arXiv subjects

F. Ronning

Publications and source records attributed to F. Ronning.

At least 19 recordsLinked to original sources

arXiv2026

Incommensurate spin fluctuations in one-dimensional Kondo metal CeCo2Ga8

We present an experimental and numerical study of the spin fluctuations in 1D Kondo metal CeCo$_2$Ga$_8$. Using inelastic neutron spectroscopy, we measure highly one-dimensional magnetism with low-energy incommensurate short-ranged magnetic fluctuations. ARPES similarly shows a highly one-dimensional electronic band structure, confirming the one-dimensional nature of the system. We use density matrix renormalization group (DMRG) simulations of the 1D Kondo lattice model to interpret the measured spectrum, which successfully reproduce the neutron scattering features. We are thus able to place CeCo$_2$Ga$_8$ within the emergent incommensurate phase of the 1D Kondo lattice phase diagram, and demonstrate that the Kondo lattice simulated non-perturbatively is an accurate microscopic model for heavy fermion physics. This shows CeCo$_2$Ga$_8$ to be one-dimensional despite its complexities, and reveals a coexistence of low-energy Kondo and magnetic features in its inelastic spectrum.

cond-mat.str-el↗

arXiv2026

Magnetic symmetry implications of the zero- and applied-field Hall effect of UNi$_4$B

The zero-field and applied-field Hall effects in noncollinear antiferromagnets provide evidence for topological states of matter and are tied to materials' magnetic symmetry. For UNi$_4$B, the antiferromagnetic state with $T_\mathrm{N}=20$ K at zero and low magnetic field is debated due to recent magnetoelectric measurements and theory work calling into question the proposed toroidal arrangement of magnetic dipole moments. For a magnetic field applied within the plane of uranium magnetic moments, the field-dependent Hall resistivity of UNi$_4$B shows a curved response for $\rho_{yz}$ ($H{\parallel}x$, $I{\parallel}z$) and $\rho_{zx}$ ($H{\parallel}y$, $I{\parallel}x$) up to $\sim$8 T at 2 K, while an out-of-plane field results in linear behavior of $\rho_{yx}$ ($H{\parallel}z$, $I{\parallel}x$) up to 16 T. Analysis using conventional empirical relationships for the Hall effect indicate that an intrinsic effect from momentum-space Berry curvature contributes significantly to the curved transverse resistivity. Moreover, a finite zero-field Hall effect emerges at the onset of magnetic order for $\rho_{yz}$ and $\rho_{zx}$, further supporting an intrinsic origin of the Hall response. Symmetry arguments for a finite Berry curvature, an observable magnetoelectric effect, and reported magnetic structures suggest that the previously proposed magnetic space groups for the zero-field magnetic structure cannot account for the observed finite zero-field effect for two Hall orientations. Instead, we propose that $Cm'$ or $Pm'$ magnetic symmetry, depending on the parent nonmagnetic space group, is consistent with Hall resistivity, neutron diffraction, and magnetoelectric effect measurements.

cond-mat.str-el↗

arXiv2026

Programmable superconducting diode from nematic domain control in FeSe

The superconducting diode effect (SDE) allows polarity-dependent critical currents when time-reversal and current-inverting spatial symmetries are broken. Superconducting diodes show promise for applications, but inversion asymmetry is usually encoded in sample geometry or non-centrosymmetric crystals, rendering them static circuit elements. Here we demonstrate a programmable superconducting diode whose functionality is encoded in correlated electronic domains. We use the nematic superconductor FeSe as a platform and report a large intrinsic SDE with efficiencies up to $\eta \sim 75\%$ due to vortices interacting with nematic twin boundaries. The domain wall configuration thus encodes the SDE of the device. Through intense microsecond current pulses to quench the nematic order at rates exceeding $10^7$ K/s, we modify the domain pattern and control the polarity and strength of the SDE. These results establish a new paradigm in which superconducting circuit elements can be programmed through patterns imprinted into correlated electronic states.

cond-mat.supr-con↗

arXiv2026

Reconciling strange metal transport in CeCoIn$_5$ through the difference of optical and cyclotron effective masses

The strange metal behavior in cuprate superconductors - characterized by linear in temperature resistivity and anomalous Hall transport - stands in stark contrast to the expectation of conventional Fermi liquid (FL) theory. Remarkably, the similar transport behavior has also been observed in the heavy fermion metal CeCoIn$_5$, whose d-wave superconducting ground state and strong antiferromagnetic fluctuations draw parallels to the cuprates. Here we have investigated the optical conductivity of the strange metal state of CeCoIn$_5$ over a wide magnetic field range using time-domain THz spectroscopy (TDTS). Using unique high-field THz spectroscopy we have shown that the current relaxation rate scales approximately as T$^2$, giving evidence for a hidden Fermi liquid state over a large field range. This result can be reconciled with linear in T resistivity with the realization that heavy quasiparticles have an optical mass that scales roughly like 1/T. This optical mass contrasts with the mass that characterizes cyclotron motion, which does not suffer the same large temperature dependent renormalization. Although by itself anomalous, this allows one to understand a number of other phenomena in CeCoIn$_5$ that have been taken to be signatures of strange metals, including the coexistence of a conventional T$^2$ dependence of the cotangent of the Hall angle with the linear in T resistivity, which with our observation also reflects FL-like physics.

cond-mat.str-el↗

arXiv2026

Pseudo Point Nodal Superconducting Gap in Spin-Triplet UTe$_2$

The unconventional superconductor UTe$_2$ represents a rare example of spin-triplet pairing with potentially topologically protected quantum states. However, conflicting reports on its gap structure, particularly regarding point nodes, have hindered understanding of the order parameter symmetry and topological properties. Here we report high-resolution thermal conductivity measurements on high-quality UTe$_2$ single crystals down to ~50 mK that resolve the gap anisotropy through bulk directional transport. The $b$-axis thermal conductivity $\kappa_b/T$ exhibits negligible residual conductivity as $T \to 0$, and its temperature dependence is consistent with a small superconducting energy gap along the $b$-axis. Under magnetic fields, the residual $\kappa_b/T$ shows only weak field-induced enhancement. Remarkably, a threshold field emerges at low fields for $H \parallel a$, characterized by a kink that signals a change in quasiparticle transport normal to the field. Below the threshold, $\kappa_b/T$ remains isotropic for all field orientations, whereas strong anisotropy between transport along and normal to the field develops above it. These signatures strongly suggest that UTe$_2$ exhibits a fully gapped state with a pseudo point-nodal structure, where gap minima approach but never reach zero. We estimate the minimal gap $\Delta_{min}/\Delta_0 \sim 0.1$ along the $b$-axis, where $\Delta_0$ is the characteristic superconducting gap. This unusual gap structure provides crucial insights into the pairing mechanism and topology of this spin-triplet superconductor and excludes non-unitary mixing of pairing symmetries.

cond-mat.supr-con↗

arXiv2025

Structural modulation, physical properties, and electronic band structure of the kagome metal UCr$_6$Ge$_6$

The chemical flexibility of the $RM_6X_6$ stoichiometry, where an $f$-block element is intercalated in the CoSn structure type, allows for the tuning of flatbands associated with kagome lattices to the Fermi level and for emergent phenomena due to interactions between the $f$- and $d$-electron lattices. Yet, 5$f$ members of the ``166" compounds are underrepresented compared with 4$f$ members. Here, we report single-crystal growth of UCr$_6$Ge$_6$, which crystallizes in a monoclinically distorted Y$_{0.5}$Co$_3$Ge$_3$-type structure. The real-space character of the modulation, which is unique within the $RM_6X_6$ family, is approximated by a 3$\times$1$\times$2 supercell of the average monoclinic cell. The compound has kagome-lattice flatbands near the Fermi level and a moderately enhanced electronic heat capacity, as evidenced by its low-temperature Sommerfeld coefficient ($\gamma=86.5$~mJ~mol$^{-1}$~K$^{-2}$) paired with band structure calculations. The small, isotropic magnetization and featureless resistivity of UCr$_6$Ge$_6$ suggest itinerant uranium 5$f$ electrons and Pauli paramagnetism. Angle-resolved photoemission spectroscopy results provide evidence for uranium 5$f$ weight at the Fermi level and for a flatband near the Fermi level associated with the chromium $3d$ kagome lattice. The isotropic magnetic behavior of the uranium 5$f$ electrons starkly contrasts with localized behavior in other uranium 166 compounds, highlighting the high tunability of the magnetic ground state across the material family.

cond-mat.str-el↗

arXiv2025

Observation of a pronounced Hebel-Slichter peak in the spin-lattice relaxation rate and implications for gap and pairing symmetry in LaNiGa$_2$

We report a pronounced Hebel-Slichter coherence peak in the zero field nuclear quadrupolar resonance (NQR) spin-lattice relaxation rate of the topological crystalline superconductor LaNiGa$_2$ in the superconducting state. Previously, a two-band internally antisymmetric non-unitary triplet pairing (INT) state was proposed for this system, with equal spin-pairing and two distinct gaps associated with different spins. A detailed examination of the temperature dependence of the NQR data shows that the data best fit an INT model if the two gaps are equal and the model is unitary. Even a tiny non-unitarity with two unequal gaps causes the coherence peak to diminish rapidly and deviate from the data. On the other hand, the data are well-fit by a two-band singlet BCS-like pairing with two distinct gaps consistent with previous measurements. This raises doubts on the identification of non-unitary triplet-pairing with time-reversal symmetry breaking in this material.

cond-mat.supr-con↗

arXiv2025

Magnetic Field Dependence of the Spin Fluctuations in CeCu$_{5.8}$Ag$_{0.2}$

Quantum phase transitions are among the most intriguing phenomena that can occur when the electronic ground state of correlated metals are tuned by external parameters such as pressure, magnetic field or chemical substitution. Such transitions between distinct states of matter are driven by quantum fluctuations, and can give rise to macroscopically coherent phases that are at the forefront of condensed matter research. However, the nature of the critical fluctuations, and thus the fundamental physics controlling many quantum phase transitions, remain poorly understood in numerous strongly correlated metals. Here we study the model material CeCu$_{5.8}$Ag$_{0.2}$ to gain insight into the implications of critical fluctuations originating from different regions in reciprocal space. By employing an external magnetic field along the crystallographic $a$- and $c$-axis as auxiliary tuning parameter we observe a pronounced anisotropy in the suppression of the quantum critical fluctuations, reflecting the spin anisotropy of the long-range ordered ground state at larger silver concentration. Coupled with the temperature dependence of the quantum critical fluctuations, these results suggest that the quantum phase transition in CeCu$_{5.8}$Ag$_{0.2}$ is driven by three-dimensional spin-density wave fluctuations.

cond-mat.str-el↗

arXiv2025

SPLENDOR: a novel detector platform to search for light dark matter with narrow-gap semiconductors

We present the design and current status of SPLENDOR, a novel detector platform that combines narrow-gap semiconductor targets with low-noise charge readout to achieve sensitivity to dark matter energy deposits well below the eV scale. SPLENDOR is designed to be a modular and scalable system able to accommodate different target materials and signal readout technologies. SPLENDOR's present strategy entails: (i) the use of strongly correlated f-electron semiconductors with anisotropic electronic structures to enable not only sub-eV energy thresholds, but also directional sensitivity to the incoming dark matter flux, allowing for signal-background discrimination via daily modulation, and (ii) custom charge readout based on cryogenic high-electron-mobility transistor (cryoHEMT) amplifiers approaching single-electron resolution. We report on the selection and characterization of Eu$_5$In$_2$Sb$_6$ as the target material for SPLENDOR's first prototype detector, as well as the development and calibration of the prototype amplifier chain, achieving a measured charge resolution of 20$\pm$7 electrons in silicon test samples, consistent with predicted performance. This provides a demonstration of the detector architecture, which is now ready for deployment in a dark matter search campaign to deliver SPLENDOR's first science results. Finally, we present estimates of sensitivity reach in the parameter space of athermally produced relic dark matter under high- and low-background environments, and for various amplifier technology upgrades with increasing performance, including planned quantum sensing upgrades in order to achieve our ultimate goal of sub-electron resolution in optimized systems. SPLENDOR provides a novel approach to dark matter direct detection, combining quantum sensing with material's design to open new avenues of exploration in the sub-MeV mass range of dark matter parameter space.

physics.ins-det↗

arXiv2025

YNiSn$_2$: A candidate Dirac semimetal

We report the synthesis and physical properties of the new compound YNiSn$_2$, which crystallizes in the orthorhombic \textit{Cmcm} structure. The material exhibits semimetallic behavior and develops a giant positive magnetoresistance approaching 1200\% at $B = 16$ T. Pronounced de Haas-van Alphen and Shubnikov-de Haas oscillations reveal a dominant quasi-two-dimensional Fermi surface with an exceptionally small cyclotron effective mass of $m^{*} = 0.08 m{0}$, indicating light carriers and a tiny Fermi surface pocket. The strong anisotropy revealed by Shubnikov-de Haas quantum oscillation measurements highlights the low-dimensional electronic character of YNiSn$_2$, positioning it as a promising Dirac semimetal candidate.

cond-mat.str-el↗

arXiv2025

Enhanced two-dimensional ferromagnetism in van der Waals $\beta$-UTe$_3$ monolayers

The discovery of local-moment magnetism in van der Waals (vdW) semiconductors down to the single-layer limit has led to a paradigm shift in the understanding of two-dimensional (2D) magnets and unleashed their potential for applications in microelectronic and optoelectronic devices. The incorporation of strong electronic and magnetic correlations in 2D vdW metals remains a sought-after platform not only to enable control of emergent quantum phases, such as superconductivity, but also to achieve more theoretically tractable microscopic models of complex materials. To date, however, there is limited success in the discovery of such metallic vdW platforms, and $f$-electron monolayers remain out of reach. Here we demonstrate that the actinide $\beta$-UTe$_3$ can be exfoliated to the monolayer limit. A sizable electronic specific heat coefficient provides the hallmark of strong correlations. Remarkably, $\beta$-UTe$_3$ remains ferromagnetic in the half-unit-cell limit with an enhanced ordering temperature of 35 K, a factor of two larger than its bulk counterpart. Our work establishes $\beta$-UTe$_3$ as a novel materials platform for investigating and modeling correlated behavior in the monolayer limit and opens numerous avenues for quantum control with, e.g., strain engineering.

cond-mat.str-el↗

arXiv2025

Microscopic investigation of enhanced Pauli paramagnetism in metallic Pu$_2$C$_3$

A combined study of the structural and electronic properties of polycrystalline Pu$_2$C$_3$ is reported based on x-ray diffraction, specific heat, magnetic susceptibility, ${}^{13}$C nuclear magnetic resonance (NMR), and band structure calculations. X-ray diffraction reveals a global noncentrosymmetric cubic lattice, with a nearest-neighbor C--C bond length of $r = 1.38$ \r{A}. ${}^{13}$C NMR measurements indicate that the global cubic symmetry is locally broken, revealing two unique carbon environments. Magnetic susceptibility suggests enhanced Pauli paramagnetism, and specific heat reveals a moderately large electronic Sommerfeld coefficient $\gamma = 45$ mJ mol$_{\mathrm{Pu}}^{-1}$ K$^{-2}$, with a Wilson ratio $R_W \approx 1.3$ further indicating moderate correlations. ${}^{13}$C nuclear spin-lattice relaxation rate ($1/T_1$) and Knight shift ($K$) measurements find metallic Korringa behavior (i.e., $T_1TK^2=$ const.) with modest ferromagnetic spin fluctuations at low temperature. Taken together, the data point to a delocalized nature of a narrow 5$f$-electron band with weak electronic correlations. Density functional theory band-structure calculations confirm the appearance of such narrow 5$f$ bands near the Fermi level. Our data provide prime evidence for a plutonium-based metallic system with weak electronic correlations, which sheds new light on the understanding of complex paramagnetism in actinide-based metallic compounds.

cond-mat.str-el↗

arXiv2025

Investigation of the Paramagnetic State of the Kagome Kondo Lattice Compound YbV$_6$Sn$_6$: a $^{51}$V Nuclear Magnetic Resonance Study

YbV$_6$Sn$_6$ is a recently discovered kagome-lattice metal that orders at $T_{\rm N}\approx0.4$~K. Its layered structure combines a triangular Kondo lattice of Yb$^{3+}$ ions with vanadium-based kagome planes, which may host an interplay between strong correlations and band topology. We report a $^{51}$V nuclear magnetic resonance (NMR) study of the paramagnetic state of YbV$_6$Sn$_6$. Detailed field-angular dependence of single-crystal NMR spectra determined the principal-axis directions of the electric field gradient tensor at the $^{51}$V sites, as well as their nuclear quadrupole frequency, $\nu_{\rm Q}$, and asymmetry parameter, $\eta$. The Knight shift, $K$, was measured for different field orientations, and the analysis of $K$ against magnetic susceptibility to extract anisotropic hyperfine couplings. Accurate spectral assignments further enabled measurements of the nuclear spin-lattice relaxation rate, $1/T_1$, for both in-plane and out-of-plane field directions. The temperature dependence of $1/T_1$ shows that out-of-plane spin fluctuations are suppressed below $\sim$20~K, whereas in-plane fluctuations are markedly enhanced, which might be understood by thermal depopulation of the low-lying crystalline electric field excited state. The notable anisotropy in $1/T_1$ indicates that the paramagnetic state of YbV$_6$Sn$_6$ is strongly affected by in-plane spin dynamics.

cond-mat.str-el↗

arXiv2025

Electronic dimensionality of UTe2

Superconductivity in the heavy-fermion metal UTe2 survives the application of very high magnetic fields, presenting both an intriguing puzzle and an experimental challenge. The strong, non-perturbative influence of the magnetic field complicates the determination of superconducting order parameters in the high-field phases. Here, we report electronic transport anisotropy measurements in precisely aligned microbars in magnetic fields to 45 T applied along the b-axis. Our results reveal a highly directional vortex pinning force in the field-reinforced phase. The critical current is significantly suppressed for currents along the c direction, whereas the flux-flow voltage is reduced with slight angular misalignments--hallmarks of vortex lock-in transitions typically seen in quasi-2D superconductors like cuprates and pnictides. These findings challenge the assumption of nearly isotropic charge transport in UTe2 and point to enhanced two-dimensionality in the high-field state, consistent with a change in the order parameter. A pair-density-wave-like state at high fields could naturally induce a layered modulation of the superfluid density, forming planar structures that confine vortices and guide their sliding in the flux-flow regime.

cond-mat.supr-con↗

arXiv2025

Magnetic order and physical properties of the Kagome metal UNb$_6$Sn$_6$

The $RM_6X_6$ family of materials ($R$ = rare-earth, $M$ = transition metal, $X$ = Ga, Si, Ge, Sn) produces an array of emergent phenomena, such as charge density waves, intrinsic Hall effects, and complex magnetic order, due to its Kagome net of transition metal atoms, its local-moment magnetic anisotropies, and its extensive chemical tunability. Here, we report a new ``166" material containing both an actinide (uranium) and a 4$d$ transition metal (niobium) to investigate the properties of a 5$f$-4$d$ electron 166 system. UNb$_6$Sn$_6$ crystallizes in the hexagonal $P$6/$mmm$ space group with a small degree of disorder due to shifts in the size of the CoSn-like cages along the $c$ axis. Upon cooling at zero magnetic field, the material undergoes two magnetic phase transitions at $T_\mathrm{2}$ = 46 K and $T_\mathrm{N}$ = 43 K. The low-temperature, zero-field phase is an antiferromagnet with ordered uranium moments and a $\textbf{k}$=(0,0,1/2) propagation vector determined by neutron diffraction. Remarkably, with a magnetic field applied along the $c$ axis, five additional magnetic transitions occur, evidenced by magnetization and resistivity data, before the moment saturates at 2.62 ${\mu}_{\mathrm{B}}$/U at 2 K and $\ge$13.6 T. In two magnetic phase regions, the Hall resistivity of UNb$_6$Sn$_6$ significantly deviates from the magnetization, suggesting that the phases have a large Berry curvature or a change in the Fermi surface. The unknown magnetic ordering of the field-dependent phases of UNb$_6$Sn$_6$ demonstrates the complexity of the 5$f$-4$d$ 166 system and encourages further study of its properties.

cond-mat.str-el↗

arXiv2025

UV$_6$Sn$_6$: a new kagome material with unusual $5f$ magnetism

Materials in the family $R$V$_{6}$Sn$_{6}$ ($R=$ rare earth) provide a unique platform to investigate the interplay between local moments from $R$ layers and nonmagnetic vanadium kagome layers. Yet, the investigation of actinide members remains scarce. Here we report the synthesis of UV$_{6}$Sn$_{6}$ single crystals through the self-flux technique. Magnetic susceptibility, specific heat, electrical resistivity, and thermal expansion measurements reveal two uranium-driven antiferromagnetic transitions at $T_{N1}=29$~K and $T_{N2}=24$~K, a complex field-temperature phase diagram, and unusual negative domain wall magnetoresistance. Specific heat measurements unveil a modest Sommerfeld coefficient of $\gamma = 40$~mJ/mol.K$^{2}$, consistent with angle-resolved photoemission spectroscopy measurements that show a moderate $f$-electron enhancement at the Fermi level ($E_{F}$). Our experiments support a modest contribution from \textit{5f} flat bands to the density of states at $E_{F}$, whereas our band structure calculations place the vanadium flat bands 0.25~eV above $E_{F}$. Our findings point to a materials opportunity to expand the uranium 166 family with the goal of enhancing correlations by tuning $5f$ and $3d$ flat bands to $E_{F}$.

cond-mat.str-el↗

arXiv2024

Derivation of low-energy Hamiltonians for heavy-fermion Materials

By utilizing a multi-orbital periodic Anderson model with parameters obtained from \textit{ab initio} band structure calculations, combined with degenerate perturbation theory, we derive effective Kondo-Heisenberg and spin Hamiltonians that capture the interaction among the effective magnetic moments. This derivation encompasses fluctuations via both nonmagnetic $4f^0$ and magnetic $4f^2$ virtual states, and its accuracy is confirmed through comparison with experimental data obtained from CeIn$_3$. The significant agreement observed between experimental results and theoretical predictions underscores the potential of deriving minimal models from first-principles calculations for achieving a quantitative description of $4f$ materials. Moreover, our microscopic derivation unveils the underlying origin of anisotropy in the exchange interaction between Kramers doublets, shedding light on the conditions under which this anisotropy may be weak compared to the isotropic contribution.

cond-mat.str-el↗

arXiv2024

Quantum Critical Scaling in Quasi-One-Dimensional YbFe$_5$P$_3$

We report measurements of the low temperature magnetization $M$ and specific heat $C$ as a function of temperature and magnetic field of the quasi-one-dimensional spin chain, heavy fermion compound YbFe$_5$P$_3$, which resides close to a quantum critical point. The results are compared to the predictions of scaling laws obtained from a generalized free energy function expected near an antiferromagnetic quantum critical point (AFQCP). The scaling behavior depends on the dimensionality $d$ of the fluctuations, the coherence length exponent $\nu$, and the dynamic exponent $z$. The free energy treats the magnetic field as a relevant renormalization group variable, which leads to a new exponent $\phi=\nu z_h$, where $z_h$ is a dynamic exponent expected in the presence of a magnetic field. When $z_h=z$, $T/H$ scaling is expected, as observed in several compounds close to a QCP; whereas in YbFe$_5$P$_3$, a $T/H^{3/4}$ dependence of the scaling is observed. This dependence reflects the relationship $z_h=(4z/3)$ and a field exponent $\phi =4/3$. A feature of the scaling law is that it restricts the possible values of the exponents to two cases for YbFe$_5$P$_3$: $d$=1, $\nu$=1, $z$=1, and $d$=2, $\nu$=1/2, $z$=2.

cond-mat.str-el↗