arXiv subjects

M. M. Bordelon

Publications and source records attributed to M. M. Bordelon.

8 recordsLinked to original sources

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 $κ_b/T$ 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 $κ_b/T$ 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, $κ_b/T$ 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 $Δ_{min}/Δ_0 \sim 0.1$ along the $b$-axis, where $Δ_0$ 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

What causes the variation in superconducting properties of UTe$_{2}$?

Reaching a consensus on the superconducting order parameter of unconventional superconductors remains a central challenge in the field of magnetically-mediated superconductivity. Though UTe$_{2}$ is largely accepted as a rare example of an odd-parity superconductor, its precise order parameter remains highly debated, even at ambient conditions. A key underlying issue is the large sample-to-sample variation in superconducting properties at zero applied pressure and magnetic field. Here, we investigate the origin of the observed variation by means of single crystal x-ray diffraction (SC-XRD) and scanning transmission electron microscopy (STEM) measurements. Our results reveal highly ordered crystalline lattices, in agreement with the expected $Immm$ structure, and no signs of uranium vacancies. Tiny amounts of interstitial defects, however, are observed on the Te2 layers that host Te chains along the b axis. We argue that these defects give rise to slightly enhanced atomic displacement parameters observed in SC-XRD data and are enough to disrupt the unconventional superconducting state in UTe$_{2}$. Our findings highlight the need to focus future order parameter determination efforts on single crystals of UTe$_{2}$ with minimal amounts of structural disorder.

cond-mat.supr-con↗

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

Coexistence of Kondo Coherence and Localized Magnetic Moments in the Normal State of Molten Salt-Flux Grown UTe2

The development of Kondo lattice coherence in UTe2 leads to the formation of a heavy Fermi liquid state from which superconductivity emerges at lower temperature. In Kondo lattice systems, the nuclear magnetic resonance (NMR) and muon Knight shift have proven to be particularly sensitive to the properties of the developing heavy-electron fluid. Here we report muon Knight shift measurements on high-quality UTe2 single crystals grown by a molten salt-flux method. Together with previous data from a single crystal grown by a chemical-vapor transport method, our results show the contribution of the heavy-electron liquid to the muon Knight shift increases below a crossover temperature T* ~ 30 K in accord with a universal scaling function of T/T* for heavy-fermion materials. An observed departure from this universal scaling below a temperature T ~ 12 K at certain muon stopping sites signifies a reversal of the Kondo hybridization and a relocalization of U 5f moments with an antiferromagnetic coupling. The preservation of universal scaling at a different muon site demonstrates a coexistence of itinerant and localized 5f electron states preceding the superconducting phase transition.

cond-mat.supr-con↗

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↗

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↗

arXiv2017

Tuning the antiferromagnetic helical pitch length and nanoscale domain size in Fe$_3$PO$_4$O$_3$ by magnetic dilution

The insulating magnetic material Fe3PO4O3 features a non-centrosymmetric lattice composed of Fe^{3+} triangular units. Frustration, due to competing near neighbor ($J_1$) and next nearest neighbor ($J_2$) antiferromagnetic interactions, was recently suggested to be the origin of an antiferromagnetic helical ground state with unusual needle-like nanoscale magnetic domains in Fe3PO4O3. Magnetic dilution is shown here to tune the ratio of these magnetic interactions, thus providing deeper insight into this unconventional antiferromagnet. Dilution of the Fe^{3+} lattice in Fe3PO4O3 was accomplished by substituting non-magnetic Ga^{3+} to form the solid solution series Fe_{3-x}Ga_xPO4O3 with $x = 0.012, 0.06, 0.25, 0.5, 1.0, 1.5$. Magnetic susceptibility and neutron powder diffraction data from this series are presented. A continuous decrease of the both the helical pitch length and the domain size is observed with increasing dilution up to at least $x = 0.25$, while for $x \ge 0.5$, the compounds lack long range magnetic order entirely. The decrease in the helical pitch length with increasing $x$ can be qualitatively understood by reduction of the ratio of $J_2/J_1$ in the Heisenberg model, consistent with mean field considerations. Intriguingly, the magnetic correlation length in the $ab$ plane remains nearly equal to the pitch length for each value of $x \le 0.25$, showing that the two quantities are intrinsically connected in this unusual antiferromagnet.

cond-mat.str-el↗