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Register a new userT/B scaling without quasiparticle mass divergence: YbCo2Ge4
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YbCo$_2$Ge$_4$ is a clean paramagnetic Kondo lattice which displays non-Fermi liquid behavior. We report a detailed investigation of the specific heat, magnetic Gruneisen parameter ($Gamma_{rm mag}$) and temperature derivative of the magnetization ($M$) on a high-quality single crystal at temperatures down to $0.1$~K and magnetic fields up to 7~T. $Gamma_{rm mag}$ and $dM/dT$ display a divergence upon cooling and obey $T/B$ scaling. Similar behavior has previously been found in several other Yb-based Kondo lattices and related to a zero-field quantum critical point without fine tuning of pressure or composition. However, in the approach of $Brightarrow 0$ the electronic heat capacity coefficient of YbCo$_2$Ge$_4$ saturates at low $T$, excluding ferromagnetic quantum criticality. This indicates that $T/B$ scaling is insufficient to prove a zero-field quantum critical point.
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We report low temperature specific heat, C, magnetization, M, susceptibility, chi, and electrical resistivity, rho, measurements on high-quality single crystals of the heavy-fermion system YbRh_2(Si_{1-x} Ge_x)_2 (x=0 and 0.05). The undoped compound shows weak antiferromagnetic (AF) order at T_N=70 mK which is suppressed to below 10 mK by a tiny volume expansion in the x=0.05 system. In the latter pronounced deviations from Landau Fermi liquid (LFL) behavior occur, e.g. Delta rho ~ T over three decades in T. Both thermodynamic and magnetic properties show a crossover at about 0.3 K: At 0.3 K <= T <= 10 K we observe C/T ~ log(T_0/T) and a non-Curie behavior chi^{-1} ~ T^alpha with alpha<1 similar to what was found for the prototypical system CeCu_{5.9} Au_{0.1}. Below 0.3 K, chi turns into a Curie-Weiss dependence chi^{-1} ~ (T-Theta) indicating large unscreened Yb^{3+} moments whereas in C(T)/T a pronounced upturn occurs. In the undoped compound the AF order is suppressed continuously by critical fields B_{c0} ~= 0.06 T and 0.7 T applied perpendicular and parallel to the c-axis, respectively. For B>B_{c0} a LFL state with Delta rho = A(B)T^2 and C(T)/T = gamma_0(B) is induced, that fulfills the Kadowaki-Woods scaling A ~ gamma_0^2. Upon reducing the magnetic field to B_{c0} a 1/(B-B_{c0}) dependence of A(B) and gamma_0^2(B) indicates singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass.
Here we provide the first clear evidence of Fermi-liquid breakdown in an intermediate valence system. We employ high precision magnetization measurements of the valence fluctuating superconductor beta-YbAlB4 to probe the quantum critical free energy down to temperatures far below the characteristic energy scale of the valence fluctuations. The observed T/B scaling in the magnetization over three decades not only indicates unconventional quantum criticality, but places an upper bound on the critical magnetic field |B_c| < 0.2 mT, a value comparable with the Earths magnetic field and six orders of magnitude smaller than the valence fluctuation scale. This tiny value of the upper bound on B_c, well inside the superconducting dome, raises the fascinating possibility that valence fluctuating beta-YbAlB4 is intrinsically quantum critical, without tuning the magnetic field, pressure, or composition: the first known example of such a phenomenon in a metal.
Two-dimensional Heisenberg antiferromagnets play a central role in quantum magnetism, yet the nature of dynamic correlations in these systems at finite temperature has remained poorly understood for decades. We solve this long-standing problem by using a novel quantum-classical duality to calculate the dynamic structure factor analytically and, paradoxically, find a broad frequency spectrum despite the very long quasiparticle lifetime. The solution reveals new multi-scale physics whereby an external probe creates a classical radiation field containing infinitely-many quanta. Crucially, it is the multi-scale nature of this phenomenon which prevents a conventional renormalization group approach. We also challenge the common wisdom on static correlations and perform Monte Carlo simulations which demonstrate excellent agreement with our theory.
Temperature dependencies of gap energies and magnon lifetimes are measured in the quasi-1-dimensional S=1/2 gapped quantum magnets IPA-CuCl3 and Sul-Cu2Cl4 using inelastic neutron scattering. The results are compared to those found in literature for S=1 Haldane spin chain materials and to theoretical calculations for the O(3)- and O(N)- quantum non-linear sigma-models. It is found that when the T=0 energy gap Delta is used as the temperature scale, all experimental and theoretical curves are identical to within system-dependent but temperature-independent scaling factors of the order of unity. This quasi-universality extends over a surprising broad T range, at least up to kappa T ~ 1.5 Delta.
We present numerical solutions of the spectral functions of $t$-$J$ models with random and all-to-all exchange and global SU($M$) spin rotation symmetry. The solutions are obtained from the saddle-point equations of the large volume limit, followed by the large $M$ limit. These saddle point equations involve Greens functions for fractionalized spinons and holons carrying emergent U(1) gauge charges, obeying relations similar to those of the Sachdev-Ye-Kitaev (SYK) models. The low frequency spectral functions are compared with an analytic analysis of the operator scaling dimensions, with good agreement. We also compute the low frequency and temperature behavior of gauge-invariant observables: the electron Greens function, the local spin susceptibility and the optical conductivity; along with the temperature dependence of the d.c. resistivity. The time reparameterization soft mode (equivalent to the boundary graviton in holographically dual models of two-dimensional quantum gravity) makes important contributions to all observables, and provides a linear-in-temperature contribution to the d.c. resistivity.
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