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Elemental tellurium is a small band-gap semiconductor, which is always p-doped due to the natural occurrence of vacancies. Its chiral non-centrosymmetric structure, characterized by helical chains arranged in a triangular lattice, and the presence of a spin-polarized Fermi surface, render tellurium a promising candidate for future applications. Here, we use a theoretical framework, appropriate for describing the corrections to conductivity from quantum interference effects, to show that a high-quality tellurium single crystal undergoes a quantum phase transition at low temperatures from an Anderson insulator to a correlated disordered metal at around 17 kbar. Such insulator-to-metal transition manifests itself in all measured physical quantities and their critical exponents are consistent with a scenario in which a pressure-induced Lifshitz transition shifts the Fermi level below the mobility edge, paving the way for a genuine Anderson-Mott transition. We conclude that previously puzzling quantum oscillation and transport measurements might be explained by a possible Anderson-Mott ground state and the observed phase transition. Elemental tellurium is a natural p-type semiconductor with a chiral structure and spin-polarized Fermi surface. Here, the authors show that the pressure-induced topological change of the Fermi surface at 17 kbar triggers an Anderson-Mott insulator-to-metal transition.

X-ray diffraction, magnetization, ac susceptibility, and specific heat have been measured on high-quality single crystals of ${\mathrm{Co}}_{3}{\mathrm{Mn}}_{3}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ ludwigite. Different from previously studied ludwigites, this compound is characterized by the existence two low-dimensional subunits each containing a unique ion. The subsystem formed by ions at sites 3-1-3, known as a three-legged ladder (3LL), contains only divalent Co ions, while the 3LL formed by ions at sites 4-2-4 contains both divalent and trivalent Mn ions. Although there is only a weak disorder caused by a very small amount of Mn at sites 1, the experimental results evidence the existence of a low-temperature spin-glass phase. A dynamic scaling analysis of ac susceptibility data according to conventional critical slowing down results in a spin-glass phase-transition temperature ${T}_{g}=31.9$ K and a dynamic exponent $z\ensuremath{\nu}=7.05$. The temperature dependence of heat capacity has a linear contribution with the coefficient $\ensuremath{\gamma}=21.9$ mJ/(mol ${\mathrm{K}}^{2}$), which shows that this compound has the highest degree of magnetic disorder among ludwigites, corroborating the spin-glass state at low temperatures. The origin of this state is discussed, taking into account the particular structure of each subunit and the competition between the different exchange interactions involved.

A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between N\'eel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use $d^{10}$-$d^0$ cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$, where the isostructural end phases Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are N\'eel and columnar type antiferromagnets, respectively. We show that magnetism in Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$ is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$, similar to several spin liquid candidates, a plateau in muon spin relaxation rate and a strong $T$-linear dependence in specific heat. Our observations for Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$ highlight the role of disorder in addition to magnetic frustration in spin liquid physics., Comment: 19+4 pages, 4+3 figures. Accepted for publication in Nature Communications

In this work, we studied the cation occupancy of bimagnetic CoO/Co1−xZnxFe2O4 core/shell nanoparticles by means of X-ray absorption and Mossbauer spectroscopies, which provide element-sensitive information at the atomic scale. Our results indicate that, although the spinel ferrite forms a multi-grain shell, the Zn cations occupy solely tetrahedral sites, while the Co cations are mostly in the octahedral site. On the other hand, the Fe cations are distributed in both tetrahedral and octahedral sites for all concentrations. Also the results provide evidence for a Zn-deficient spinel with an excess of Co cations in the shell, whose origin is further rationalized in terms of the two-step synthesis process. In overall, this work gives a description of the cation occupancy in the core/shell nanoparticles and can serve as a guide to the interpretation of the magnetic properties of complex bimagnetic systems for future technological applications.

The magnetic profile of the Si(100)/NiO(35 nm)/NiFe(10 nm)/Ta(1 nm) sample has been obtained by X-ray absorption spectroscopy (XAS) and the X-ray magnetic circular dichroism (XMCD). Two experimental procedures were used. In the procedure 1, the magnetic depth profile has been determines using samples deposited with different NiFe thicknesses, Si(100)/NiO(35 nm)/NiFe(t)/Ta(1 nm), t = 1 , 3 , 5 , 7 , 10 nm . In procedure 2, the sample (NiFe=10 nm), was thinned by several in situ sputtering cycles with Ar+ ions, followed by XAS and XMCD analysis. In both procedures, the calculated magnetic moments values tend to decrease close to interface with the NiO antiferromagnetic (AF) layer, however, this decreasement is more evidenced in the sputtered sample. There is no charge transfer between Ni and Fe in the inner part of the NiFe layer, a reduction of the m orb / m spin eff has been found at the NiFe/NiO interface. Procedure 1 emerged as the most indicated to analyse the interface region.

This work presents a study of the oxyborate ${\mathrm{Ni}}_{5}\mathrm{Ti}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ using single-crystal x-ray diffraction and magnetic and specific-heat measurements. This material has in its crystalline structure subunits in the form of three-legged ladders. Despite the magnetic ions occupying these low-dimensional structures, we find that this system has long-range magnetic order at low temperatures. The analysis of the magnetization and specific-heat data shows that the process of establishing this order is complex and occurs in several stages upon lowering the temperature. Initially at 92 K a partial magnetic ordering of the Ni moments in one of the two ladders sets in. As the system is further cooled, weakly coupled clusters of spins order, and, finally, at 78 K the whole system of spins in the two ladders are magnetically ordered. Since the Ti ions are found in the nonmagnetic valence state ${\mathrm{Ti}}^{4+}$, the magnetic order is exclusively due to the ${\mathrm{Ni}}^{2+}$ ions with spin $S=1$. We discuss the ferrimagnetic nature of the long-range order, which is consistent with our knowledge of the magnetic interactions, the atomic distances, and bond angles obtained from the x-ray data.

We report how the sequence of deposition of Ni and Bi affects the final morphology of Ni–Bi bilayers. Two samples have been prepared by sputtering with the same thickness of Ni (4 nm) and Bi (40 nm) but reversed sequence of deposition. The sample with Bi deposited first formed a smooth layer of NiBi3 intermetallic, while the sample Ni deposited first formed mainly NiBi3 nanoparticles with its out-of-plane dimensions bigger than the thickness of the total deposited bilayer. The NiBi3 phase in the two samples results from differing nucleation and growth processes leading to differing morphologies. The volume of NiBi3 is smaller than the pure Bi volume needed for the reaction and this volume contraction is a thermodynamic factor that favors the NiBi3 formation. Both samples are superconducting with the critical temperature Tc of ~4.0 K. The Ni–Bi system offers an example of the formation of a superconducting compound under a volume contraction of its constituents, thus being of interest for both researchers in physics and material sciences.

Two superconducting intermetallic compounds in the Ni-Bi system, namely NiBi and NiBi3, can be formed as result of interdiffusion, if Ni/Bi are produced as bilayers at room temperature by pulsed laser deposition (PLD). These two equilibrium phases are responsible for the observed superconductivity in this kind of bilayer material. Due to different behavior in external magnetic fields, there appear two-step superconducting transitions in the electrical transport measurements. In this work we studied the superconducting and magnetic properties of the Ni-Bi bilayers with the formation of the two intermetallic compounds, NiBi and NiBi3, which have similar Tc ~ 4.0 K but very different upper critical magnetic fields at zero temperature [Bc2(0)]. The magnetic hysteresis loops of the pure NiBi3 phase show the behavior of a weak type II superconductor without strong pinning effect. Upon formation of the NiBi phase caused by highly energetic atoms during pulsed laser deposition, strong pinning effects appeared in the hysteresis loops below Tc. The isotherm V(I) curves show that the residual Ni layer does not induce spontaneous vortices in the NiBi or NiBi3 phases.

The effect of mechanical milling in i-Al64Cu23Fe13 quasicrystalline and ω-Al70Cu20Fe10 crystalline phases is systematically investigated in the present work. The Al–Cu–Fe samples were obtained by arc furnace technique and then nanostructured by means of mechanical milling. The results indicate that the solid samples present a weak ferromagnetic behavior at 300 K, showing a saturation magnetization of 0.124 emu g−1 for the icosahedral phase (i-phase) and 0.449 emu g−1 for the tetragonal phase (ω-phase). These small values could be an indication that only a few percentage of Fe atoms carry magnetic moment. The magnetic response in the nanostructured ω-phase increases up to 3.5 times higher than its corresponding solid counterpart. Whereas for the i-phase this increment is about 16 times higher. Moreover, the speed of the variation of the studied physical parameters after reducing the average grain size has been obtained from the exponent (α) of a power law fit of the experimental data. The values of α, corresponding to the magnetic response, are slightly different in each phase, which should be related to the different chemical composition and/or the type of long range order. Additionally, we also search for a critical grain size. However, this critical value has not been observed in the studied samples.

We present a study of thermal conductivity, $\kappa$, in undoped and doped strontium titanate in a wide temperature range (2-400 K) and detecting different regimes of heat flow. In undoped SrTiO$_{3}$, $\kappa$ evolves faster than cubic with temperature below its peak and in a narrow temperature window. Such a behavior, previously observed in a handful of solids, has been attributed to a Poiseuille flow of phonons, expected to arise when momentum-conserving scattering events outweigh momentum-degrading ones. The effect disappears in presence of dopants. In SrTi$_{1-x}$Nb$_{x}$O$_{3}$, a significant reduction in lattice thermal conductivity starts below the temperature at which the average interdopant distance and the thermal wavelength of acoustic phonons become comparable. In the high-temperature regime, thermal diffusivity becomes proportional to the inverse of temperature, with a prefactor set by sound velocity and Planckian time ($\tau_{p}=\frac{\hbar}{k_{B}T}$)., Comment: 5 pages including 4 figures

Phase separation and suppression of critical dynamics at quantum phase transitions of MnSi and (Sr 1− x Ca x )RuO 3

Static magnetic order of quasi-two-dimensional FeAs compounds Sr${}_{4}$$A$${}_{2}$O${}_{6\ensuremath{-}x}$Fe${}_{2}$As${}_{2}$, with $A=$ Sc and V, has been detected by $^{57}\mathrm{Fe}$ M\"ossbauer and muon spin relaxation ($\ensuremath{\mu}$SR) spectroscopies. The nonsuperconducting stoichiometric ($x=0$) $A=$ Sc system exhibits a static internal/hyperfine magnetic field both at the $^{57}\mathrm{Fe}$ and ${\ensuremath{\mu}}^{+}$ sites, indicating antiferromagnetic order of Fe moments below ${T}_{\mathrm{N}}$ $=$ 35 K with $~$0.1 bohr magneton per Fe at $T=$ 2 K. The superconducting and oxygen deficient ($x=0.4$) $A=$ V system exhibits a static internal field only at the ${\ensuremath{\mu}}^{+}$ site below ${T}_{\mathrm{N}}~40$ K, indicating static magnetic order of V moments coexisting with superconductivity without freezing of Fe moments. These results suggest that the 42622 FeAs systems belong to the same paradigm with the 1111 and 122 FeAs systems with respect to magnetic behavior of Fe moments.

We have studied the magnetism in superconducting single crystals of EuFe2As1.4P0.6 by using the local probe techniques of zero-field muon spin rotation/relaxation and 151Eu/57Fe Mossbauer spectroscopy. All of these measurements reveal magnetic hyperfine fields below the magnetic ordering temperature T M = 18 K of the Eu2+ moments. The analysis of the data shows that there is a coexistence of antiferromagnetism, resulting from Eu2+ moments ordered along the crystallographic c-axis, and superconductivity below T SC ≈ 10 K. We find indications for a change in the dynamics of the small Fe magnetic moments (~0.07 μ B ) at T ⁎ ≃ 15 K that may be triggering the onset of superconductivity: below T⁎ the Fe magnetic moments seem to be “frozen” within the ab-plane.

We report muon spin rotation ($\mu$SR) measurements of single crystal Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. From measurements of the magnetic field penetration depth $\lambda$ we find that for optimally- and over-doped samples, $1/\lambda(T\to 0)^2$ varies monotonically with the superconducting transition temperature T$_{\rm C}$. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied.

We have performed transverse field muon spin rotation measurements of single crystals of $\text{Ba}{({\text{Fe}}_{0.926}{\text{Co}}_{0.074})}_{2}{\text{As}}_{2}$ with the applied magnetic field along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{c}$ direction. Fourier transforms of the measured spectra reveal an anisotropic line-shape characteristic of an Abrikosov vortex lattice. We have fit the $\ensuremath{\mu}\text{SR}$ spectra to a microscopic model in terms of the penetration depth $\ensuremath{\lambda}$ and the Ginzburg-Landau parameter $\ensuremath{\kappa}$. We find that as a function of temperature, the penetration depth varies more rapidly than in standard weak-coupled BCS theory. For this reason we first fit the temperature dependence to a power law where the power varies from 1.6 to 2.2 as the field changes from 0.02 to 0.1 T. Due to the surprisingly strong field dependence of the power and the superfluid density we proceeded to fit the temperature dependence to a two-gap model, where the size of the two gaps is field independent. From this model, we obtained gaps of $2{\ensuremath{\Delta}}_{1}=3.77{k}_{B}{T}_{C}$ and $2{\ensuremath{\Delta}}_{2}=1.57{k}_{B}{T}_{C}$, corresponding to roughly 6 and 3 meV, respectively.

By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the $A{\text{Fe}}_{2}{\text{As}}_{2}$ $(A=\text{Ca},\text{Ba},\text{Sr})$ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of $({\text{Ba}}_{0.5}{\text{K}}_{0.5}){\text{Fe}}_{2}{\text{As}}_{2}$ exhibits a nearly linear variation in $T$ at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.

Recently, high-transition-temperature (high-T(c)) superconductivity was discovered in the iron pnictide RFeAsO(1-x)F(x) (R, rare-earth metal) family of materials. We use neutron scattering to study the structural and magnetic phase transitions in CeFeAsO(1-x)F(x) as the system is tuned from a semimetal to a high-T(c) superconductor through fluorine (F) doping, x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a collinear antiferromagnetic order with decreasing temperature. With increasing fluorine doping, the structural phase transition decreases gradually and vanishes within the superconductivity dome near x D 0 : 10, whereas the antiferromagnetic order is suppressed before the appearance of superconductivity for x > 0.06, resulting in an electronic phase diagram remarkably similar to that of the high-T(c) copper oxides. Comparison of the structural evolution of CeFeAsO(1-x)F(x) with other Fe-based superconductors suggests that the structural perfection of the Fe-As tetrahedron is important for the high-T(c) superconductivity in these Fe pnictides.