Session T08: Phases and phase transitions in strongly correlated metals

Effects of elastic correlations on random field disorder in electronic nematics

Electronic nematicity is a phase seen in many quantum materials in which the electrons collectively lower the point group symmetry of the underlying crystal. Due to nemato-elastic coupling, local nematic order parameter is tied to any spatial inhomogeneity within the crystal that locally lowers the same symmetry. These inhomogeneities are due to local internal strains which then serve as random field disorder for the nematic. Typically, the random field Ising model is used to model the low-energy physics of these systems. However, there is growing experimental evidence that conventional treatments with uncorrelated random fields cannot describe even the paranematic phase. Since internal strains are due to crystalline defects, we instead calculate the distribution of strains with an ensemble of dislocations via elasticity theory. These elastic defects produce correlations in the strain distribution not only through the medium, but also between different components of the strain tensor. We demonstrate the effects of these correlations in the paranematic phase of TmVO₄, treating it as an Ising model in the presence of both random longitudinal and random transverse fields generated by the same ensemble of dislocations.   

Interplay between electronic nematicity and acoustic phonons in cubic lattices

Electronic nematicity, a state that spontaneously breaks rotational symmetry, has been widely investigated in tetragonal and hexagonal systems, where it is described by an Ising or a 3-state Potts order parameter, respectively. The nematic critical properties in these lattices are known to be strongly impacted by the underlying crystalline medium via the nemato-elastic coupling, since acoustic phonons mediate anisotropic long range nematic interactions. In this work, we investigate the intrinsically coupled nematic and elastic phenomena on the relatively unexplored cubic lattice. We find that two types of nematic order emerge: 3-state Potts (Z₃) nematicity, associated with the breaking of threefold rotational symmetry, and 4-state Potts (Z₄) nematicity, related to fourfold rotations. We calculate the contribution of the acoustic phonons to the nematic self-energy and find that, although in both cases the nematic mass softens only along certain momentum-space directions, in the Z₄ case the nematic director is significantly rotated by the phonon-mediated interactions whereas in the Z₃ case the nematic director is unaffected by them.   

The Effects of Ni and Pt Doping on the Thermoelectric Properties of YbIr2Zn20

Thermoelectricity is a direct conversion between heat energy and electrical power, making it an attractive renewable energy source. Other advantages include that thermoelectric modules have no moving parts, which makes them well-suited to a variety of specialized applications (e.g., space exploration and cryogenic refrigeration). However, such materials also face limitations due to an intrinsic competition between their electronic and lattice behaviors. This is especially true for low-temperature thermoelectrics which, so far, have not been widely used in technologies. Intriguingly, it was recently shown that some of the compounds YbTM₂Zn₂₀ (TM = transition metal) exhibit competitive thermoelectric properties at low temperatures, that are likely related to strong hybridization between the f- and conduction electron states [1, 2]. Expanding on this, we investigate chemical substitution on the transition metal site of YbIr₂Zn₂₀ using nickel and platinum. Large single crystals were synthesized using the molten metal flux growth technique, where the structure was characterized using powder X-ray diffraction and the stoichiometry was determined using energy dispersive spectroscopy. Through magnetic, electrical, and thermal properties measurements, we will reveal changes in the electronic and structural states, and their influence on the thermoelectric properties.

[1] Mun, et. al. Phys. Rev. B, 115110 (2012).

[2] Wei, et. al. Sci. Adv. 5, eaaw6183 (2019).   

Magnetic field and temperature dependence of anomalous transport properties in YbCuAS2 Kondo lattice compound

YbCuAs₂ has been characterized as an antiferromagnetic Kondo Lattice system with T_N ~ 3.7 K. Previously, the magnetic field dependences of the electrical resistivity, specific heat, and magnetization show the unusual hysteresis loop, persisting even above the magnetic ordering temperature [1]. Motivated by the magnetic field dependent hysteresis, Hall resistivity and thermoelectric power measurements are performed to understand the origin of the hysteresis. Interestingly, both Hall and thermoelectric power indicate a large hysteresis loop over the wide range of temperature, which was not observed in earlier thermodynamic and transport measurements. In this talk, we will discuss the anomalous hysteresis loop in regarding a valence instability of Yb-ions in YbCuAs₂.

[1] David Evans, et al., PHYSICAL REVIEW B 105, 085105 (2022)   

Improved Thermoelectric Properties of YbCo2Zn20 through Ni Doping

As global energy demands escalate and environmental concerns intensify, thermoelectricity emerges as a sustainable and innovative solution for harnessing waste heat and reducing our ecological footprint. While there are now a variety of effective high-temperature thermoelectrics, those that are useful at low temperatures are less common. To address this, we focus on the heavy-fermion compounds YbTM₂Zn₂₀ (TM = Co, Rh, Ir), which previously were shown to exhibit attractive thermoelectric figures of merit at low temperatures [1,2]. In particular, we present an investigation on the chemical substitution series YbCo_2-xNi_xZn₂₀, where we find that modest Ni doping rapidly increases the low-temperature Seebeck coefficient. As a result, the thermoelectric figure of merit is also improved. This is compared to what is seen for Co→Rh→Ir tuning, which also results in a rapid enhancement of the Seebeck coefficient. By comparing the distinct tuning axes of isoelectronic volume expansion (Co → Ir) and isovolume electronic tuning (Co → Ni) using magnetization, heat capacity, electrical resistivity, and Hall effect measurements, we clarify strategies for optimizing the thermoelectric properties of these materials and provide insights into the f-electron ground state.

 

[1] Mun et al., Phys. Rev. B 86, 115110 (2012).

[2] Wei, et. al. Sci. Adv. 5, eaaw6183 (2019).   

Magnetic structure and resistivity minimum in GdCuAs2

The electrical resistivity of GdCuAs₂ single crystals exhibits an anomalous Kondo-like resistivity minimum above the antiferromagnetic ordering temperature T_N1 ≈ 10.6 K, which is unusual for highly localized 4f- moment (Gd³⁺) system. Using X-ray resonant magnetic scattering, we determined the magnetic structure of GdCuAs₂, where Gd moments are antiferromagnetically aligned along the crystallographic a axis and (++−−) arrangement in the c direction and ferromagnetically arranged in the b direction. The antiferromagnetic order appears first at q = (δ, 0, 0.5) below 10 K, with an incommensurate modulation along the a axis, and locks into a commensurate position at q = (1/3, 0, 0.5) below T_N2 ≈ 6 K. Our high-resolution X-ray diffraction measurements show a two-peak structure at Q = (2, 0, 6) above the resistivity minimum, suggesting a lower symmetry crystal structure than the reported tetragonal structure, and the Q = (2, 0, 6) peak becomes a sharp one-peak structure below the resistivity minimum, implying a magnetoelastic coupling above TN. Our findings suggest a complex interplay between the crystal structure and antiferromagnetic structure through a magnetoelastic cou- pling, associated with the anomalous resistivity minimum above the T_N1.   

Zero-field Coulomb blockade thermometers

Coulomb Blockade Thermometers (CBTs) exploit the temperature dependence of conductance in normal metal tunnel junction arrays for primary thermometry at cryogenic temperatures. Traditionally, tunnel junctions at millikelvin temperatures utilize superconducting materials, leading to the use of external magnetic field to suppress the superconductivity. To this end, we describe our latest efforts to realize CBTs at zero magnetic field. Fabrication process relying on the side wall passivation by spacer isolation (SWAPS) [1] was used to manufacture tunnel junctions, followed by electroplating of copper with thicknesses up to tens of micrometers on thermalization islands, ensuring good scalability of the process. The large Copper islands enhance the electron-phonon coupling and enable the usage of nuclear demagnetization refrigeration techniques while pursuing temperatures in the microkelvin regime [2]. We discuss CBT designs for different temperature ranges and show measurement results that verify their operation at the lowest temperatures.   

[1] L. Grönberg et al,” Side-wall spacer passivated sub-µm Josephson junction fabrication process”, Supercond. Sci. Technol. 30, 125016 (2017). 

[2] Samani, M et al. ”Microkelvin electronics on a pulse-tube cryostat with a gate Coulomb-blockade thermometer”. Physical Review Research, 4(3), 033225 (2022).    

Geometric frustration produces Bose metal as a failed insulator and probing it with electrons produces generic non-Fermi liquid scattering and pseudogap phenomenaphysics

Two of the most prominent phases of bosonic matter are the superfluid with perfect flow and the insulator with no flow. A now decades-old mystery unexpectedly arose when experimental observations indicated that bosons could organize into the formation of an entirely different intervening third phase: the Bose metal with dissipative flow. We recently proposed a universal homogeneous theory for a Bose metal in which geometric frustration confines the essential quantum coherence to a lower dimension resulting in a gapless insulator characterized by dissipative flow that vanishes in the low-energy limit. Even more recently, we have demonstrated that non-Fermi liquid behavior and pseudogap formation are inevitable consequences when fermions couple to our Bose metal at mean field level due to its unconventional lower-dimensional coherence. Not only do we find both exotic phenomena, but also a host of other features that have been observed e.g. in the cuprates including nodal anti-nodal dichotomy and pseudogap asymmetry (symmetry) in momentum (real) space. Obtaining these exotic and heretofore mysterious phenomena offers a simple, universal, and therefore widely applicable explanation for their ubiquitous empirical appearance.   

Obstructed Atomic Insulators and Superfluids of Fermions Coupled to $mathbb{Z}_2$ Gauge Fields

We study spin-$frac{1}{2}$ fermions coupled to $mathbb{Z}_2$ gauge fields on the square lattice and show how a spatial modulation of the fermion hopping amplitude allows for the realization of various obstructed atomic insulators that host higher-order band topology. Up on including the effect of quantum dynamics of the gauge fields within a simplified model, we find a rich phase diagram of this system with a number of superfluid phases (which host distinct types of topological defects) arising from the attractive interactions meditated by the gauge fields. The evolution of the superfluid obtained by the destabilization of the obstructed atomic insulators from the Bardeen-Cooper-Schrieffer (BCS) type to a Bose-Einstein condensate (BEC) of tightly bound pairs occurs via the realization of these different superfluid phases separated by first-order transitions.   

Singular-mode functional renormalization-group approach to the electron nematic state and unconventional superconductivity in two-dimensional electron systems

The electron nematic states have been the recent main topics in the strongly-correlated electron systems [1,2]. For the theoretical description of the electron nematic states, we have to go beyond the standard theory, e.g., RPA. The functional renormalization group method is a powerful theoretical tool that can treat the correlation effects in an unbiased way [3]. In the present study, we employ the singular-mode functional renormalization group (SM-FRG) method [4] to analyze the electron nematic states. In the SM-FRG, the interaction vertex is decomposed into the spin, charge, and superconducting channels, and the physical interpretation of the mutual interactions among them becomes apparent. We found that the SM-FRG can describe the spin-fluctuation-induced nematic fluctuations and studied the interference between the superconducting and nematic fluctuations in detail. In the present study, we also focused on the staggered d-wave superconducting state in the spinless two-band model with two fermi pockets [5]. We applied the SM-FRM method to the spinful Hubbard model and found that the staggered d-wave state is also enhanced in more realistic models.

[1] M. Tsuchiizu, K. Kawaguchi, Y. Yamakawa and H. Kontani, Phys. Rev. B 97, 165131 (2018).

[2] R. Tazai, Y. Yamakawa, M. Tsuchiizu, and H. Kontani, J. Phys. Soc. Jpn. 90, 111012 (2021).

[3] W. Metzner, M. Salmhofer, C. Honerkamp, V. Meden, and K. Schoenhammer, Rev. Mod. Phys. 84, 299 (2012).

[4] C. Husemann and M. Salmhofer, Phys. Rev. B 79, 195125 (2009); C. Husemann and W. Metzner, Phys. Rev. B 86, 085113 (2012).

[5] Y. He, K. Yang, J. B. Hauck, E. J. Bergholtz, and D. M. Kennes, Phys. Rev. Research 5, L012009 (2023).