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Author(s): B. Michon [sup.1] [sup.2] [sup.3] , C. Girod [sup.1] [sup.2] [sup.3] , S. Badoux [sup.2] , J. Kacmarcík [sup.4] , Q. Ma [sup.5] , M. Dragomir [sup.6] , H. [...], The three central phenomena of cuprate (copper oxide) superconductors are linked by a common doping level p*--at which the enigmatic pseudogap phase ends and the resistivity exhibits an anomalous linear dependence on temperature, and around which the superconducting phase forms a dome-shaped area in the phase diagram.sup.1. However, the fundamental nature of p* remains unclear, in particular regarding whether it marks a true quantum phase transition. Here we measure 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.sup.2 that includes p*. As a function of doping, we find that C.sub.el/T is strongly peaked at p* (where C.sub.el is the electronic contribution to C) and exhibits a log(1/T) dependence as temperature T tends to zero. These are the classic thermodynamic signatures of a quantum critical point.sup.3-5, as observed in heavy-fermion.sup.6 and iron-based.sup.7 superconductors at the point where their antiferromagnetic phase comes to an end. We conclude that the pseudogap phase of cuprates ends at a quantum critical point, the associated fluctuations of which are probably involved in d-wave pairing and the anomalous scattering of charge carriers. Measurements of the specific heat of two cuprate materials at low temperature in magnetic fields large enough to suppress superconductivity and over a wide doping range reveal that the pseudogap phase of cuprates ends at a quantum critical point.

Results of systematic high-frequency electron spin resonance (ESR) studies of BaCrO, a weakly coupled $S=\frac{1}{2}$ dimer system, in magnetic fields up to 25 T are reported. Two pairs of ESR gapped modes corresponding to transitions from a spin-singlet ground state to the first excited triplet states with gaps, Δ=563 GHz and Δ=399 GHz, are revealed below H=12.5 T. The detection of the ground-state excitations by means of ESR clearly indicates the presence of a non-secular term allowing these transitions. A complex structure of the microwave absorption spectrum in magnetic fields above H is observed, those peculiarities are discussed. [ABSTRACT FROM AUTHOR]

The frequency, temperature, and the dc and ac dependences of the magnetic susceptibility of the YbCoGaO4 and LuCoGaO4 single crystals are investigated. The YbCoGaO4 behaves like an Ising spin glass with a strong uniaxial anisotropy. In contrast LuCoGaO4 is Heisenberg-like spin glass. Dynamical scaling reveals a three dimensional phase transition near Tg and yields critical exponent values between those of Heisenberg- and Ising-Like systems. [ABSTRACT FROM AUTHOR]

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 μ0Hc1~6.25 T and μ0Hc2~7 T is discontinuous, while the final quantum critical point at μ0Hc3~13 T is continuous. [ABSTRACT FROM AUTHOR]

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 to 0.08 meV over most of the Brillouin zone. 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 [110] 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 [110] 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 dipole correlations. [ABSTRACT FROM AUTHOR]

Magnetic excitations in the isostructural spin-dimer systems Sr3Cr2O8 and Ba3Cr2O8 are probed by means of high-field electron spin resonance at subterahertz 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. [ABSTRACT FROM AUTHOR]

Thermal transport measurements have been made on the spin-ice material Ho2Ti2O7 m an applied magnetic field with both the heat current and the field parallel to the [111] direction for temperatures from 50 mK to 1.2 K. A large magnetic field >6 T is applied to suppress the magnetic contribution to the thermal conductivity in order to extract the lattice conductivity. The low field thermal conductivity thus reveals a magnetic field dependent contribution to the conductivity which both transfers heat and scatters phonons. We interpret these magnetic excitations as monopolelike excitations and describe their behavior via existing Debye-Hiickel theory. [ABSTRACT FROM AUTHOR]

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 plateaus as anticipated for so-called quantum spin ice, predicted theoretically for [111] when quantum fluctuations renormalize the interactions. Instead, the magnetization for T ≪ T* agrees semiquantitatively 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. [ABSTRACT FROM AUTHOR]

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 (μSR) experiment [S. T. Bramwell et al., Nature (London) 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 μSR experiments in Dy2Ti2O7 that such an effect is not observable in a TF μSR 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. [ABSTRACT FROM AUTHOR]