arXiv subjects

P. Gegenwart

Publications and source records attributed to P. Gegenwart.

At least 19 recordsLinked to original sources

arXiv2026

Complex Magnetic Behavior of the Ce sawtooth chains in CeRhSn$_2$

Conflicting reports exist on the ground state of the intermetallic compound CeRhSn$_2$. This can be rooted in the sawtooth-like arrangement of two inequivalent Ce sites in the unit cell, which suggests potential geometric magnetic frustration. To resolve, we conducted a comprehensive study on high-quality single crystals of CeRhSn$_2$ by means of magnetization ($M$), specific heat ($C_p/T$), and resistivity ($ρ$). The system exhibits strong magnetic anisotropy, confirming the $b$-axis as the easy magnetic axis. We establish three successive transitions, an AFM order at $T_{N} = 3.65$K, a first-order FM order at $T_{C} = 1.7$K and final transition, at $T = 1.5$K. The transition temperatures are highly field-directional dependent: in a magnetic field, the lowest transition is immediately suppressed while $\mathbf{H} \parallel b$ rapidly merges $T_{C}$ and $T_{N}$ into a single second-order transition. Conversely, $\mathbf{H}\parallel c$ suppresses the FM order and reduces $T_{N}$. Additional ab initio calculations affirm the FM ground state of CeRhSn$_2$. The observation of an enhancement of the Sommerfeld coefficient ($γ= 76.5$mJ/mol$\cdot$K$^2$) may arise from geometric frustration, but it is most consistently attributed to weak Kondo hybridization as frustration cannot be conclusively established through our data.

cond-mat.str-el↗

arXiv2026

High-Resolution Capacitance Dilatometry of Microscopically Thin Samples Using a Miniature Dilatometer

We present a novel application of our high-resolution capacitance dilatometer, specifically engineered for the precise characterization of quantum materials. These materials, which often appear as ultrathin, platelet-shaped crystals, are known for exotic phenomena such as superconductivity, topological order and quantum spin liquid. However, these crystals seldom reach macroscopic dimensions, making them unsuitable for conventional dilatometry techniques. By introducing a modified sample-mounting configuration, our design enables high-resolution measurements of thermal expansion and magnetostriction along in-plane crystallographic directions in samples with thicknesses well below 500 $μ$m. Validation measurements using a Quantum Design PPMS system confirm reliable performance for a 300 $μ$m-thick silver platelet, relatively hard ferromagnetic EuB$_6$ single crystals down to 50 $μ$m, and a 40 $μ$m-thin, soft AgCrS$_2$ single crystal. This advancement significantly broadens the applicability of capacitance dilatometry, providing a powerful platform for investigating emergent phenomena in reduced-dimensional quantum systems.

cond-mat.mtrl-sci↗

arXiv2025

Trigonal distortion in the Kitaev candidate honeycomb magnet BaCo2(AsO4)2

We conducted x-ray absorption (XAS) and magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ edges on single crystals of the Kitaev candidate honeycomb lattice compound BaCo$_2$(AsO$_4$)$_2$. The measurements employed the inverse partial fluorescence yield technique, which is ideal for acquiring reliable x-ray absorption spectra from highly insulating samples, enabling precise quantitative analysis. Our experimental results revealed a significant linear dichroic signal, indicating strong trigonal distortion in the CoO$_{6}$ octahedra in BaCo$_2$(AsO$_4$)$_2$. We performed a detailed analysis of the experimental XAS and XMCD spectra using a full-multiplet configuration-interaction cluster model. This analysis unveiled that the $t_{2g}$ hole density is predominantly localized in the $a_{1g}$ orbital. Through XMCD sum rules and theoretical calculations, we quantified both the spin and orbital magnetic moments. Our study demonstrates that the local electronic structure of the CoO$_{6}$ octahedra displays an effective trigonal distortion of approximately $-0.114$ eV. This distortion is larger than the Co $3d$ spin-orbit coupling constant, emphasizing the crucial impact of local structural distortions on the electronic and magnetic properties of BaCo$_2$(AsO$_4$)$_2$.

cond-mat.str-el↗

arXiv2025

Quantum disordered ground state and relative proximity to an exactly solvable model in the frustrated magnet CeMgAl$_{11}$O$_{19}$

The magnetic properties of the triangular magnet CeMgAl$_{11}$O$_{19}$ were investigated by magnetization and specific heat measurements down to $T=0.03\,$K on single crystals grown by the floating zone method. The formation of effective spins $S_\mathrm{eff}= 1/2$ below $T < 10$\,K was confirmed both by DFT calculations and specific heat measurements. No magnetic order was found down to $T=0.03\,$K despite the formation of magnetic correlations observed in specific heat. The measured magnetization was compared with DMRG computation and their agreement supports the proposal of a strongly anisotropic magnetic interaction antiferromagnetically coupling the spin components in the $ab$ plane and ferromagnetically coupling the spin component along the $c$ axis. However, our quantitative study of the magnetization indicates a weaker proximity to quantum criticality between ferromagnetism and antiferromagnetism than the previous inelastic neutron scattering study. Finally, we propose that the absence of magnetic order in CeMgAl$_{11}$O$_{19}$ would most probably be related to the structural disorder revealed by single-crystal X-ray diffraction.

cond-mat.str-el↗

arXiv2025

Large magnetoreflectance and optical anisotropy due to $4f$ flat bands in the frustrated kagome magnet HoAgGe

We report peculiar optical properties of the frustrated itinerant magnet HoAgGe, which exhibits multiple magnetically ordered states obeying the kagome spin-ice rule. The optical conductivity is surprisingly higher for light polarization perpendicular to the kagome plane, both for the free carrier response and the interband transitions. The latter ones have strong contributions from Ho $4f$ flat bands located near the Fermi level, as revealed by our \textit{ab initio} calculations, explaining the unusual anisotropy of the optical properties and the pronounced temperature dependence of the interband transitions for out--of--plane light polarization. The key role of Ho $4f$ states is further supported by the large variation of the reflectivity upon the metamagnetic transitions, that follows the field dependence of the magnetization, in contrast to that of the dc magnetotransport data. Such heavy-electron bands near the Fermi level offer an efficient way to control transport and optical properties.

cond-mat.str-el↗

arXiv2025

Adiabatic demagnetization refrigeration with antiferromagnetically ordered NaGdP$_2$O$_7$

We present a comprehensive study of the structural, magnetic, and thermodynamic properties, as well as the adiabatic demagnetization refrigeration (ADR) performance of NaGdP$_2$O$_7$. Although NaGdP$_2$O$_7$ exhibits antiferromagnetic ordering at a Néel temperature of $T_{\rm N} = 570$ mK in zero field, ADR experiments achieved a minimum temperature of 220 mK starting from $T = 2$ K under an applied magnetic field of $μ_0H = 5$ T. The warm-up time back to $T = 2$ K exceeds 60 hours, which is roughly 50 times longer than that of its Yb-based analogue, underscoring the potential of NaGdP$_2$O$_7$ as an efficient precooling stage in double-stage ADR systems. We show that NaGdP$_2$O$_7$ can be seen as a network of ferromagnetic spin chains with antiferromagnetic interchain couplings and also investigate the influence of antiferromagnetic ordering on the magnetic entropy. We find that the temperature dependence of the entropy plays a more dominant role than its magnetic field dependence in the magnetically ordered state.

cond-mat.str-el↗

arXiv2024

Optical conductivity of the metallic pyrochlore iridate Pr$_2$Ir$_2$O$_7$: Influence of spin-orbit coupling and electronic correlations on the electronic structure

The synergy of strong spin-orbit coupling and electron-electron interactions gives rise to unconventional topological states, such as topological Mott insulator, Weyl semimetal, and quantum spin liquid. In this study, we have grown single crystals of the pyrochlore iridate Pr$_2$Ir$_2$O$_7$ and explored its magnetic, lattice dynamical, and electronic properties. While Raman spectroscopy data reveal six phonon modes confirming the cubic \textit{Fd$\bar{3}$m} crystal symmetry, dc magnetic susceptibility data show no anomalies and hence indicate the absence of magnetic phase transitions down to 2~K. Both temperature-dependent electric transport and optical conductivity data reveal the metallic character of Pr$_2$Ir$_2$O$_7$. The optical conductivity spectrum contains a mid-infrared absorption band, which becomes more pronounced with decreasing temperature due to spectral weight transfer from high to low energies. The presence of the mid-infrared band hints at the importance of correlation physics. The optical response furthermore suggests that Pr$_2$Ir$_2$O$_7$ is close to the Weyl semimetal phase.

cond-mat.str-el↗

arXiv2023

Strong terahertz third-harmonic generation by kinetic heavy quasiparticles in CaRuO$_3$

We report on time-resolved nonlinear terahertz spectroscopy of a strongly correlated ruthenate, CaRuO$_3$, as a function of temperature, frequency and terahertz field strength. Third-harmonic radiation for frequencies up to 2.1 THz is observed evidently at low temperatures below 80 K, where the low-frequency linear dynamical response deviates from the Drude model and a coherent heavy quasiparticle band emerges by strong correlations associated with the Hund's coupling.Phenomenologically, by taking an experimentally observed frequency-dependent scattering rate, the deviation of the field driven kinetics from the Drude behavior is reconciled in a time-dependent Boltzmann description, which allows an attribution of the observed third-harmonic generation to the terahertz field driven nonlinear kinetics of the heavy quasiparticles.

cond-mat.str-el↗

arXiv2023

Adiabatic demagnetization refrigeration to mK temperatures with the distorted square lattice magnet NaYbGeO$_{4}$

We report the synthesis, characterization, low-temperature magnetic, and thermodynamic measurements of the novel milli-Kelvin adiabatic demagnetization refrigeration (mK-ADR) candidate material NaYbGeO$_4$ which exhibits a distorted square lattice arrangement of YbO$_{6}$ magnetic units. Magnetization and specific heat indicate weakly interacting effective spin-1/2 moments below 10~K, with a Curie-Weiss temperature of only 15~mK, that can be polarized by magnetic fields of order 1~T. For the ADR performance test, we start the demagnetization from 5~T at a temperature of $\sim 2$~K and reach a minimum temperature of 150~mK at zero field. The warming curve indicates a sharp magnetic transition in the heat capacity at 210~mK, implying only weak magnetic frustration. The entropy density of $S_{\rm GS}\simeq 101$ mJ K$ ^{-1}$cm$^{-3}$ and hold time below 2~K of 220~min are competitive while the minimal temperature is higher compared to frustrated Ytterbium-oxide ADR materials studied under similar conditions.

cond-mat.str-el↗

arXiv2023

Quantum disordered ground state in the spin-orbit coupled Jeff = 1/2 distorted honeycomb magnet BiYbGeO5

We delineate quantum magnetism in the strongly spin-orbit coupled, distorted honeycomb-lattice antiferromagnet BiYbGeO$_{5}$. Our magnetization and heat capacity measurements reveal that its low-temperature behavior is well described by an effective $J_{\rm eff}=1/2$ Kramers doublet of Yb$^{3+}$. The ground state is nonmagnetic with a tiny spin gap. Temperature-dependent magnetic susceptibility, magnetization isotherm, and heat capacity could be modeled well assuming isolated spin dimers with anisotropic exchange interactions $J_{\rm Z} \simeq 2.6$~K and $J_{\rm XY} \simeq 1.3$~K. Heat capacity measurements backed by muon spin relaxation suggest the absence of magnetic long-range order down to at least 80\,mK both in zero field and in applied fields. This sets BiYbGeO$_5$ apart from Yb$_2$Si$_2$O$_7$ with its unusual regime of magnon Bose-Einstein condensation and suggests negligible interdimer couplings, despite only a weak structural deformation of the honeycomb lattice.

cond-mat.str-el↗

arXiv2023

Efficient adiabatic demagnetization refrigeration to below 50 mK with UHV compatible Ytterbium diphosphates $A$YbP$_2$O$_7$ ($A=$Na, K)

Attaining milli-Kelvin temperatures is often a prerequisite for the study of novel quantum phenomena and the operation of quantum devices. Adiabatic demagnetization refrigeration (ADR) is an effective, easy and sustainable alternative to evaporation or dilution cooling with the rare and super-expensive $^3$He. Paramagnetic salts, traditionally used for mK-ADR, suffer from chemical instability related to water of crystallization. We report synthesis, characterization as well as low-temperature magnetization and specific heat measurements of two new UHV compatible candidate materials NaYbP$_2$O$_7$ and KYbP$_2$O$_7$. Utilizing the PPMS at 2 K, the ADR of sintered pellets with Ag powder admixture starting at 5 T yields base temperatures (warm-up times) of 45 mK (55 min) and 37 mK (35 min) for NaYbP$_2$O$_7$ and KYbP$_2$O$_7$, respectively, slightly advantageous to KBaYb(BO$_3$)$_2$ (45 mK and 40 min) studied under similar conditions.

cond-mat.mtrl-sci↗

arXiv2023

Adiabatic demagnetization cooling well below the magnetic ordering temperature in the triangular antiferromagnet KBaGd(BO3)2

Crystal structure, thermodynamic properties, and adiabatic demagnetization refrigeration (ADR) effect in the spin-7/2 triangular antiferromagnet KBaGd(BO3)2 are reported. With the average nearest-neighbor exchange coupling of 44 mK, this compound shows magnetic order below $T_N = 263$\,mK in zero field. The ADR tests reach the temperature of $T_{min} = 122$ mK, more than twice lower than $T_N$, along with the entropy storage capacity of 192 mJK$^{-1}$cm$^{-3}$ and the hold time of more than 8 hours in the PPMS setup, both significantly improved compared to the spin-1/2 Yb$^{3+}$ analog. We argue that KBaGd(BO3)2 shows a balanced interplay of exchange and dipolar couplings that together with structural randomness and geometrical frustration shift $T_{min}$ to well below the ordering temperature $T_N$, therefore facilitating the cooling.

cond-mat.str-el↗

arXiv2023

RIXS observation of bond-directional nearest-neighbor excitations in the Kitaev material Na$_2$IrO$_3$

Spin-orbit coupling locks spin direction and spatial orientation and generates, in semi-classical magnets, a local spin easy-axis and associated ordering. Quantum spin-1/2's defy this fate: rather than spins becoming locally anisotropic, the spin-spin interactions do. Consequently interactions become dependent on the spatial orientation of bonds between spins, prime theoretical examples of which are Kitaev magnets. Bond-directional interactions imply the existence of bond-directional magnetic modes, predicted spin excitations that render crystallographically equivalent bonds magnetically inequivalent, which yet have remained elusive experimentally. Here we show that resonant inelastic x-ray scattering allows us to explicitly probe the bond-directional character of magnetic excitations. To do so, we use a scattering plane spanned by one bond and the corresponding spin component and scan a range of momentum transfer that encompasses multiple Brillouin zones. Applying this approach to Na$_2$IrO$_3$ we establish the different bond-directional characters of magnetic excitations at 10 meV and 45 meV. Combined with the observation of spin-spin correlations that are confined to a single bond, this experimentally validates the Kitaev character of exchange interactions long proposed for this material.

cond-mat.str-el↗

arXiv2022

Quasimolecular electronic structure of the spin-liquid candidate Ba$_3$InIr$_2$O$_9$

The mixed-valent iridate Ba3InIr2O9 has been discussed as a promising candidate for quantum spin-liquid behavior. The compound exhibits Ir$^{4.5+}$ ions in face-sharing IrO6 octahedra forming Ir2O9 dimers with three t2g holes per dimer. Our results establish Ba3InIr2O9 as a cluster Mott insulator. Strong intra-dimer hopping delocalizes the three t2g holes in quasi-molecular dimer states while inter-dimer charge fluctuations are suppressed by Coulomb repulsion. The magnetism of Ba3InIr2O9 emerges from spin-orbit entangled quasi-molecular moments with yet unexplored interactions, opening up a new route to unconventional magnetic properties of 5d compounds. Using single-crystal x-ray diffraction we find the monoclinic space group C2/c already at room temperature. Dielectric spectroscopy shows insulating behavior. Resonant inelastic x-ray scattering (RIXS) reveals a rich excitation spectrum below 1.5 eV with a sinusoidal dynamical structure factor that unambiguously demonstrates the quasi-molecular character of the electronic states. Below 0.3 eV, we observe a series of excitations. According to exact diagonalization calculations, such low-energy excitations reflect the proximity of Ba3InIr2O9 to a hopping-induced phase transition based on the condensation of a quasi-molecular spin-orbit exciton. The dimer ground state roughly hosts two holes in a bonding j=1/2 orbital and the third hole in a bonding j=3/2 orbital.

cond-mat.str-el↗

arXiv2022

Spin-liquid signatures in the quantum critical regime of pressurized CePdAl

CePdAl is a prototypical frustrated Kondo lattice with partial long-range order (LRO) at $T_\mathrm{N}=2.7$ K. Previous bulk experiments under hydrostatic pressure found signatures for a quantum critical regime that extends from $p_\mathrm{c} \approx 0.9$ GPa, where LRO disappears, up to $\sim 1.7$ GPa. We employed extensive muon spin relaxation and rotation ($μ$SR) experiments under pressure. The continuous and complete suppression of LRO at $p_\mathrm{c}$ is confirmed. Above $T_\mathrm{N}(p)$ and beyond $p_\mathrm{c}$, an additional crossover scale $T^\ast(p)$ characterizes the change from pure to stretched exponential relaxation in zero field. Remarkably $T^\ast(p)$ agrees with previously determined signatures of entropy accumulation above LRO. This coincidence microscopically evidences fluctuating frustrated spins at $T\leq T^\ast$ with spin-liquid behavior. Power-law divergences of the temperature and longitudinal field dependences of the relaxation rate, with time-field scaling, at pressures between $p_c$ and 1.7 GPa characterize this regime as quantum critical.

cond-mat.str-el↗

arXiv2022

Divergent Thermal Expansion and Grüneisen Ratio in a Quadrupolar Kondo Metal

We report on the low-temperature thermal expansion and magnetostriction of the single-impurity quadrupolar Kondo candidate Y$_{1-x}$Pr$_{x}$Ir$_2$Zn$_{20}$. In the dilute limit, we find a quadrupolar strain that possesses a singular dependence on temperature $T$, $\varepsilon_{\mathrm{u}} \sim H^2 \log 1/T$, for a small but finite magnetic field $H$. Together with the previously reported anomalous specific heat $C$, this implies a quadrupolar Grüneisen ratio $Γ_u = \partial_T \varepsilon_{\mathrm{u}}/C \sim H^2/(T^2 \log 1/T)$ whose divergence for finite $H$ is consistent with the scenario of a quadrupolar Kondo effect. In addition, we find a singular behavior of the isotropic strain $\varepsilon_{\mathrm{B}}$ in zero magnetic field resulting in a divergence of both the volume thermal expansion and the volume Grüneisen parameter. We speculate that this behavior might be also induced by putative Kondo correlations via elastic anharmonicities or static strain disorder.

cond-mat.str-el↗

arXiv2021

Antiferromagnetic correlations in strongly valence fluctuating CeIrSn

CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of $T_{\mathrm{K}}\sim 480$\,K. We report a negative in-plane thermal expansion $α/T$ below 2\,K, which passes through a broad minimum near 0.75\,K. Volume and $a$-axis magnetostriction for $B \parallel a$ are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6\,T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1\,K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at $T\ll T_{\mathrm{K}}$ result from geometrical frustration.

cond-mat.str-el↗

arXiv2021

Linkage between scattering rates and superconductivity in doped ferropnictides

We report an angle-resolved photoemission study of a series of hole and electron doped iron-based superconductors, their parent compound BaFe2As2, and their cousins BaCr2As2 and BaCo2As2. We focus on the energy (E) dependent scattering rate Gamma(E) as a function of the 3d count and on the renormalization function Z(E) of the inner hole pocket, which is the hot spot in these compounds. We obtain a non-Fermi-liquid-like linear in energy scattering rate Gamma(E>> kBT), independent of the dopant concentration. The main result is that the slope beta=Gamma(E >> kBT)/E, reaches its maxima near optimal doping and scales with the superconducting transition temperature. This supports the spin fluctuation model for superconductivity for these materials. In the optimally hole-doped compound, the slope of the scattering rate of the inner hole pocket is about three times bigger than the Planckian limit Gamma(E)/E~1. This result together with the energy dependence of the renormalization function Z(E) signals very incoherent charge carriers in the normal state which transform at low temperatures to a coherent unconventional superconducting state.

cond-mat.supr-con↗