the Mo5+ sites of an undistorted face-centered cubic (FCC) lattice. Previous low-temperature characterization revealed an absence of static long-range magnetic order and suggested a non-magnetic spin-singlet ground state. We report new time-of-flight and triple-axis neutron spectroscopy of Ba2YMoO6 that shows a 28 meV spin excitation with a bandwidth of ~4 meV, which vanishes above ~125 K. We identify this as the singlet-triplet excitation out of a singlet ground state, and further identify a weaker continuum of magnetic states within the gap, reminiscent of spin-polaron states arising due to weak disorder., Comment: Accepted to Phys. Rev. B

Low temperature measurements of the ac magnetic susceptibility along the [110] direction of a single crystal of the dipolar spin ice material Ho2Ti2O7, in zero static field, are presented. While behavior that is qualitatively consistent with previous work on Ho2Ti2O7 and the related material Dy2Ti2O7 is observed, this work extends measurements to appreciably lower temperatures and frequencies. In the freezing regime, below 1 K, the dynamics are found to be temperature activated, thus well described by an Arrhenius law with an activation energy close to 6J_eff, a result that is not easily explained with the current model of magnetic monopole excitations in dipolar spin ice. The form and temperature dependence of the ac susceptibility spectra are found to be nontrivial and distinct from standard glassy relaxation. Particular attention has been paid to correcting for the demagnetization effect, which is quite significant in these materials and has important, even qualitative, effects on the susceptibility spectra., Comment: 10 pages, 8 figures

We report high resolution single crystal x-ray diffraction measurements of the frustrated pyrochlore magnet Tb2Ti2O7, collected using a novel low temperature pulsed magnet system. This instrument allows characterization of structural degrees of freedom to temperatures as low as 4.4 K, and in applied magnetic fields as large as 30 Tesla. We show that Tb2Ti2O7 manifests intriguing structural effects under the application of magnetic fields, including strongly anisotropic giant magnetostriction, a restoration of perfect pyrochlore symmetry in low magnetic fields, and ultimately a structural phase transition in high magnetic fields. It is suggested that the magnetoelastic coupling thus revealed plays a significant role in the spin liquid physics of Tb2Ti2O7 at low temperatures., Comment: 4 pages, 4 figures, submitted for publication

Single crystals of the spin dimer system Sr3Cr2O8 have been grown for the first time. Magnetization, heat capacity, and magnetocaloric effect data up to 65 T reveal magnetic order between applied fields of Hc1 ~ 30.4 T and Hc2 ~ 62 T. This field-induced order persists up to ~ 8 K at H ~ 44 T, the highest observed in any quantum magnet where Hc2 is experimentally-accessible. We fit the temperature-field phase diagram boundary close to Hc1 using the expression Tc = A(H-Hc1)^v. The exponent v = 0.65(2), obtained at temperatures much smaller than 8 K, is that of the 3D Bose-Einstein condensate (BEC) universality class. This finding strongly suggests that Sr3Cr2O8 is a new realization of a triplon BEC where the universal regimes corresponding to both Hc1 and Hc2 are accessible at He-4 temperatures., Comment: 4 pages, 3 figures, accepted by PRL

Neutron scattering measurements show the ferromagnetic XY pyrochlore Yb2Ti2O7 to display strong quasi-two dimensional (2D) spin correlations at low temperature, which give way to long range order (LRO) under the application of modest magnetic fields. Rods of scattering along < 111 > directions due to these 2D spin correlations imply a magnetic decomposition of the cubic pyrochlore system into decoupled kagome planes. A magnetic field of ~0.5 T applied along the [1-10] direction induces a transition to a 3D LRO state characterized by long-lived, dispersive spin waves. Our measurements map out a complex low temperature-field phase diagram for this exotic pyrochlore magnet., Comment: 5 pages, 4 figures

By performing heat capacity, magnetocaloric effect, torque magnetometry and force magnetometry measurements up to 33 T, we have mapped out the T-H phase diagram of the S = 1/2 spin dimer compound Ba3Cr2O8. We found evidence for field-induced magnetic order between Hc1 = 12.52(2) T and Hc2 = 23.65(5) T, with the maximum transition temperature Tc = 2.7 K at H = 18 T. The lower transition can likely be described by Bose-Einstein condensation of triplons theory, and this is consistent with the absence of any magnetization plateaus in our magnetic torque and force measurements. In contrast, the nature of the upper phase transition appears to be quite different as our measurements suggest that this transition is actually first order., Comment: 4 pages, 4 figures, Accepted by PRB

Elastic and inelastic neutron scattering studies have been carried out on the pyrochlore magnet Ho2Ti2O7. Measurements in zero applied magnetic field show that the disordered spin ice ground state of Ho2Ti2O7 is characterized by a pattern of rectangular diffuse elastic scattering within the [HHL] plane of reciprocal space, which closely resembles the zone boundary scattering seen in its sister compound Dy2Ti2O7. Well-defined peaks in the zone boundary scattering develop only within the spin ice ground state below ~ 2 K. In contrast, the overall diffuse scattering pattern evolves on a much higher temperature scale of ~ 17 K. The diffuse scattering at small wavevectors below [001] is found to vanish on going to Q=0, an explicit signature of expectations for dipolar spin ice. Very high energy-resolution inelastic measurements reveal that the spin ice ground state below ~ 2 K is also characterized by a transition from dynamic to static spin correlations on the time scale of 10^{-9} seconds. Measurements in a magnetic field applied along the [1${\bar1}$0] direction in zero-field cooled conditions show that the system can be broken up into orthogonal sets of polarized alpha chains along [1${\bar1}$0] and quasi-one-dimensional beta chains along [110]. Three dimensional correlations between beta chains are shown to be very sensitive to the precise alignment of the [1${\bar1}$0] externally applied magnetic field., Comment: 11 pages, 10 figures. Submitted for publication

We report detailed measurements of the low temperature magnetic phase diagram of Er$_2$Ti$_2$O$_7$. Heat capacity and time-of-flight neutron scattering studies of single crystals, subject to magnetic fields applied along the crystallographic [110] direction, reveal unconventional low energy states. Er$^{3+}$ magnetic ions reside on a pyrochlore lattice in Er$_2$Ti$_2$O$_7$, where local XY anisotropy and antiferromagnetic interactions give rise to a unique frustrated system. In zero field, the ground state exhibits coexisting short and long range order, accompanied by soft collective spin excitations previously believed to be absent. The application of finite magnetic fields tunes the ground state continuously through a landscape of non-collinear phases, divided by a zero temperature phase transition at $\mu_0 H_c \sim$ 1.5 T. The characteristic energy scale for spin fluctuations is seen to vanish at the critical point, as expected for a second order quantum phase transition driven by quantum fluctuations., Comment: 5 pages, 4 figures, submitted for publication

The critical behaviour associated with cooperative Jahn-Teller phase transitions in TbV$_{1-x}$As$_{x}$O$_{4}$ (where \textit{x} = 0, 0.17, 1) single crystals have been studied using high resolution x-ray scattering. These materials undergo continuous tetragonal $\to$ orthorhombic structural phase transitions driven by Jahn-Teller physics at T$_C$ = 33.26(2) K, 30.32(2) K and 27.30(2) K for \textit{x} = 0, 0.17 and 1 respectively. The orthorhombic strain was measured close to the phase transition and is shown to display mean field behavior in all three samples. Pronounced fluctuation effects are manifest in the longitudinal width of the Bragg scattering, which diverges as a power law, with an exponent given by $x=0.45 \pm 0.04$, on approaching the transition from either above or below. All samples exhibited twinning; however the disordered x = 0.17 sample showed a broad distribution of twins which were stable to relatively low temperatures, well below T$_C$. This indicates that while the orthorhombic strain continues to develop in a conventional mean field manner in the presence of disorder, twin domains are easily pinned by the quenched impurities and their associated random strains., Comment: 8 pages, 6 figures

We have studied incommensurate spin ordering in single crystal underdoped La_{2-x}Ba_{x}CuO_{4} with x~0.08, 0.05 and 0.025 using neutron scattering techniques. Static incommensurate magnetic order is observed in the La_{2-x}Ba_{x}CuO_{4} (x=0.05 and 0.025) compounds with ordering wavevectors which are rotated by 45 degree about the commensurate (0.5,0.5,0) position, with respect to that in the superconducting x=0.08 material. These spin modulations are one dimensional in the x=0.05 and 0.025 samples, with ordering wavevectors lying along the orthorhombic b* direction. Such a rotation in the orientation of the static spin ordering as a function of increasing Ba doping, from diagonal to collinear, is roughly coincident with the transition from an insulating to a superconducting ground state and is similar to that observed in the related La_{2-x}Sr_{x}CuO_{4} system. This phenomenon is therefore a generic property of underdoped La-214 cuprates., Comment: 5 pages, 4 figures, submitted to PRL

Single crystals of the lightly-doped spin-Peierls system Cu(1-x)Cd(x)GeO3 have been studied using bulk susceptibility, x-ray diffraction, and inelastic neutron scattering techniques. We investigate the triplet gap in the magnetic excitation spectrum of this quasi-one dimensional quantum antiferromagnet, and its relation to the spin-Peierls dimerisation order parameter. We employ two different theoretical forms to model the inelastic neutron scattering cross section and chi''(Q,omega), and show the sensitivity of the gap energy to the choice of chi''(Q,omega). We find that a finite gap exists at the spin-Peierls phase transition., Comment: 15 Pages, 7 Figures, Submitted to J. Phys. :Condensed Matter

Using neutron scattering techniques, we have studied incommensurate spin ordering as well as low energy spin dynamics in single crystal underdoped \LBCO with x$\sim$0.095 and 0.08; high temperature superconductors with T$_C \sim$ 27 K and 29 K respectively. Static two dimensional incommensurate magnetic order appears below T$_N$=39.5 $\pm$ 0.3 K in \LBCO (x=0.095) and a similar temperature for x=0.08 within the low temperature tetragonal phase. The spin order is unaffected by either the onset of superconductivity or the application of magnetic fields of up to 7 Tesla applied along the c-axis in the x=0.095 sample. Such magnetic field {\it independent} behaviour is in marked contrast with the field induced enhancement of the staggered magnetisation observed in the related \LSCO system, indicating this phenomenon is not a universal property of cuprate superconductors. Surprisingly, we find that incommensurability $\delta $ is only weakly dependent on doping relative to \LSCO. Dispersive excitations in \LBCO (x=0.095) at the same incommensurate wavevector persist up to at least 60 K. The dynamical spin susceptibility of the low energy spin excitations saturates below \tc, in a similar manner to that seen in the superconducting state of La$_2$CuO$_{4+y}$., Comment: 9 pages, 7 figures, submitted to PRB, figures updated

We have studied structural phase transitions in high quality underdoped La$_{2-x}$Ba$_x$CuO$_4$ single crystals using high resolution x-ray scattering techniques. Critical properties associated with the continuous High Temperature Tetragonal (HTT, $I4/mmm$) to Middle Temperature Orthorhombic (MTO, $Cmca$) phase transition were investigated in single crystal samples with x=0.125, 0.095, and 0.08 and we find that all behavior is consistent with three dimensional XY criticality, as expected from theory. Power law behavior in the orthorhombic strain, 2(a-b)/(a+b), is observed over a remarkably wide temperature range, spanning most of the MTO regime in the phase diagram. Low temperature measurements investigating the Low Temperature Tetragonal (LTT, $P4_{2}/ncm$) phase, below the strongly discontinuous MTO$\to$LTT phase transition, in x=0.125 and x=0.095 samples show that the LTT phase is characterized by relatively broad Bragg scattering, compared with that observed at related wavevectors in the HTT phase. This shows that the LTT phase is either an admixture of tetragonal and orthorhombic phases, or that it is orthorhombic with very small orthorhombic strain, consistent with the ``less orthorhombic" low temperature structure previously reported in mixed La$_{2-x}$Sr$_{x-y}$Ba$_y$CuO$_4$ single crystals. We compare the complex temperature-composition phase diagram for the location of structural and superconducting phase transitions in underdoped La$_{2-x}$Ba$_x$CuO$_4$ and find good agreement with results obtained on polycrystalline samples., Comment: 8 pages, 7 figures, 1 table

We measure a sequence of quantum Hall-like plateaux at 1/q: 9 >= q >= 2 and p/q = 2/9 fractions in the magnetisation with increasing magnetic field in the geometrically frustrated spin system SrCu2(BO3)2. We find that the entire observed sequence of plateaux is reproduced by solving the Hofstadter problem on the system lattice when short-range repulsive interactions are included, thus providing a sterling demonstration of bosons confined by a magnetic and lattice potential mimicking fermions in the extreme quantum limit., Comment: 14 pages, 3 figures

High resolution X-ray scattering measurements on single crystal Tb2Ti2O7 reveal finite structural correlations at low temperatures. This geometrically frustrated pyrochlore is known to exhibit a spin liquid, or cooperative paramagnetic state, at temperatures below ~ 20 K. Parametric studies of structural Bragg peaks appropriate to the Fd$\bar{3}$m space group of Tb2Ti2O7 reveal substantial broadening and peak intensity reduction in the temperature regime 20 K to 300 mK. We also observe a small, anomalous lattice expansion on cooling below a density maximum at ~ 18 K. These measurements are consistent with the development of fluctuations above a cooperative Jahn-Teller, cubic-tetragonal phase transition at very low temperatures., Comment: 5 pages, 4 figures, submitted for publication

We have performed $\mu$SR studies on single crystals of SrCu$_2$(BO$_3$)$_2$, a quasi-two-dimensional spin system with a spin singlet ground state. We observe two different muon sites which we associate with muons located adjacent to the two inequivalent O sites. One site, presumed to be located in the Cu-O-Cu superexchange path, exhibits a large increase in the frequency shift with decreasing temperature which is unaffected by the singlet formation, indicating that the muon has locally broken the singlet bond. We have also performed $\mu$SR on single crystals of SrMg$_{0.05}$Cu$_{1.95}$(BO$_3$)$_2$, Sr$_{0.96}$La$_{0.04}$Cu$_2$(BO$_3$)$_2$, and Sr$_{0.95}$Na$_{0.05}$Cu$_2$(BO$_3$)$_2$. We have found that the frequency shifts of these doped samples are equivalent and contain three branches at low temperatures. Two of these branches map on to the branches observed in the pure sample reasonably well and so we attribute the third branch to effect of the dopants. Specifically, this third branch represents the case when the muon sits at a site in the superexchange path lacking a corresponding singlet, due to it already being broken as a result of doping. This then leads to the conclusion that singlets are broken in this system when it is doped both in and out of the CuBO$_3$ planes., Comment: 7 pages, 6 figures