arXiv subjects

S. M. Thomas

Publications and source records attributed to S. M. Thomas.

At least 37 records · Page 2Linked to original sources

arXiv2024

Thermodynamic evidence for polaron stabilization inside the antiferromagnetic order of Eu$_5$In$_2$Sb$_6$

Materials exhibiting electronic inhomogeneities at the nanometer scale have enormous potential for applications. Magnetic polarons are one such type of inhomogeneity which link the electronic, magnetic and lattice degrees of freedom in correlated matter and often give rise to colossal magnetoresistance. Here, we investigate single crystals of Eu$_5$In$_2$Sb$_6$ by thermal expansion and magnetostriction along different crystallographic directions. These data provide compelling evidence for the formation of magnetic polarons in Eu$_5$In$_2$Sb$_6$ well above the magnetic ordering temperature. More specifically, our results are consistent with anisotropic polarons with varying extent along the different crystallographic directions. A crossover revealed within the magnetically ordered phase can be associated with a surprising stabilization of ferromagnetic polarons within the global antiferromagnetic order upon decreasing temperature. These findings make Eu$_5$In$_2$Sb$_6$ a rare example of such coexisting and competing magnetic orders and, importantly, shed new light on colossal magnetoresistive behavior beyond manganites.

cond-mat.str-el↗

arXiv2024

Magnetic edge fields in UTe$_2$ near zero background fields

Chiral superconductors are theorized to exhibit spontaneous edge currents. Here, we found magnetic fields at the edges of UTe$_2$, a candidate odd-parity chiral superconductor, that seem to agree with predictions for a chiral order parameter. However, we did not detect the chiral domains that would be expected, and recent polar Kerr and muon spin relaxation data in nominally clean samples argue against chiral superconductivity. Our results show that hidden sources of magnetism must be carefully ruled out when using spontaneous edge currents to identify chiral superconductivity.

cond-mat.supr-con↗

arXiv2024

Quantum Order by Disorder: A Key to Understanding the Magnetic Phases of BaCo$_2$(AsO$_4$)$_2$

BaCo$_2$(AsO$_4$)$_2$ (BCAO), a honeycomb cobaltate, is considered a promising candidate for materials displaying the Kitaev quantum spin liquid state. This assumption is based on the distinctive characteristics of Co$^{2+}$ ions (3$d^7$) within an octahedral crystal environment, resulting in spin-orbit-coupled $J_{\rm eff}$~=~1/2 doublet states. However, recent experimental observations and theoretical analyses have raised questions regarding this hypothesis. Despite these uncertainties, reports of continuum excitations reminiscent of spinon excitations have prompted further investigations. In this study, we explore the magnetic phases of BCAO under both in-plane and out-of-plane magnetic fields, employing dc and ac magnetic susceptibilities, capacitance, and torque magnetometry measurement. Our results affirm the existence of multiple field-induced magnetic phases, with strong anisotropy of the phase boundaries between in-plane and out-of-plane fields. To elucidate the nature of these phases, we develop a minimal anisotropic exchange model. This model, supported by combined first principles calculations and theoretical modeling, quantitatively reproduces our experimental data. In BCAO, the combination of strong bond-independent XXZ anisotropy and geometric frustration leads to significant quantum order by disorder effects that stabilize collinear phases under both zero and finite magnetic fields.

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 $ν$, 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↗

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

Absence of Spontaneous Magnetic Fields Due to Time-Reversal Symmetry Breaking in Bulk Superconducting UTe2

We have investigated the low-temperature local magnetic properties in the bulk of molten salt-flux (MSF) grown single crystals of the candidate odd-parity superconductor UTe2 by zero-field muon spin relaxation (muSR). In contrast to previous muSR studies of UTe2 single crystals grown by a chemical vapour transport (CVT) method, we find no evidence of magnetic clusters or electronic moments fluctuating slow enough to cause a discernible relaxation of the zero-field muSR asymmetry spectrum. Consequently, our measurements on MSF-grown single crystals rule out the generation of spontaneous magnetic fields in the bulk that would occur near impurities or lattice defects if the superconducting state of UTe2 breaks time-reversal symmetry. This result suggests UTe2 is characterized by a single-component superconducting order parameter.

cond-mat.supr-con↗

arXiv2023

Metastable phase of UTe$_2$ formed under high pressure above 5 GPa

Uranium ditelluride (UTe$_2$) has attracted recent interest due to its unique superconducting properties, which include the potential for a topological odd-parity superconducting state. Recently, ac-calorimetry measurements under pressure indicate a change in the ground state of UTe$_2$ from superconducting to antiferromagnetic at 1.4 GPa. Here, we investigate the effect of pressure on the crystal structure of UTe$_2$ up to 25 GPa at room temperature using x-ray diffraction. We find that UTe$_2$, which at ambient conditions has an orthorhombic ($Immm$) structure, transforms to a body-centered tetragonal ($I4/mmm$) structure at 5 GPa in a quasi-hydrostatic neon (Ne) pressure transmitting medium. In the absence of a pressure-transmitting medium, this transformation occurs between 5 and 8 GPa. The data were fit with a third-order Birch-Murnaghan equation of state resulting in values of $B_0$=46.0 $\pm$ 0.6 GPa, $B^{\prime}$=9.3 $\pm$ 0.5 (no pressure medium) and $B_0$=42.5 $\pm$ 2.0 GPa, $B^{\prime}$=9.3 (fixed) (neon pressure medium) for the $Immm$ phase. For the $I4/mmm$ phase, $B_0$=78.9 $\pm$ 0.5 GPa and $B^{\prime}$=4.2 $\pm$ 0.1 (no pressure transmitting medium), and $B_0$=70.0 $\pm$ 1.1 GPa and $B^{\prime}$=4.1 $\pm$ 0.2 (neon pressure medium). The high-pressure tetragonal phase is retained after decompression to ambient pressure, with approximately 30% remaining after 2 days. We argue that the observed phase transition into a higher symmetry structure at P~5 GPa (orthorhombic to tetragonal), is accompanied by an increase in the shortest distance between uranium atoms from 3.6 Angstrom (orthorhombic) to 3.9 Angstrom (tetragonal), which suggests localization of the 5f electrons, albeit with a 10.7% decrease in volume.

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

Anisotropic field-induced changes in the superconducting order parameter of UTe2

UTe2 is a newly discovered unconventional superconductor, where electron Cooper pairs combine into a spin-triplet ground state. Here we report the specific heat C(H,T) of a high-quality single crystal of UTe2 with a single specific heat anomaly at the superconducting transition temperature T_c {\approx} 2 K and a small zero-field residual Sommerfeld coefficient γ_0 = C/T (T=0) = 10 mJ/mol-K^2. We applied magnetic field up to 12 T along the three principal crystallographic axes of UTe2 to probe the nature of the superconducting state. The evolution of the residual Sommerfeld coefficient as a function of magnetic field, γ_0 (H), is highly anisotropic and reveals distinct regions. In magnetic field up to 4 T applied along a, b, and c axes, we find γ_0{\approx}α_i {\square}H, with i=a,b,c, as expected for an unconventional superconductor with nodes (zeros) of the superconducting order parameter on the Fermi surface. A pronounced kink in γ_0(H), however, is observed at roughly 4 T for field applied along both a and b axes, whereas a smooth change from square-root to linear behaviour is observed at 4 T for H//c. These results strongly indicate that a zero-field ground state is stable up to 4 T and undergoes a field-induced evolution above 4 T. {α_c} > {α_a} > {α_b}, indicating that the nodes in the low-field state are predominantly located in the vicinity of the a-b plane. The modification of the order parameter is strongest when field is applied in the a-b plane, which causes nodes to move away from the direction of the applied field. Both d_(B_2u)+id_(B_1u) and d_(B_2u)+id_(A_u) two-component order parameters can account for our observations, with d_(B_2u)+id_(B_1u) a more likely candidate. In either scenario, our measurements indicate that B_2u is the primary superconducting order parameter in UTe2.

cond-mat.supr-con↗

arXiv2023

Localized f-electron magnetism in the semimetal Ce3Bi4Au3

Ce$_{3}$Bi$_{4}$Au$_{3}$ crystallizes in the same non-centrosymmetric cubic structure as the prototypical Kondo insulator Ce$_{3}$Bi$_{4}$Pt$_{3}$. Here we report the physical properties of Ce$_{3}$Bi$_{4}$Au$_{3}$ single crystals using magnetization, thermodynamic, and electrical-transport measurements. Magnetic-susceptibility and heat-capacity data reveal antiferromagnetic (AFM) order below $T_N=3.2$ K. The magnetic entropy $S_{\rm mag}$ reaches $R$ln2 slightly above $T_N$, which suggests localized $4f$-moments in a doublet ground state. Multiple field-induced magnetic transitions are observed at temperatures below $T_N$, which indicate a complex spin structure with competing interactions. Ce$_{3}$Bi$_{4}$Au$_{3}$ shows semimetallic behavior in electrical resistivity measurements in contrast to the majority of reported Cerium-based 343 compounds. Electrical-resistivity measurements under hydrostatic pressure reveal a slight enhancement of $T_N$ under pressures up to 2.3 GPa, which supports a scenario wherein Ce$_{3}$Bi$_{4}$Au$_{3}$ belongs to the far left of the Doniach phase diagram dominated by Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Using realistic many-body simulations, we confirm the semi-metallic electronic structure of Ce$_{3}$Bi$_{4}$Au$_{3}$ and quantitatively reproduce its local moment behavior in the paramagnetic state.

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↗

arXiv2023

$μ^+$ Knight Shift in UTe$_2$: Evidence for Relocalization in a Kondo Lattice

The local magnetic susceptibility of the spin-triplet superconductor UTe$_2$ has been investigated by positive muon ($μ^+$) Knight shift measurements in the normal state. Three distinct $μ^+$ Knight shift components are observed for a magnetic field applied parallel to the $c$ axis. Two of these exhibit a breakdown in the linear relationship with the bulk magnetic susceptibility ($χ$) below a temperature $T^* \! \sim \! 30$ K, which points to a gradual emergence of a correlated Kondo liquid. Below $T_{\rm r} \! \sim \! 12$ K linearity is gradually restored, indicating partial relocalization of the Kondo liquid quasiparticles. The third Knight shift component is two orders of magnitude larger, and despite the $c$-axis alignment of the external field, scales with the $a$-axis $χ$ above $T_{\rm r} \! \sim \! 12$ K. We conjecture that this component is associated with magnetic clusters and the change in the temperature dependence of all three Knight shift components below $T_{\rm r}$ is associated with a change in magnetic correlations. Our findings indicate that prior to the onset of superconductivity the development of the itinerant heavy-electron fluid is halted by a gradual development of local U $5f$-moment fluctuations.

cond-mat.str-el↗

arXiv2023

Ubiquitous Spin Freezing in the Superconducting State of UTe2

In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either phonons or by long-range interactions such as spin fluctuations. The superconductor UTe$_2$ is a rare material wherein electrons are believed to form pairs in a unique spin-triplet state with potential topological properties. While spin-triplet pairing may be mediated by ferromagnetic or antiferromagnetic fluctuations, experimentally, the magnetic properties of UTe$_2$ are unclear. By way of muon spin rotation/relaxation ($μ$SR) measurements on independently grown UTe$_2$ single crystals we demonstrate the existence of magnetic clusters that gradually freeze into a disordered spin frozen state at low temperatures. Our findings suggest that inhomogeneous freezing of magnetic clusters is linked to the ubiquitous residual linear term in the temperature dependence of the specific heat ($C$) and the low-temperature upturn in $C/T$ versus $T$. The omnipresent magnetic inhomogeneity has potential implications for experiments aimed at establishing the intrinsic low-temperature properties of UTe$_2$.

cond-mat.supr-con↗

arXiv2022

Differences in the Resistive and Thermodynamic Properties of the Single Crystalline Chiral Superconductor Candidate SrPtAs

$\require{mediawiki-texvc}$The locally non-centrosymmetric superconductor SrPtAs is proposed to host a topological chiral $d$-wave state, but experimental reports have been limited to polycrystalline samples. Here we report the synthesis of single crystalline SrPtAs grown from Pb flux. SrPtAs crystallizes in the hexagonal space group $P6_{3}$/$mmc$ with lattice parameters $a$ = 4.2445(4) $Å$ and $c$ = 8.9513(18) $Å$. Magnetic susceptibility and electrical resistivity measurements reveal a superconducting transition at T$_c$ $\sim$2.2 K, in agreement with previous reports on polycrystalline samples. Surprisingly, heat capacity data show only a small bulk transition at 0.7 K. We discuss the possible origins of the discrepancy between the various measurements.

cond-mat.supr-con↗

arXiv2022

Microscopic imaging homogeneous and single phase superfluid density in UTe$_2$

The spin-triplet superconductor UTe$_2$ shows spontaneous time-reversal symmetry breaking and multiple superconducting phases in some crystals, implying chiral superconductivity. Here we microscopically image the local magnetic fields and magnetic susceptibility near the surface of UTe$_2$, observing a homogeneous superfluid density $n_s$ and homogeneous pinned vortices. The temperature dependence of $n_s$ is consistent with an anisotropic gap and shows no evidence for an additional kink that would be expected at any second phase transition. Our findings are consistent with a dominant $B_{3u}$ superconducting order parameter in the case of a quasi-2D Fermi surface and provide no evidence for multiple phase transitions in $n_s(T)$ in 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↗