Quantum criticality describes the collective fluctuations of matter undergoing a second‐order phase transition at zero temperature. It is being discussed in a number of strongly correlated electron ...systems. A prototype case occurs in the heavy fermion metals, in which antiferromagnetic quantum critical points (QCPs) have been explicitly observed. Here, I address two types of antiferromagnetic QCPs. In addition to the standard description based on the fluctuations of the antiferromagnetic order, a local QCP is also considered. It contains inherently quantum modes that are associated with a critical breakdown of the Kondo effect. Across such a QCP, there is a sudden collapse of a large Fermi surface to a small one. I also consider the proximate antiferromagnetic and paramagnetic phases, and these considerations lead to a global phase diagram. Finally, I discuss the pertinent experiments and outline some directions for future studies.
Motivated by the properties of the iron chalcogenides, we study the phase diagram of a generalized Heisenberg model with frustrated bilinear-biquadratic interactions on a square lattice. We identify ...zero-temperature phases with antiferroquadrupolar and Ising-nematic orders. The effects of quantum fluctuations and interlayer couplings are analyzed. We propose the Ising-nematic order as underlying the structural phase transition observed in the normal state of FeSe, and discuss the role of the Goldstone modes of the antiferroquadrupolar order for the dipolar magnetic fluctuations in this system. Our results provide a considerably broadened perspective on the overall magnetic phase diagram of the iron chalcogenides and pnictides, and are amenable to tests by new experiments.
Heavy Fermions and Quantum Phase Transitions Si, Qimiao; Steglich, Frank
Science (American Association for the Advancement of Science),
09/2010, Letnik:
329, Številka:
5996
Journal Article
Recenzirano
Odprti dostop
Quantum phase transitions arise in many-body systems because of competing interactions that promote rivaling ground states. Recent years have seen the identification of continuous quantum phase ...transitions, or quantum critical points, in a host of antiferromagnetic heavy-fermion compounds. Studies of the interplay between the various effects have revealed new classes of quantum critical points and are uncovering a plethora of new quantum phases. At the same time, quantum criticality has provided fresh insights into the electronic, magnetic, and superconducting properties of the heavy-fermion metals. We review these developments, discuss the open issues, and outline some directions for future research.
We address the global magnetic phase diagram of Kondo lattice systems. Through the distinct Fermi surface properties of the various phases at zero temperature, we argue that the phase diagram ...supports two quantum critical point (QCP) classes. One of these describes a direct transition from a magnetic metal phase with localized f-electrons to a paramagnetic one with itinerant f-electrons. This result provides the context for the picture of local quantum criticality, in which the Fermi surface jumps across the QCP and the quasiparticle residue vanishes as the QCP is approached from either side. Some of the unusual experiments, concerning the phases and QCPs of heavy fermion metals, are discussed from the present perspective. These developments have implications in broader contexts. In particular, they form a part of the growing evidence for quantum criticality that goes beyond the orthodox description in terms of order-parameter fluctuations.
Strongly correlated electron systems at the border of magnetism are of active current interest, particularly because the accompanying quantum criticality provides a route towards both strange‐metal ...non‐Fermi liquid behavior and unconventional superconductivity. Among the many important questions is whether the magnetism acts simply as a source of fluctuations in the textbook Landau framework, or instead serves as a proxy for some unexpected new physics. We put into this general context the recent developments on quantum phase transitions in antiferromagnetic (AF) heavy fermion metals. Among these are the extensive recent theoretical and experimental studies on the physics of Kondo destruction in a class of beyond‐Landau quantum critical points (QCPs). Also discussed are the theoretical basis for a global phase diagram of AF heavy fermion metals, and the recent surge of materials suitable for studying this phase diagram. Furthermore, we address the generalization of this global phase diagram to the case of Kondo insulators, and consider the future prospect to study the interplay among Kondo coherence, magnetism, and topological states. Finally, we touch upon related issues beyond the AF settings, arising in mixed valent, ferromagnetic, quadrupolar, or spin glass f‐electron systems, as well as some general issues on emergent phases near QCPs.
Abstract
Recent experiments in multiband Fe-based and heavy-fermion superconductors have challenged the long-held dichotomy between simple
s
- and
d
-wave spin-singlet pairing states. Here, we ...advance several time-reversal-invariant irreducible pairings that go beyond the standard singlet functions through a matrix structure in the band/orbital space, and elucidate their naturalness in multiband systems. We consider the
s
τ
3
multiorbital superconducting state for Fe-chalcogenide superconductors. This state, corresponding to a
d
+
d
intra- and inter-band pairing, is shown to contrast with the more familiar
d
+ i
d
state in a way analogous to how the B- triplet pairing phase of
3
He superfluid differs from its A- phase counterpart. In addition, we construct an analog of the
s
τ
3
pairing for the heavy-fermion superconductor CeCu
2
Si
2
, using degrees-of-freedom that incorporate spin-orbit coupling. Our results lead to the proposition that
d
-wave superconductors in correlated multiband systems will generically have a fully-gapped Fermi surface when they are examined at sufficiently low energies.
We consider the scaling behavior of thermodynamic quantities in the one-dimensional transverse field Ising model near its quantum critical point (QCP). Our study has been motivated by the question ...about the thermodynamical signatures of this paradigmatic quantum critical system and, more generally, by the issue of how quantum criticality accumulates entropy. We find that the crossovers in the phase diagram of temperature and (the nonthermal control parameter) transverse field obey a general scaling ansatz, and so does the critical scaling behavior of the specific heat and magnetic expansion coefficient. Furthermore, the Grüneisen ratio diverges in a power-law way when the QCP is accessed as a function of the transverse field at zero temperature, which follows the prediction of quantum critical scaling. However, at the critical field, upon decreasing the temperature, the Grüneisen ratio approaches a constant instead of showing the expected divergence. We are able to understand this unusual result in terms of a peculiar form of the quantum critical scaling function for the free energy; the contribution to the Grüneisen ratio vanishes at the linear order in a suitable Taylor expansion of the scaling function. In spite of this special form of the scaling function, we show that the entropy is still maximized near the QCP, as expected from the general scaling argument. Our results establish the telltale thermodynamic signature of a transverse-field Ising chain, and will thus facilitate the experimental identification of this model quantum-critical system in real materials.
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