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Register a new userMultiple regions of quantum criticality in YbAgGe
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Dilation and thermopower measurements on YbAgGe, a heavy-fermion antiferromagnet, clarify and refine the magnetic field-temperature (H-T) phase diagram and reveal a field-induced phase with T-linear resistivity. On the low-H side of this phase we find evidence for a first-order transition and suggest that YbAgGe at 4.5 T may be close to a quantum critical end point. On the high-H side our results are consistent with a second-order transition suppressed to a quantum critical point near 7.2 T. We discuss these results in light of global phase diagrams proposed for Kondo lattice systems.
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Bicritical points, at which two distinct symmetry-broken phases become simultaneously unstable, are typical for spin-flop metamagnetism. Interestingly, the heavy-fermion compound YbAgGe also possesses such a bicritical point (BCP) with a low temperature T_BCP ~ 0.3 K at a magnetic field of mu_0 H_BCP ~ 4.5 T. In its vicinity, YbAgGe exhibits anomalous behavior that we attribute to the influence of a quantum bicritical point (QBCP), that is close in parameter space yet can be reached by tuning T_BCP further to zero. Using high-resolution measurements of the magnetocaloric effect, we demonstrate that the magnetic Grueneisen parameter Gamma_H indeed both changes sign and diverges as required for quantum criticality. Moreover, Gamma_H displays a characteristic scaling behavior but only on the low-field side, H < H_BCP, indicating a pronounced asymmetry with respect to the critical field. We speculate that the small value of T_BCP is related to the geometric frustration of the Kondo-lattice of YbAgGe.
We consider 2+1 dimensional conformal gauge theories coupled to additional degrees of freedom which induce a spatially local but long-range in time $1/(tau-tau)^2$ interaction between gauge-neutral local operators. Such theories have been argued to describe the hole-doped cuprates near optimal doping. We focus on a SU(2) gauge theory with $N_h$ flavors of adjoint Higgs fields undergoing a quantum transition between Higgs and confining phases: the $1/(tau-tau)^2$ interaction arises from a spectator large Fermi surface of electrons. The large $N_h$ expansion leads to an effective action containing fields which are bilocal in time but local in space. We find a strongly-coupled fixed point at order $1/N_h$, with dynamic critical exponent $z > 1$. We show that the entropy preserves hyperscaling, but nevertheless leads to a linear in temperature specific heat with a co-efficient which has a finite enhancement near the quantum critical point.
In fermionic systems with different types of quasi-particles, attractive interactions can give rise to exotic superconducting states, as pair density wave (PDW) superconductivity and breached pairing. In the last years the search for these new types of ground states in cold atom and in metallic systems has been intense. In the case of metals the different quasi-particles may be the up and down spin bands in an external magnetic field or bands arising from distinct atomic orbitals that coexist at a common Fermi surface. These systems present a complex phase diagram as a function of the difference between the Fermi wave-vectors of the different bands. This can be controlled by external means, varying the density in the two-component cold atom system or, in a metal, by applying an external magnetic field or pressure. Here we study the zero temperature instability of the normal system as the Fermi wave-vectors mismatch of the quasi-particles (bands) is reduced and find a second order quantum phase transition to a PDW superconducting state. From the nature of the quantum critical fluctuations close to the superconducting quantum critical point (SQCP), we obtain its dynamic critical exponent. It turns out to be $z=2$ and this allows to fully characterize the SQCP for dimensions $d ge 2$.
Iridates provide a fertile ground to investigate correlated electrons in the presence of strong spin-orbit coupling. Bringing these systems to the proximity of a metal-insulator quantum phase transition is a challenge that must be met to access quantum critical fluctuations with charge and spin-orbital degrees of freedom. Here, electrical transport and Raman scattering measurements provide evidence that a metal-insulator quantum critical point is effectively reached in 5 % Co-doped Sr$_2$IrO$_4$ with high structural quality. The dc-electrical conductivity shows a linear temperature dependence that is successfully captured by a model involving a Co acceptor level at the Fermi energy that becomes gradually populated at finite temperatures, creating thermally-activated holes in the $J_{text {eff}}=1/2$ lower Hubbard band. The so-formed quantum critical fluctuations are exceptionally heavy and the resulting electronic continuum couples with an optical phonon at all temperatures. The magnetic order and pseudospin-phonon coupling are preserved under the Co doping. This work brings quantum phase transitions, iridates and heavy-fermion physics to the same arena.
During the last few years, investigations of Rare-Earth materials have made clear that not only the heavy fermion phase in these systems provides interesting physics, but the quantum criticality where such a phase dies exhibits novel phase transition physics not fully understood. Moreover, attempts to study the critical point numerically face the infamous fermion sign problem, which limits their accuracy. Effective action techniques and Callan-Symanzik equations have been very popular in high energy physics, where they enjoy a good record of success. Yet, they have been little exploited for fermionic systems in condensed matter physics. In this work, we apply the RG effective action and Callan-Symanzik techiques to the heavy fermion problem. We write for the first time the effective action describing the low energy physics of the system. The f-fermions are replaced by a dynamical scalar field whose nonzero expected value corresponds to the heavy fermion phase. This removes the fermion sign problem, making the effective action amenable to numerical studies as the effective theory is bosonic. Renormalization group studies of the effective action can be performed to extract approximations to nonperturbative effects at the transition. By performing one-loop renormalizations, resummed via Callan-Symanzik methods, we describe the heavy fermion criticality and predict the heavy fermion critical dynamical susceptibility and critical specific heat. The specific heat coefficient exponent we obtain (0.39) is in excellent agreement with the experimental result at low temperatures (0.4).
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