Session S44: Heavy Fermions in f-electron Systems

Mechanism of strange metal state near a heavy fermion quantum critical point

Unconventional metallic or strange metal (SM) behavior with non-Fermi liquid (NFL) properties, generic features of heavy fermion (HF) systems near quantum phase transitions, are yet to be understood microscopically. A paradigmatic example is the magnetic field-tuned quantum critical HF metal YbRh₂Si₂, revealing a possible SM state over a finite range of fields at low T when substituted with Ge. Above a critical field, the SM state gives way to a heavy Fermi liquid with Kondo correlation. The NFL behavior, suc as a linear-in-temperature (T) electrical resistivity and a logarithmic-in-T followed by a power-law singularity in the specific heat coefficient at low T, still lacks a definite understanding. We propose a quasi-2d fluctuating short-ranged resonating-valence-bond spin liquid competing with the Kondo correlation as the mechanism of the experimentally observed behavior. Applying a field-theoretical renormalization group analysis on an effective field theory beyond a large-N approach to an antiferromagnetic Kondo-Heisenberg model, we identify the critical point and explain well the SM behavior.   

Emerging magnetic order in YbFe2Zn20 under pressure

YbFe₂Zn₂₀ is one of six closely related heavy fermion compounds, YbT₂Zn₂₀ (T = Fe, Ru, Os, Co, Rh, Ir) [1] with the Sommerfeld coefficient γ > 500 mJ/mol K² and Kondo temperature T_K ~ 35 K. Recent measurements under pressure up to ~8 GPa show strong suppression of T_K and Fermi-liquid region [2]. We report measurements of electrical resistivity of YbFe₂Zn₂₀ single crystals in zero and applied magnetic field under pressures up to ~25 GPa. The data suggest transition from heavy fermion to magnetically ordered ground state under pressure. Possible nature of the magnetic order will be discussed based on the measurements in magnetic field and comparison with the other members of the family.
[1] M.S. Torikachvili et al., PNAS 104 (2007) 9960.
[2] S. K. Kim, et al., Phys. Rev. B 88 (2013) 045116.
  

Thermodynamic and Transport Properties of Single Crystal YbNi4Cd

Single crystals of YbNi₄Cd and LuNi₄Cd, which crystallize in the cubic MgCu₄Sn-type structure, were grown using a Ni-Cd flux. We have studied the thermal, magnetic, and electrical properties of these compounds by means of specific heat, magnetization, and resistivity measurements. For YbNi₄Cd, a long-range magnetic order detected below ~ 1 K can be suppressed by the applied magnetic field of 5 kOe. In the paramagnetic state, the resistivity measurements show anomalous temperature dependencies with n > 2 (ρ = ρ_o + ATⁿ). We will discuss the observed physical properties in relation to the cubic heavy fermion compounds YbCu₄X (X = Ag, Cd, In).   

Field-induced quantum criticality in the heavy-fermion metal Yb2Pt2Pb

We study non-Fermi-liquid behavior due to local criticality in the heavy-fermion metal Yb2Pt2Pb. In this material fractionalized local moments are hardly hybridized with itinerant conduction electrons and behave like quasi one-dimensional spin-1/2 chains with an excitation gap. The gap can be tuned to zero via an external magnetic field. As an advantage of the one-dimensionality, the exact form of the corresponding soft modes is known so that we can account for the non-trivial dynamics of the local moments in a clear, analytical way. The weak coupling to three-dimensional electrons induces non-Fermi-liquid behavior. We calculate the resulting electronic scattering rate and resistivity in the critical regime with perturbation theory. We find a linear scaling with temperature, which agrees with experimental data.   

Quantum Oscillations in PrNi2Cd20 and PrPd2Cd20 Compounds

Our group recently discovered a variation of the RT₂X₂₀ (R is a rare earth element, T a transition metal) compounds with X = Cd that contain both d- and f- electron ions that can generate correlated electron behavior which may act together to produce novel emergent quantum phenomena. Magnetic measurements on PrNi₂Cd₂₀ and PrPd₂Cd₂₀ do not show signs of magnetic order down to 2 K and these compounds appear to have complex Fermi surfaces. To probe the Fermi surface and test relevant theories, we have grown single crystals of PrNi₂Cd₂₀ and PrPd₂Cd₂₀ and have measured Shubnikov de Haas quantum oscillations using the Proximity Detector Oscillator (PDO) method under high magnetic fields up to 60 T at low temperatures of around 1.5 K. Samples were mounted on a rotation stage, allowing the field to be applied at many different orientations with respect to the crystal axis.   

Model for Ferromagnetic Quantum Critical Point in a 1D Kondo Lattice

Motivated by recent experiments, we study a quasi-one dimensional model of a Kondo lattice with Ferromagnetic coupling between the spins. Using bosonization and dynamical large-N techniques we establish the presence of a Fermi liquid and a magnetic phase separated by a local quantum critical point, governed by the Kondo breakdown picture. Thermodynamic properties are studied and a gapless charged mode at the quantum critical point is highlighted.   

Quantum criticality in a one-dimensional Ising-anisotropic Kondo lattice model

Heavy-fermion systems have recently emerged as a playground to study quantum critical points. Here we investigate a modified Kondo lattice model in one-dimension with an additional Ising-type interaction between the localized spins. The Ising-type interaction between local moments plays the role to induce the magnetic order, and drive a magnetic phase transition from Kondo insulator phase to magnetic ordered phase. We first identify the magnetic phase transition as a continuous type. The evolution of the dynamical susceptibility across the critical point is then numerically studied by density-matrix renormalization group. We also compare our results with the locally critical point scenario governed by the Kondo breakdown picture.
  

Disordered Ferromagnetic Quantum Critical Point in CePd2P2 with Pd → Ni substitution

We report an investigation of the structural, thermodynamic, and electrical transport properties of the chemical substitution series Ce(Pd_1-xNi_x)₂P₂. Pd → Ni substitution results in (1) contraction of the unit cell volume, which tunes the relative strengths of the Kondo and RKKY interactions, and (2) the introduction of disorder through alloying. The ferromagnetic ordering temperature T_C is monotonically suppressed with increasing x, resulting in a quantum phase transition near x_cr ≈ 0.75. In this region there is a logarithmic divergence in the electronic component of the heat capacity divided by temperature that is comparable to that seen for similar Ce-based systems near antiferromagnetic quantum critical points: e.g., CeCu₂Si₂. Together with a sub-quadratic temperature dependence of the electrical resistivity, this suggests a breakdown of Fermi-liquid behavior as would be expected near a quantum critical point. This behavior is examined in the context of the Belitz-Kirkpatrick-Vojta (BKV) theory, where the disorder causes the electronic soft modes to be diffusive and, as a result, the phase transition remains second order down to zero temperature. Based on this, we suggest that this is a model system for understanding ferromagnetic quantum criticality in a disordered metal.   

Visualization of simultaneous hybridization and magnetic exchange in the semimetal CeSb

Understanding the many-body physics that leads to coexisting quantum ground states is at the center of some of the deepest problems in condensed matter physics, from high-temperature superconductivity to quantum magnetism. The so-called heavy fermion compounds are an archetype of such correlated phenomena, manifesting signatures of both magnetically ordered and many-body entangled states. The compound CeSb, as a semimetal, is an unusual example where such states not only coexist but are coincident, and understanding the mechanism behinds this has eluded a complete theory for decades. Using on-resonance angle-resolved photoemission spectroscopy, we are able to highlight Ce f-bands and provide a direct visualization of how they interact with delocalized electrons and holes in the system. We find there are two distinct modes of interaction, whose action is allowed to occur simultaneously due to their separation in momentum space. This study illustrates a simple but rare example of material how the quantum mechanical independence of momentum space allows distinct many-body phenomena to coexist.   

Spin density wave order under uniaxial stress: The strain-temperature phase diagram of CeAuSb2

We present results of electrical transport studies on the magnetic phases of the antiferromagnet CeAuSb₂ under uniaxial pressure. Unstressed CeAuSb₂ develops incommensurate spin density waves at 6.5 K, and has two metamagnetic transitions, at H₁ = 2.9 T and H₂ = 5.9 T. In the unstressed crystal, the magnetic order spontaneously lifts the C₄ symmetry of the lattice, with <110> principal axes. This spontaneous symmetry breaking is clearly seen in the response to uniaxial stress, however within our resolution C₄ symmetry is restored above H₁; we observe neither (110)/(1-10) nor (100)/(010) symmetry breaking. With compression above ~0.5%, the field-temperature phase diagram becomes qualitatively different from its form in the unstressed crystal.   

Synthesis and Investigations of the Kondo Lattice Compound CeSbS

Recent work reveals intriguing behavior in the family of layered tetragonal materials that crystallize with the formula LnSbCh (Ln = lanthanide and Ch = chalcogenide). CeSbTe has been investigated as a possible host for topologically protected electronic states [1] and CeSbSe exhibits complex magnetic ordering at T_M = 3 K, where the application of a magnetic field results in a cascade of magnetically ordered states [2]. Inspired by these results, we have synthesized a variety of f-electron analogues where Ln = La, Ce, and Yb and Ch = S, Se, and Te. We report these results and focus on the electrical transport and magnetic properties of CeSbS, which is the volume compressed analogue to CeSbSe.
[1] L. M. Schoop et al., arXiv:1707.03408 (2017).
[2] K.-W. Chen et al, Phys. Rev. B 96, 014421 (2017).   

Tuning RScT (R = rare-earth and T = transition metal) materials to the edge of stability

We report here the interplay between the crystal structure and magnetism in the series of RTX materials (R = rare-earth, T = transition metal and X = p-block element). We demonstrate that the CeScSi-type adopted by GdScGe and CeFeSi-type by GdScSb coexist over a limited range of compositions. Sb for Ge substitutions in GdScGe result in an anisotropic expansion of the unit cell of the parent that is most pronounced along the c-axis. Such expansion acts as the driving force for the instability of the double layer CeScSi-type structure of GdScGe. Extensive, yet limited Sb substitutions lead to a strong reduction of the Curie temperature, but without affecting the saturation magnetization. At x_Sb = 0.7, the appearance of an antiferromagnetic (AFM) phase coincides with the CeFeSi-type . The electronic structure calculations for x_Sb = 0.75 indicate that the key factor in the conversion of the ferromagnetic CeScSi-type to the antiferromagnetic CeFeSi-type structure is the disappearance of the induced magnetic moment on Sc.   

RIXS study of the Anderson Lattice compound CePd3

Intermediate valence (IV) materials represent one of the longest standing issues in quantum materials physics. In CePd₃, the correlation of localized and itinerant electrons results in an ambivalent state that strongly fluctuates between the Ce⁴⁺ (4f ⁰) and Ce³⁺ (4f ¹) configurations. The most striking manifestation of this Kondo hybridization of 4f electrons with a sea of transition metal states occurs at low temperatures, where the valence fluctuations may develop coherency. The few available neutron spectroscopic studies of such coherent “Anderson Lattices” have revealed that the phenomenon is associated with unusual quasiparticle excitations across the hybridization gap. The dynamic magnetic susceptibility thus acquires a characteristic energy-/momentum-space texture. The recent advances in soft x-ray resonant inelastic scattering (RIXS) are among the most anticipated developments at 3^rd generation synchrotrons. With an energy resolution on the order of E/dE=30000, soft RIXS is now able to address the small energy scale of the IV phenomenon. We present the first use of Ce M-edge RIXS to reveal the evolution of a low-temperature coherently hybridized state in CePd₃.   

Scanning Tunneling Microscopy Studies of CeTe2

The RTe₂ (where R = rare-earth ion) compounds are the less-well-studied relatives of the RTe₃ compounds. Whereas the RTe₃ compounds are composed of alternating layers of conducting double Te-planes and insulating R-Te block layers, the RTe₂ compounds are composed of alternating single conducting Te-planes and insulating R-Te block layers. Similar to the RTe₃ compounds, the RTe₂ compounds are quasi 2-dimensional and host charge density wave (CDW) states. A number of techniques, including ARPES and TEM, have been employed to gain insight into the Fermi surface and CDW states in RTe₂ compounds. Whereas STM has been successful in imaging and characterizing CDW states in RTe₃ compounds, including CeTe₃ and TbTe₃, similar comprehensive STM studies have not yet been conducted on the RTe₂ compounds. Here we present our room-temperature scanning tunneling microscopy measurements of CDW states in CeTe₂.
  

Incommensurate magnetism near quantum criticality in CeNiAsO

CeNiAsO is isotructural to the 1111 Fe-based superconductors but exhibits Rare-earth based heavy fermion magnetism. Two phase transitions in the tetragonal strongly correlated electron system CeNiAsO were probed by neutron scattering and zero field muon spin rotation ( μSR) [1]. For T < T_N1 = 8.7(3) K, a second order phase transition yields an incommensurate spin density wave with wave vector k = (0.44(4), 0, 0). For T < T_N2 = 7.6(3) K, we find co-planar commensurate order with a moment of 0.37(5) μB. By substitution of P for As or by applying pressure, the material can be driven into a paramagnetic Fermi-liquid phase [2]. Non-Fermi-liquid transport is found up to the critical phosphorus concentration xc = 0.4(1). We employed μSR to study several compositions across the phase diagram. CeNiAs1−xPxO shows the commensurate order only exists for x ≤ 0.1 so the transition at xc = 0.4(1) is from an incommensurate longitudinal spin density wave to a paramagnetic Fermi liquid. Within the conventional Hertz-Millis framework this suggests the volume of the Fermi-surface is sustained through the Quantum critical point and yet the anomalous transport properties are incompatible with this framework.
[1] Shan Wu, et al. arXiv:1707.09645 (2017)
[2] Luo, Y. et al. Nature Material 13,777-781(2014)