Session B22: Quantum Criticality and Novel Phases in f-electron Systems

Quasi-1D heavy fermion magnet Yb$_2$Pt$_2$Pb in Magnetic Field

The Yb$^{3+}$ ions in Yb$_2$Pt$_2$Pb form large, seemingly classical Ising magnetic moments, with the large spin-orbit coupling of the 4f-electrons and the crystal electric field forming a $J=\pm 7/2$ Yb ground state doublet [1]. However, from this unlikely host, emerges a continuum of quantum excitations -- spinons on one dimensional chains -- in good agreement with the behavior expected for nearly isotropic, $S=\pm 1/2$, d-electron magnetic moments [2]. These spinons, in a system with orbitally dominated magnetic constituents, are robust and at low temperatures exist up to 2.3 T, when all Yb magnetic moments become saturated. In magnetic fields larger than 0.5 T, the spinon gap is closed, modifying the quantum continuum through the formation of a fermi surface and spinon bound states between the Yb chains. The ground state doublet nature of the Yb ions ensures that at all fields, transverse excitations are virtually nonexistent, giving unprecedented access to only the longitudinal excitation channel without the presence of spin waves or other transverse damping mechanisms.\newline [1] M. S. Kim, \textit{et al.}, \textit{Phys. Rev. B.} \textbf{77}, 144425 (2008); K. Iwakawa \textit{et al.}, \textit{J. Phys. Soc. Jpn.} \textbf{81}, SB058 (2012);Y. Shimura \textit{et al.}, \textit{J. Phys. Soc. Jpn.} \textbf{81} 103601 (2012); M. S. Kim and M. C. Aronson, \textit{Phys. Rev. Lett} \textbf{110}, 017201 (2013); W. Miiller \textit{et al.}, \textit{Phys. Rev. B} \textbf{93}, 104419 (2016). [2] L. S. Wu \textit{et al.}, \textit{Science} \textbf{352}, 1206 (2016).   

Quantum criticality in geometrically frustrated heavy-fermion systems

Study of geometrically frustrated Kondo lattices has been motivated by the prediction of unconventional quantum criticality and metallic quantum spin liquid states. We focus on heavy-fermion metals YbAgGe, CePdAl and CeRhSn. All of them crystallize in the hexagonal ZrNiAl structure with 4f moments forming a distorted kagom\'{e} network. Using magnetic field, chemical substitution as well as uniaxial pressure, various quantum critical points are investigated by millikelvin thermodynamic experiments. In YbAgGe quantum-bicriticality is induced by magnetic field, leading drastic non-Fermi liquid effects [1,2]. For CePd$_{\mathrm{1-x}}$Ni$_{\mathrm{x}}$Al two-dimensional (2D) AF quantum criticality arises at the suppression of 3D magnetic order and signatures of magnetic frustration remain even beyond the quantum critical point [3]. In CeRhSn evidence for quantum criticality induced by geometrical frustration has been found [4] and frustration is modified using uniaxial pressure. [1] J. Dong, Y. Tokiwa, S. L. Bud'ko, P. C. Canfield, P. Gegenwart, Phys. Rev. Lett. 110 (2013) 176402. [2] Y. Tokiwa, M. Garst, P. Gegenwart, S.L. Bud'ko, P.C. Canfield, Phys. Rev. Lett. 111 (2013) 116401. [3] A. Sakai, S. Lucas, P. Gegenwart, O. Stockert, H.v. L\"{o}hneysen, V. Fritsch, arXiv:1609.00816. [4] Y. Tokiwa, C. Stingl, M.-S. Kim, T. Takabatake, P. Gegenwart, Sci. Adv. 1, e1500001 (2015).   

Global phase diagram and quantum criticality of the Ising-anisotropic Kondo lattice

Heavy fermion systems provide a prototype setting to study quantum-phase transitions in strongly-correlated systems. Experiments on many quantum critical heavy-fermion metals have provided strong evidence for quantum criticality beyond the Landau framework, with magnetic order developing at a breakdown of the underlying Kondo effect and involving an abrupt collapse of the entire Fermi surface. To account for this local quantum criticality along with other possible types of quantum phase transitions, a global phase diagram has emerged [1] which, in addition to the competition between the Kondo effect and the tendency towards magnetism, also considers the effects of the quantum fluctuations of the local moments. Motivated by these developments, we investigated the effect of enhanced quantum fluctuations induced by a transverse magnetic field in an Ising-anisotropic Kondo lattice model [2]. Solved within an extended dynamical mean field theory using the numerical renormalization group, our work represents one of the very first concrete theoretical studies on the interplay between the Doniach tuning of RKKY vs. Kondo interactions and the local-moment quantum fluctuations. We demonstrate that local quantum criticality survives along a line of critical points with unchanged critical exponents. In addition to elucidating the theory of the Global Phase Diagram, our results also motivate new experimental tests of this T$=$0 phase diagram. Work done in collaboration with Kevin Ingersent and Qimiao Si. [1] Q. Si, Physica B 378, 23 (2006); P. Coleman and A. Nevidomsky, J. Low. Temp. Phys. 161, 182 (2010). [2] E. M. Nica, K. Ingersent, and Q. Si, arXiv:1603.03829 (2016).   

Preserved Entropy, quantum criticality and fragile magnetism

A large swath of strongly correlated electron systems can be associated with the phenomenon of preserved entropy and fragile magnetism. In this talk I will present our thoughts and plans for the discovery and development of lanthanide and transition metal based, strongly correlated systems that are revealed by suppressed, fragile magnetism or grow out of preserved entropy. This talk is based on work published in Rep. Prog. Phys. 79 (2016) 084506   

Emergent Phases in Heavy Fermions: a Magnetic Substitution Study

Unconventional superconductivity frequently is found as an antiferromagnetic transition is tuned by chemical substitution or applied pressure toward a zero-temperature phase transition, a magnetic quantum-critical point. Different classes of unconventional superconductors (e.g. heavy-fermions, cuprates, and Fe-based) display a collective magnetic excitation in their superconducting state, and a universal relationship exists between the energy of this spin resonance mode and the superconducting gap. In this talk, I will focus on the heavy-fermion family of unconventional superconductors Ce$T$In$_{5}$ ($T =$ Co, Rh). I will show evidence for an emergent spin-density wave phase below the superconducting dome in pressurized CeRhIn$_{5}$ containing small amounts of magnetic substitution in the Ce site. This phase can be understood as the condensation of the spin resonance mode, providing a universal scenario for these materials.