arXiv subjects

Eric D. Bauer

Publications and source records attributed to Eric D. Bauer.

At least 19 recordsLinked to original sources

arXiv2026

High-quality single crystals of the kagome metals Ni$_3$In and Ni$_3$Sn grown from Pb flux

The bilayer kagome metal Ni$_3$In has recently attracted attention due to the presence of a flat band located near the Fermi level, which has been associated with unconventional thermodynamic and electronic transport properties [Ye et al., Nat. Phys. 20, 610-614 (2024)]. However, further investigation of the intrinsic properties of this system has been hindered by the lack of large, high-quality single crystals. Here, we report the successful growth of Ni$_3$(In, Sn) single crystals using a Pb-flux technique. By optimizing the growth conditions, competing binary phases can be effectively suppressed, enabling the synthesis of single crystals with dimensions reaching several millimeters. We compare the physical properties of our Pb-flux-grown crystals to previously reported samples prepared by iodine-assisted chemical vapor transport and molecular beam epitaxy as well as to first-principles resistivity calculations. We find a significantly lower electrical resistivity in our crystals, in excellent agreement with calculations of resistivity from electron-phonon scattering, a sizeable non-saturating magnetoresistance, and a reduced Sommerfeld coefficient and magnetic susceptibility compared to previous experimental findings, which are likely related to differences in the Fermi level position. Our results establish Pb-flux growth as a reliable route for obtaining large single crystals of the bilayer kagome metals Ni$_3$(In, Sn) that are suitable for further thermodynamic and spectroscopic investigations of their intrinsic electronic and magnetic properties.

cond-mat.mtrl-sci↗

arXiv2026

The Kondo effect in ferromagnetic quantum critical CeRh$_6$Ge$_4$

The mechanism of a pressure-induced quantum critical point in the heavy fermion ferromagnet CeRh$_6$Ge$_4$ has attracted interest, as ferromagnetic quantum criticality in a clean itinerant Ce compound is typically avoided. The localized versus itinerant character of the 4\textit{f} electrons is a key aspect for understanding this behavior. We investigated the electronic structure of the 4\textit{f} shell in CeRh$_6$Ge$_4$ using core-level photoelectron and x-ray absorption spectroscopy, demonstrating the hybridization of Ce 4\textit{f} with the conduction electrons. Linearly polarized x-ray absorption reveals a temperature-dependent linear dichroism consistent with the crystal-electric-field (CEF) sequence as inferred from the static susceptibility. This dichroism cannot be described by an ionic full-multiplet model alone, but is reproduced by including the Kondo effect within a single-impurity Anderson model in the non crossing approximation (SIAM/NCA). The Kondo effect mixes higher lying crystal-field states into a resulting multiorbital ground state with 4\textit{f} occupancy \textit{n}$_f$\,$\sim$\,0.9. Deviations at low temperatures between the measured linear dichroism and calculated dichroism suggest an orbital-dependent Kondo effect. A scenario in which there is a multiorbital ground state and orbital-dependent Kondo hybridization should be a starting point for a model of pressure-induced criticality in CeRh$_6$Ge$_4$.

cond-mat.str-el↗

arXiv2026

UCd$_{11}$: A strongly localized 5$f^3$ material

UCd$_{11}$ is an antiferromagnetic uranium intermetallic compound ($T_{\rm N}$ = 5.3K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5$f$ electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd$_{11}$ adopts the formal U$^{3+}$ 5$f^3$ configuration, while core-level photoemission spectroscopy (PES) data of UCd$_{11}$ reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of UCd$_{11}$, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that UCd$_{11}$ is a highly localized uranium 5$f^3$ system. Furthermore, core-level spectra obtained from a DFT+DMFT Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant 5$f$ behavior.

cond-mat.str-el↗

arXiv2025

Exploring the apparent violation of the Mott relation in a noncentrosymmetric kagome ferromagnet

In magnetic topological materials, time-reversal symmetry breaking gives rise to topological point and line nodes with distinctive signatures in the anomalous Hall and anomalous Nernst conductivity that satisfy the well-known Mott relation. However, this relationship can fail for doping-dependent transport measurements of materials with complex magnetism, topology, and electronic correlations. In this work, we present transport measurements of the correlated topological metal UCoAl doped with Ru, which appear to violate the Mott relation. We develop a model that captures the evolution of Stoner magnetism and topological Weyl points as a function of doping. Using this model, we show how the correlated flat band in this material pins the Weyl points to the Fermi energy, and demonstrate how this explains the unusual doping-dependent behavior of the anomalous Hall and anomalous Nernst conductivities in this material, while the Mott relation is in fact satisfied at each doping level.

cond-mat.str-el↗

arXiv2025

Evidence for easy-plane XY ferromagnetism in heavy-fermion quantum-critical CeRh6Ge4

We report $^{73}$Ge nuclear quadrupole resonance (NQR) and magnetic resonance (NMR) spectroscopy in the heavy-fermion quantum-critical ferromagnet CeRh$_6$Ge$_4$. NQR and NMR spectral measurements at the two non-equivalent Ge sites reveal electric field gradient tensors and the directions of their principal axes relative to the hexagonal basal plane. The spin-lattice relaxation rate $1/T_1$ experiments reveal a clear critical slowing down approaching the ferromagnetic transition. $1/T_1$ in the paramagnetic state is found to be predominantly caused by fluctuating 4$f$-local moments. The Knight shift shows Curie-Weiss behavior at high temperature and a deviation from this below $T^\ast$ $\approx$ 25 K possibly due to a mixture of crystalline electric field effects and Kondo screening. Order-parameter-like behavior of hyperfine fields at the Ge sites and ferromagnetic signal enhancement are observed in Zeeman-perturbed NQR, with uniform ferromagnetic order and a small ordered moment of $\approx$ 0.26 $\mu_B$/Ce confined within the $ab$-plane. The ordered moment shows a notable in-plane magnetic stiffness against out-of-plane radiofrequency fields and has an (XY-type) in-plane isotropic nature. Our results reveal a strong easy-plane anisotropy of 4$f$-electron moment and suggest an involved interplay of hybridization and local moment physics in this quantum critical heavy-fermion ferromagnet.

cond-mat.str-el↗

arXiv2025

Tracking flat bands via phonon-mediated interband scattering

Flat-band (FB) materials have emerged as promising platforms for exploring exotic quantum phases. While numerous candidates have recently been identified through spectroscopic techniques such as angle-resolved photoemission spectroscopy, central challenges remain on how to tune FBs towards the Fermi level $E_F$ and to understand their impact on low-energy excitations probed in electronic transport experiments. Here, we show that, by attributing the temperature dependence of the electrical resistivity at elevated temperatures to electron-phonon interband scattering, one can infer the position of FBs near $E_F$ across diverse material classes. As charge carriers scatter off phonons, interband transitions into FB states lead to distinctive sub- or superlinear resistivity at elevated temperatures, governed by the proximity of the FB to $E_F$. Our phenomenological model captures these universal transport behaviors observed across several recently studied FB compounds and offers a simple, broadly applicable method for detecting flat bands.

cond-mat.str-el↗

arXiv2025

Evolution of magnetic bubble domains in the uniaxial ferromagnet CeRu$_2$Ga$_2$B inferred from the Hall effect and ac magnetic susceptibility

We study the Hall effect, AC magnetic susceptibility ($\chi_{\rm ac}$), and magnetic force microscopy of the uniaxial ferromagnet CeRu$_2$Ga$_2$B with a centrosymmetric crystal structure. We observe a finite topological Hall effect (THE) within the ordered phase before the magnetization is polarized by applied field. By comparing the field dependences of the area fraction of the magnetic bubbles, the derivative of $\chi_{\rm ac}$, and the THE signal, we deduce that the magnetic bubbles of CeRu$_2$Ga$_2$B evolve from the trivial to topological spin texture with field. Our findings will be utilized to expand the search for magnetic materials hosting the topological spin textures to ones with uniaxial anisotropy, and open a new possibility to tailor the topological spin texture.

cond-mat.str-el↗

arXiv2025

Fabrication of microstructured devices of the unconventional superconductor CeCoIn5 for investigations of isolated grain boundaries

Grain boundaries are critical for determining the functionality of polycrystalline materials. Here we present on the structural $\&$ transport properties of grain boundaries in the unconventional superconductor CeCoIn$_5$. We provide a detailed recipe for the fabrication of isolated grain boundary devices from of as-grown polycrystalline samples of CeCoIn$_5$. Electron backscattered diffraction imaging of polycrystalline CeCoIn$_5$ samples reveals an abundance of $90^\circ$ misorientation grain boundaries suggesting a preferential nucleation of CeCoIn$_5$ grains with 90$^\circ$ misorientation over a random distribution of grain orientations. Transport measurements across grain boundary devices establish coherence of superconductivity and allows us to establish a lower bound on the critical current density for the grain boundaries. Our work opens new possibilities for fabrication of quantum devices such as Josephson-junctions out of bulk unconventional superconducting materials.

cond-mat.supr-con↗

arXiv2025

Hydrostatic pressure studies on non-superconducting UTe2

We report the temperature-pressure phase diagram of a UTe$_2$ single crystal that does not undergo a bulk superconducting transition but shows filamentary superconductivity with a critical transition temperature of 1 K at ambient pressure. Electrical-resistivity measurements reveal that the evolution of the filamentary superconducting state under pressure resembles the behavior observed in previous reports on bulk superconducting samples. AC calorimetry, however, does not show evidence for either bulk superconductivity or magnetism for pressures up to 1.6 GPa. Our results highlight the role of inhomogeneity in chemical-vapor-transport-grown UTe$_2$ samples and serve as a cautionary tale when probing electrical resistivity alone.

cond-mat.str-el↗

arXiv2024

Observation of the Yamaji effect in a cuprate superconductor

The pseudogap state of high-$T_{\rm c}$ cuprates, known for its partial gapping of the Fermi surface above the superconducting transition temperature $T_{\rm c}$, is believed to hold the key to understanding the origin of Planckian relaxation and quantum criticality. However, the nature of the Fermi surface in the pseudogap state has remained a fundamental open question. Here, we report the observation of the Yamaji effect above $T_{\rm c}$ in the single layer cuprate HgBa$_2$CuO$_{4+\delta}$. This observation is direct evidence of closed Fermi surface pockets in the normal state of the pseudogap phase. The small size of the pockets determined from the Yamaji effect (occupying approximately $1.3\%$ of the Brillouin zone area) is all the more surprising given the absence of evidence for long-range broken translational symmetry that can reconstruct the Fermi-surface.

cond-mat.str-el↗

arXiv2024

Unveiling the 5$f$ electron hybridization process in UPd$_2$Al$_3$ via ARPES and Time-resolved PES

This study investigates the 5$f$-electron-conduction electron hybridization process in the heavy fermion superconductor UPd$_2$Al$_3$ using a combination of angle-resolved photoemission spectroscopy (ARPES) and time-resolved photoemission spectroscopy (tr-PES). ARPES measurements reveal the formation of a hybridization gap at a temperature of approximately 75 K, which becomes more pronounced as the temperature decreases. Notably, the persistence of a flat U 5$f$ band at temperatures well above the hybridization onset challenges conventional understanding. Our findings demonstrate a non-monotonic temperature dependence of the quasiparticle relaxation time, with an anomalous decrease at 20 K, suggesting complex electronic and magnetic interactions. These findings provide detailed insights into the 5$f$-electron hybridization process in UPd$_2$Al$_3$, with significant implications for the understanding of heavy fermion superconductivity and the role of 5$f$-electron hybridization in uranium-based materials.

cond-mat.str-el↗

arXiv2024

Magnetic Phase Diagram of ErB$_4$ as Explored by Neutron Scattering

The tetragonal $4f$-electron intermetallic ErB$_4$ is characterized by strong Ising anisotropy along the tetragonal $c$ axis. The magnetic moments on the erbium sites can be mapped onto a Shastry-Sutherland (SSL) lattice resulting in geometrical frustration. At zero magnetic field ErB$_4$ exhibits collinear columnar antiferromagnetic (CAFM) order below $T_\text{N} = 15.4$ K. In the presence of a magnetic field parallel to the $c$ axis, ErB$_4$ exhibits a plateau at $1/2$ of the saturation magnetization $M_\text{S}$, which arises at a spin flip transition at $H_1$ $=$ 1.9 T. Fractional magnetization plateaus and other exotic spin phases are a well-established characteristic feature of frustrated spin systems. Monte Carlo simulations propose that ErB$_4$ is an ideal candidate to feature a spin supersolid phase in close vicinity of $H_1$ between the CAFM and $M/M_\text{S}=1/2$ plateau (HP) phase. Here we combine single-crystal neutron diffraction and inelastic neutron scattering to study the magnetic phase diagram and the crystal electric field (CEF) ground state of ErB$_4$. Our measurements as a function of magnetic field find no signature of the spin supersolid phase but allow us to determine the magnetic structure of the HP phase to be of the uuud type consistent with an Ising material. The magnetic moment $\mu_{\mathrm{CEF}}$ $=$ 8.96 $\mu_B$ expected from the CEF configuration determined by our inelastic neutron scattering measurements is also consistent with the ordered moment observed in neutron diffraction showing that the moments are fully ordered and close to the Er$^{3+}$ free ion moment (9.6 $\mu_B$).

cond-mat.str-el↗

arXiv2024

Designing Pu Intermetallics with First Principle Calculations

We present the ab initio supported discovery of two new Pu based intermetallic compounds, PuNiSn and PuPtSn. Using density functional theory, the formation energies within the relevant ternary phase diagrams were calculated to predict the stability of both compounds. Simultaneously, Pu-Ni-Sn and Pu-Pt-Sn materials were arc-melted and subsequently characterized with magnetization, specific heat, and resistivity measurements from 2-300 K. Magnetization measurements show that PuNiSn and PuPtSn order antiferromagnetically at TN = 11 K and TN = 15 K, respectively. Specific heat measurements show an enhanced residual electronic specific heat that is indicative of strong electron correlations. Resistivity measurements are indicative of Kondo behavior for PuNiSn while crystal field effects may play a role in the observed temperature dependence for PuPtSn.

cond-mat.mtrl-sci↗

arXiv2024

Local aging effects in PuB$_{4}$: Growing inhomogeneity and slow dynamics of local-field fluctuations probed by $^{239}$Pu NMR

The effect of self-irradiation damage can influence many properties of a radioactive material. Actinide materials involving the decay through alpha radiation have been frequently studied using techniques such as transport, thermodynamics, and x-ray diffraction. The use of nuclear magnetic resonance (NMR) spectroscopy to study such effects, however, has seen relatively little attention. Here, we use $^{239}$Pu NMR to study the local influence of self-damage in a single crystal of the candidate topological insulator plutonium tetraboride (PuB$_{4}$). We first characterize the anisotropy of the $^{239}$Pu resonance in a single crystal and confirm the local axial site symmetry inferred from previous polycrystalline measurements. Aging effects are then evaluated over the timeframe of six years. We find that, though the static NMR spectra may show a slight modulation in their shape, their field-rotation pattern reveals no change in Pu local site symmetry over time, suggesting that aging has a surprisingly small impact on the spatial distribution of the static hyperfine field. By contrast, aging has a prominent impact on the NMR relaxation processes and signal intensity. Specifically, aging-induced damage manifests itself as an increase in the spin-lattice relaxation time $T_{1}$, an increased distribution of $T_{1}$, and a signal intensity that decreases linearly by 20 % per year. The spin-spin relaxation time $T_{2}$ in the aged sample shows a strong variation across the spectrum as well as a drastic shortening towards lower temperature, suggesting growth of slow fluctuations of the hyperfine field that are linked to radiation-damage-induced inhomogeneity and could be responsible for the signal wipeout that develops over time.

cond-mat.mtrl-sci↗

arXiv2024

Anisotropic hybridization in CeRhSn

The optical conductivity $σ(ω,T)$ of CeRhSn was studied by broadband infrared spectroscopy. Temperature-dependent spectral weight transfer occurs over high energy ($0.8\,$eV) and temperature (${\sim}500\,$K) scales, classifying CeRhSn as a mixed valent compound. The optical conductivity reveals a substantial anisotropy in the electronic structure. Renormalization of $σ(ω,T)$ occurs as a function of temperature to a coherent Kondo state with concomitant effective mass generation. Associated spectroscopic signatures were reproduced remarkably well by the combination of density functional theory and dynamical mean field theory using a momentum-independent self energy. The theory shows that the anisotropy for energies $>10\,$meV is mainly driven by the bare three-dimensional electronic structure that is renormalized by local electronic correlations. The possible influence of magnetic frustration and quantum criticality is restricted to lower energies.

cond-mat.str-el↗

arXiv2024

Relocalization of Uranium 5f Electrons in Antiferromagnetic Heavy Fermion Superconductor UPd$_2$Al$_3$: Insights from Angle-Resolved Photoemission Spectroscopy

We investigate the antiferromagnetic heavy fermion superconductor UPd$_2$Al$_3$, employing angle-resolved photoemission spectroscopy to unravel the complex electronic structure of its U 5f electrons. We observe unexpected characteristics that challenge the conventional temperature-dependent behavior of heavy fermion systems, revealing unexpected characteristics. At temperatures above the anticipated coherence temperature (T$^*$), we observe itinerant U 5f electrons at temperatures higher than previously postulated. Additionally, a previously unidentified dispersionless band emerges around 600 meV below the Fermi energy, potentially linked to spin-orbit splitting within the U 5f states. Hybridization between the 5f electrons and conduction band was observed with an energy dispersion of 10 meV at low temperatures, suggesting that U 5f electrons near and at the Fermi surface have an itinerant nature. Temperature-dependent 5d-5f resonance spectra reveal that the 5f electron spectrum weight increases with lowering temperature and begins to decrease at temperatures significantly higher than the Neel temperature (T$_N$). We further show that the competition between the Kondo effect and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions may be responsible for the relocalization of 5f electrons, making relocalization a precursor to the establishment of magnetic order at lower temperatures. Our experiments also provide evidence that 5f electrons with the same orbital are involved in both the Kondo effect and RKKY interactions, suggesting that the two coexist at lower temperatures.

cond-mat.str-el↗

arXiv2024

Normal Fermi Surface in the Nodal Superconductor CeCoIn$_5$ Revealed via Thermal Conductivity

The thermal conductivity of heavy-fermion superconductor CeCoIn$_5$ was measured with a magnetic field rotating in the tetragonal a-b plane, with the heat current in the anti-nodal direction, $J$ || [100]. We observe a sharp resonance in thermal conductivity for the magnetic field at an angle $\theta$ $\sim$ 12$^{\circ}$, measured from the heat current direction [100]. This resonance corresponds to the reported resonance at an angle $\theta'$ $\sim$ 33$^{\circ}$ from the direction of the heat current applied along the nodal direction, $J$ || [110]. Both resonances, therefore, occur when the magnetic field is applied in the same crystallographic orientation in the two experiments, regardless of the direction of the heat current, proving conclusively that these resonances are due to the structure of the Fermi surface of CeCoIn$_5$. We argue that the uncondensed Landau quasiparticles, emerging with field, are responsible for the observed resonance. We support our experimental results with density-functional-theory model calculations of the density of states in a rotating magnetic field. Our calculations, using a model Fermi surface of CeCoIn$_5$, reveal several sharp peaks as a function of the field direction. Our study demonstrates that the thermal-conductivity measurement in rotating magnetic field can probe the normal parts of the Fermi surface deep inside the superconducting state.

cond-mat.supr-con↗

arXiv2024

Stabilization of U 5$f^2$ configuration in UTe$_2$ through U 6d dimers in the presence of Te2 chains

We investigate the topological superconductor candidate UTe$_2$ using high-resolution valence-band resonant inelastic x-ray scattering at the U $M_{4,5}$-edges. We observe atomic-like low-energy excitations that support the correlated nature of this unconventional superconductor. These excitations originate from the U $5f^2$ configuration, which is unexpected since the short Te2-Te2 distances exclude Te2 being 2-. By utilizing the photoionization cross-section dependence of the photoemission spectra in combination with band structure calculations, we infer that the stabilization of the U $5f^2$ configuration is due to the U $6d$ bonding states in the U-dimers acting as a charge reservoir. Our results emphasize that the description of the physical properties should commence with a $5f^2$ $ansatz$.

cond-mat.str-el↗