arXiv subjects

F. Ronning

Publications and source records attributed to F. Ronning.

At least 37 records · Page 2Linked to original sources

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 $ν$, and the dynamic exponent $z$. The free energy treats the magnetic field as a relevant renormalization group variable, which leads to a new exponent $ϕ=νz_h$, 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 $ϕ=4/3$. 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, $ν$=1, $z$=1, and $d$=2, $ν$=1/2, $z$=2.

cond-mat.str-el↗

arXiv2024

Probing quantum criticality in ferromagnetic CeRh6Ge4

CeRh$_6$Ge$_4$ is unusual in that its ferromagnetic transition can be suppressed continuously to zero temperature, i.e., to a ferromagnetic quantum-critical point (QCP), through the application of modest hydrostatic pressure. This discovery has raised the possibility that the ferromagnetic QCP may be of the Kondo-breakdown type characterized by a jump in Fermi volume, to which thermopower S measurements should be sensitive. Though $S/T$ changes both sign and magnitude around the critical pressure P$_{c}\approx{}0.8$ GPa, these changes are not abrupt but extend over a pressure interval from within the ferromagnetic state up to P$_c$. Together with temperature and pressure variations in electrical resistivity and previously reported heat capacity, thermopower results point to the near coincidence of two sequential effects near P$_c$, delocalization of 4f degrees-of-freedom through orbital-selective hybridization followed by quantum criticality of itinerant ferromagnetism.

cond-mat.str-el↗

arXiv2024

Two-Stage Cryogenic HEMT Based Amplifier For Low Temperature Detectors

To search for dark matter candidates with masses below $\mathcal{O}$(MeV), the SPLENDOR (Search for Particles of Light dark mattEr with Narrow-gap semiconDuctORs) experiment is developing novel narrow-bandgap semiconductors with electronic bandgaps on the order of 1-100 meV. In order to detect the charge signal produced by scattering or absorption events, SPLENDOR has designed a two-stage cryogenic HEMT-based amplifier with an estimated charge resolution approaching the single-electron level. A low-capacitance ($\sim$1.6 pF) HEMT is used as a buffer stage at $T=10\,\mathrm{mK}$ to mitigate effects of stray capacitance at the input. The buffered signal is then amplified by a higher-capacitance ($\sim$200 pF) HEMT amplifier stage at $T=4\,\mathrm{K}$. Importantly, the design of this amplifier makes it usable with any insulating material - allowing for rapid prototyping of a variety of novel detector materials. We present the two-stage cryogenic amplifier design, preliminary voltage noise performance, and estimated charge resolution of 7.2 electrons.

physics.ins-det↗

arXiv2024

Excess heat capacity in magnetically ordered Ce heavy fermion metals

We study the magnetic heat capacity of a series of magnetically ordered Ce-based heavy fermion materials, which show an anomalous $T^3$ heat capacity in excess of the phonon contribution in many materials. For compounds for which magnon models have been worked out, we show that the local-moment magnon heat capacity derived from the measured magnon spectra underestimates the experimental specific heat. The excess heat capacity reveals increasing density of states with increasing energy, akin to a pseudogap. We show that this anomalous temperature-dependent term is not associated with proximity to a quantum critical point (QCP), but is strongly correlated with $T_N$, indicating the anomalous excitations are governed by the magnetic exchange interaction. This insight may hold key information for understanding magnetically ordered heavy fermions.

cond-mat.str-el↗

arXiv2024

A microscopic Kondo lattice model for the heavy fermion antiferromagnet CeIn$_3$

Electrons at the border of localization generate exotic states of matter across all classes of strongly correlated electron materials and many other quantum materials with emergent functionality. Heavy electron metals are a model example, in which magnetic interactions arise from the opposing limits of localized and itinerant electrons. This remarkable duality is intimately related to the emergence of a plethora of novel quantum matter states such as unconventional superconductivity, electronic-nematic states, hidden order and most recently topological states of matter such as topological Kondo insulators and Kondo semimetals and putative chiral superconductors. The outstanding challenge is that the archetypal Kondo lattice model that captures the underlying electronic dichotomy is notoriously difficult to solve for real materials. Here we show, using the prototypical strongly-correlated antiferromagnet CeIn$_3$, that a multi-orbital periodic Anderson model embedded with input from ab initio bandstructure calculations can be reduced to a simple Kondo-Heisenberg model, which captures the magnetic interactions quantitatively. We validate this tractable Hamiltonian via high-resolution neutron spectroscopy that reproduces accurately the magnetic soft modes in CeIn$_3$, which are believed to mediate unconventional superconductivity. Our study paves the way for a quantitative understanding of metallic quantum states such as unconventional superconductivity.

cond-mat.str-el↗

arXiv2023

Unusual magnetism of the axion-insulator candidate Eu$_5$In$_2$Sb$_6$

Eu$_5$In$_2$Sb$_6$ is a member of a family of orthorhombic nonsymmorphic rare-earth intermetallics that combines large localized magnetic moments and itinerant exchange with a low carrier density and perpendicular glide planes. This may result in special topological crystalline (wallpaper fermion) or axion insulating phases. Recent studies of Eu$_5$In$_2$Sb$_6$ single crystals have revealed colossal negative magnetoresistance and multiple magnetic phase transitions. Here, we clarify this ordering process using neutron scattering, resonant elastic X-ray scattering, muon spin-rotation, and magnetometry. The nonsymmorphic and multisite character of Eu$_5$In$_2$Sb$_6$ results in coplanar noncollinear magnetic structure with an Ising-like net magnetization along the $a$ axis. A reordering transition, attributable to competing ferro- and antiferromagnetic couplings, manifests as the onset of a second commensurate Fourier component. In the absence of spatially resolved probes, the experimental evidence for this low-temperature state can be interpreted either as an unusual double-$q$ structure or in a phase separation scenario. The net magnetization produces variable anisotropic hysteretic effects which also couple to charge transport. The implied potential for functional domain physics and topological transport suggests that this structural family may be a promising platform to implement concepts of topological antiferromagnetic spintronics.

cond-mat.str-el↗

arXiv2023

Structural and physical properties of the chiral antiferromagnet CeRhC$_2$

We report a study of the structural, magnetic, transport, and thermodynamic properties of polycrystalline samples of CeRhC$_2$. CeRhC$_2$ crystallizes in a tetragonal structure with space group $P4_1$ and it orders antiferromagnetically below $T_\textrm{N1} \approx$ 1.8 K. Powder neutron diffraction measurements reveal a chiral magnetic structure with a single propagation vector $Q_m = (1/2,1/2,0.228(5))$, indicating an antiferromagnetic arrangement of Ce magnetic moments in the $ab$-plane and incommensurate order along the $c$-axis with a root-mean-square ordered moment of $m_\textrm{ord}$= 0.68 $μ_\textrm{B}$/Ce. Applying a magnetic field suppresses the Néel temperature $T_\textrm{N1}$ to zero near $μ_0H_\textrm{c1}\sim$0.75 T. A second antiferromagnetic phase ($T_\textrm{N2}$), however, becomes apparent in electrical resistivity, Hall and heat capacity measurements in fields above 0.5 T and extrapolates to zero temperature at $μ_0H_\textrm{c2}\sim$ 1 T. Electrical resistivity measurements reveal that LaRhC$_2$ is a semiconductor with a bandgap of $E_\textrm{g}\sim24$ meV; whereas, resistivity and Hall measurements indicate that CeRhC$_2$ is a semimetal with a low carrier concentration of $n\sim10^{20}$ cm$^{-3}$. With applied hydrostatic pressure, the zero-field antiferromagnetic transition of CeRhC$_2$ is slightly enhanced and CeRhC$_2$ becomes notably more metallic up to 1.36 GPa. The trend toward metallicity is in line with density-functional calculations that indicate that both LaRhC$_2$ and CeRhC$_2$ are semimetals, but the band overlap is larger for CeRhC$_2$, which has a smaller unit cell volume that its La counterpart. This suggests that the bandgap closes due to a lattice contraction when replacing La with Ce in RRhC$_2$ (R = rare-earth), in agreement with experimental results.

cond-mat.str-el↗

arXiv2023

The fate of time-reversal symmetry breaking in UTe2

Topological superconductivity is a long-sought state of matter in bulk materials, and odd-parity superconductor UTe$_2$ is a prime candidate. The recent observation of a field-trainable spontaneous Kerr signal in UTe$_2$ at the onset of superconductivity provides strong evidence that the superconducting order parameter is multicomponent and breaks time-reversal symmetry. Here, we perform Kerr effect measurements on a number of UTe$_2$ samples -- grown $via$ both chemical vapor transport and the molten-salt-flux methods -- that show a single superconducting transition between 1.6~K and 2.1~K. Our results show no evidence for a spontaneous Kerr signal in zero field measurements. This implies that the superconducting state of UTe$_2$ does not intrinsically break time-reversal symmetry. Instead, we observe a field-trainable signal that varies in magnitude between samples and between different locations on a single sample, which is a sign of inhomogeneous magnetic regions. Our results provide an examination of representative UTe$_2$ samples and place strong constraints on the superconducting order parameter of UTe$_2$.

cond-mat.supr-con↗

arXiv2022

Physical properties of the layered $f$-electron van der Waals magnet Ce$_2$Te$_5$

We report a detailed study of the magnetic, transport, and thermodynamic properties of Ce$_2$Te$_5$ single crystals, a layered $f$-electron van der Waals magnet. Four consecutive transitions at $\sim$ 5.2, 2.1, 0.9, and 0.4 K were observed in the $ac$-plane electrical resistivity $ρ$(T), which were further confirmed in specific heat $C_\textrm{p}$(T) measurements. Analysis of the magnetic susceptibility $χ$(T), the magnetic-field variation of $ρ$(T), and the increase of the first transition temperature ($T_\textrm{c} \sim$ 5.2 K) with applied magnetic field indicates ferromagnetic order, while the decrease of the other transitions with field suggests different states with dominant antiferromagnetic interactions below $T_2 \sim$ 2.1 K, $T_3 \sim$ 0.9 K, and $T_4$ = 0.4 K. Critical behavior analysis around $T_\textrm{c}$ that gives critical exponents $β= 0.31(2)$, $γ= 0.99(2)$, $δ= 4.46(1)$, $T_\textrm{c} = 5.32(1)$ K indicates that Ce$_2$Te$_5$ shows a three-dimensional magnetic critical behavior. Moreover, the Hall resistivity $ρ_{\textrm{xy}}$ indicates that Ce$_2$Te$_5$ is a multi-band system with a relatively high electron mobility $\sim 2900$ cm$^2$ V$^{-1}$ s$^{-1}$ near $T_\textrm{c}$, providing further opportunities for future device applications.

cond-mat.str-el↗

arXiv2022

Kondo quasiparticle dynamics observed by resonant inelastic x-ray scattering

Effective models focused on pertinent low-energy degrees of freedom have substantially contributed to our qualitative understanding of quantum materials. An iconic example, the Kondo model, was key to demonstrating that the rich phase diagrams of correlated metals originate from the interplay of localized and itinerant electrons. Modern electronic structure calculations suggest that to achieve quantitative material-specific models, accurate consideration of the crystal field and spin-orbit interactions is imperative. This poses the question of how local high-energy degrees of freedom become incorporated into a collective electronic state. Here, we use resonant inelastic x-ray scattering (RIXS) on CePd$_3$ to clarify the fate of all relevant energy scales. We find that even spin-orbit excited states acquire pronounced momentum-dependence at low temperature - the telltale sign of hybridization with the underlying metallic state. Our results demonstrate how localized electronic degrees of freedom endow correlated metals with new properties, which is critical for a microscopic understanding of superconducting, electronic nematic, and topological states.

cond-mat.str-el↗

arXiv2022

Persistence of correlation-driven surface states in SmB6 under pressure

The proposed topological Kondo insulator SmB$_{6}$ hosts a bulk Kondo hybridization gap that stems from strong electronic correlations and a metallic surface state whose effective mass remains disputed. Thermopower and scanning tunneling spectroscopy measurements argue for heavy surface states that also stem from strong correlations, whereas quantum oscillation and angle-resolved photoemission measurements reveal light effective masses that would be consistent with a Kondo breakdown scenario at the surface. Here we investigate the evolution of the surface state via electrical and thermoelectric transport measurements under hydrostatic pressure, a clean symmetry-preserving tuning parameter that suppresses the Kondo gap and increases the valence of Sm from 2.6+ towards a 3+ magnetic metallic state. Electrical resistivity measurements reveal that the surface carrier density increases with increasing pressure, whereas thermopower measurements show an unchanged Fermi energy under pressure. As a result, the effective mass of the surface state charge carriers linearly increases with pressure as the Sm valence approaches 3+. Our results are consistent with the presence of correlation-driven surface states in SmB$_{6}$ and suggest that the surface Kondo effect persists under pressure to 2 GPa.

cond-mat.str-el↗

arXiv2022

Colossal piezoresistance in narrow-gap Eu5In2Sb6

Piezoresistance, the change of a material's electrical resistance ($R$) in response to an applied mechanical stress ($σ$), is the driving principle of electromechanical devices such as strain gauges, accelerometers, and cantilever force sensors. Enhanced piezoresistance has been traditionally observed in two classes of uncorrelated materials: nonmagnetic semiconductors and composite structures. We report the discovery of a remarkably large piezoresistance in Eu$_5$In$_2$Sb$_6$ single crystals, wherein anisotropic metallic clusters naturally form within a semiconducting matrix due to electronic interactions. Eu$_5$In$_2$Sb$_6$ shows a highly anisotropic piezoresistance, and uniaxial pressure along [001] of only 0.4~GPa leads to a resistivity drop of more than 99.95\% that results in a colossal piezoresistance factor of $5000\times10^{-11}$Pa$^{-1}$. Our result not only reveals the role of interactions and phase separation in the realization of colossal piezoresistance, but it also highlights a novel route to multi-functional devices with large responses to both pressure and magnetic field.

cond-mat.str-el↗

arXiv2022

Possible quadrupole-order-driven commensurate-incommensurate phase transition in B20 CoGe

The B20-type cobalt germanide CoGe was investigated by measuring the specific heat, resistivity, and $^{59}$Co nuclear magnetic resonance (NMR). We observed a phase transition at $T_Q=13.7$ K, evidenced by a very narrow peak of the specific heat and sharp changes of the nuclear spin-spin ($T_2^{-1}$) and spin-lattice ($T_1^{-1}$) relaxation rates. The fact that the entropy release is extremely small and the Knight shift is almost independent of temperature down to low temperatures as anticipated in a paramagnetic metal indicates that the $T_Q$ transition is of non-magnetic origin. In addition, we detected a crossover scale $T_0\sim30$ K below which the resistivity and the NMR linewidth increase, and $T_1^{-1}$ is progressively distributed in space, that is, a static and dynamical spatial inhomogeneity develops. While the order parameter for the $T_Q$ transition remains an open question, a group-theoretical analysis suggests that the finite electric quadrupole density arising from the low local site symmetry at cobalt sites could drive the crystal symmetry lowering from the P2$_1$3 symmetry that is commensurate to the R3 symmetry with an incommensurate wavevector, which fairly well accounts for the $T_Q$ transition. The quadrupole-order-driven commensurate-incommensurate phase transition may be another remarkable phenomenon arising from the structural chirality inherent in the noncentrosymmetric B20 family.

cond-mat.str-el↗

arXiv2022

Microscopic probe of magnetic polarons in antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$

Colossal magnetoresistance (CMR) emerges from intertwined spin and charge degrees of freedom in the form of ferromagnetic clusters also known as trapped magnetic polarons. As a result, CMR is rarely observed in antiferromagnetic materials. Here we use electron spin resonance (ESR) to reveal microscopic evidence for the formation of magnetic polarons in antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$. First, we observe a reduction of the Eu$^{2+}$ ESR linewidth as a function of the applied magnetic field consistent with ferromagnetic clusters that are antiferromagnetically coupled. Additionally, the Eu$^{2+}$ lineshape changes markedly below T' ~ 200 K, a temperature scale that coincides with the onset of CMR. The combination of these two effects provide strong evidence that magnetic polarons grow in size below T' and start influencing the macroscopic properties of the system.

cond-mat.str-el↗

arXiv2021

Ground state of Ce$_{3}$Bi$_{4}$Pd$_{3}$ unraveled by hydrostatic pressure

Noncentrosymmetric Ce$_{3}$Bi$_{4}$Pd$_{3}$ has attracted a lot of attention as a candidate for strongly correlated topological material, yet its experimental ground state remains a matter of contention. Two conflicting scenarios have emerged from a comparison to prototypical Kondo insulator Ce$_{3}$Bi$_{4}$Pt$_{3}$: either Ce$_{3}$Bi$_{4}$Pd$_{3}$ is a spin-orbit-driven topological semimetal or a Kondo insulator with smaller Kondo coupling than its Pt counterpart. Here we determine the ground state of Ce$_{3}$Bi$_{4}$Pd$_{3}$ via electrical resistivity measurements under hydrostatic pressure, which is a clean symmetry-preserving tuning parameter that increases hybridization but virtually preserves spin-orbit coupling. Ce$_{3}$Bi$_{4}$Pd$_{3}$ becomes more insulating under pressure, which is a signature of Ce-based Kondo insulating materials. Its small zero-pressure gap increases quadratically with pressure, similar to the behavior observed in the series Ce$_{3}$Bi$_{4}$(Pt$_{1-x}$Pd$_{x}$)$_{3}$, which indicates that Pt substitution and applied pressure have a similar effect. Our result not only demonstrates that Kondo coupling, rather than spin-orbit coupling, is the main tuning parameter in this class of materials, but it also establishes that Ce$_{3}$Bi$_{4}$Pd$_{3}$ has a narrow-gap Kondo insulating ground state.

cond-mat.str-el↗

arXiv2021

Single-component superconducting state in UTe2 at 2 K

UTe2 is a newly-discovered unconventional superconductor wherein multicomponent topological superconductivity is anticipated based on the presence of two superconducting transitions and time-reversal symmetry breaking in the superconducting state. The observation of two superconducting transitions, however, remains controversial. Here we demonstrate that UTe2 single crystals displaying an optimal superconducting transition temperature at 2 K exhibit a single transition and remarkably high quality supported by their small residual heat capacity in the superconducting state and large residual resistance ratio. Our results shed light on the intrinsic superconducting properties of UTe2 and bring into question whether UTe2 is a multicomponent superconductor at ambient pressure.

cond-mat.supr-con↗

arXiv2021

Spatially inhomogeneous superconductivity in UTe2

Newly-discovered superconductor UTe$_2$ is a strong contender for a topological spin-triplet state wherein a multi-component order parameter arises from two nearly-degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that UTe$_2$ at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature (T$_c$). Notably, samples with higher T$_c$ only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line in the pressure-temperature phase diagram but also provide a unified scenario for the sample dependence of UTe$_{2}$.

cond-mat.str-el↗

arXiv2021

Narrow-gap semiconducting behavior in antiferromagnetic Eu$_{11}$InSb$_9$

Here we investigate the thermodynamic and electronic properties of Eu$_{11}$InSb$_9$ single crystals. Electrical transport data show that Eu$_{11}$InSb$_9$ has a semiconducting ground state with a relatively narrow band gap of $320$~meV. Magnetic susceptibility data reveal antiferromagnetic order at low temperatures, whereas ferromagnetic interactions dominate at high temperature. Specific heat, magnetic susceptibility, and electrical resistivity measurements reveal three phase transitions at $T_{N1}=9.3$~K, $T_{N2} =8.3$~K, and $T_{N3} =4.3$~K. Unlike Eu$_{5}$In$_{2}$Sb$_6$, a related europium-containing Zintl compound, no colossal magnetoresistance (CMR) is observed in Eu$_{11}$InSb$_9$. We attribute the absence of CMR to the smaller carrier density and the larger distance between Eu ions and In-Sb polyhedra in Eu$_{11}$InSb$_9$. Our results indicate that Eu$_{11}$InSb$_9$ has potential applications as a thermoelectric material through doping or as a long-wavelength detector due to its narrow gap.

cond-mat.str-el↗