arXiv subjects

Peter Czajka

Publications and source records attributed to Peter Czajka.

8 recordsLinked to original sources

arXiv2026

Limited coincidence between ultrahigh-field superconductivity and line of metamagnetic endpoints in UTe$_2$

The field-dependent magnetization of UTe$_2$ was measured through the metamagnetic transition at a variety of field angles, tracking how the step in magnetization evolves with fields tilted away from the $b$ axis. For fields oriented within the $ab$ plane, jumps in both $M_a$ and $M_b$ vanish approximately 18{\deg} away from the $b$ axis. From contactless conductivity measurements, we find that the halo-like high-field superconducting region extends to the $ab$ plane, where it exists only within a very narrow ($<$1{\deg}) angular range near the termination of the metamagnetic phase boundary and extends beyond the highest measured field of 73 T. As the field orientation tilts towards the $c$ axis, the superconducting and metamagnetic phase boundaries no longer coincide and exhibit distinct trends.

cond-mat.supr-con↗

arXiv2025

High-Field Superconducting Halo in UTe$_2$

Heavy fermion UTe$_2$ is a promising candidate for topological superconductivity that also exhibits multiple high-field superconducting phases. The SC$_{\rm{FP}}$ phase has only been observed in off-axis magnetic fields in the $bc$ plane at fields greater than 40 teslas, a striking scale given its critical temperature of only 2 kelvins. Here, we extend measurements of this unique superconducting state outside of the $bc$ plane and reveal its core structure. The SC$_{\rm{FP}}$ phase is not confined to fields in the $bc$ plane and in fact wraps around the $b$ axis in a halo-like fashion. In other words, this superconducting state, which exists in fields above 73 teslas, is stabilized by a field component perpendicular to the magnetic easy axis. These remarkable field scales further underscore UTe$_2$'s unique magnetophilic superconducting tendencies and suggest an underlying pairing mechanism that is qualitatively distinct from known theories for field-enhanced superconductivity. Phenomenological modeling points to a two-component, non-unitary spin triplet order parameter with finite orbital momentum of the Cooper pairs as a natural explanation for the field-angle dependence of the upper critical field of the SC$_{\rm{FP}}$ phase.

cond-mat.supr-con↗

arXiv2024

Scaling behavior and giant field-enhancement of the thermal conductivity in the honeycomb antiferromagnet BaCo2(AsO4)2

The layered honeycomb material BaCo$_2$(AsO$_4$)$_2$ (BCAO) is of topical interest because its magnetic state is related to that of the Kitaev magnet $α$-RuCl$_3$. Using thermal transport to probe how magnetic excitations interact with phonons in the magnetically disordered regime, we have uncovered an unusually large enhancement of the thermal conductivity $κ_{xx}$ in an in-plane magnetic field ${\bf H}$. Just above the Néel temperature $T_{\rm N}$, a field of 13 T increases $κ_{xx}$ by a factor $\sim 211$, much larger than reported previously in any magnetic insulator. Interestingly, $κ_{xx}(H,T)$ exhibits a scaling behavior in the entire magnetically disordered region that surrounds the ordered zigzag state. The ratio $Δκ_{xx}(H,T)/κ_{xx}(13,T)$, measured throughout the disordered region, collapses to a one-parameter scaling function ${\rm exp}(-1/gx)$ (where $x = μ_{\rm B}B/k_{\rm B}T$ and $g$ is a constant).

cond-mat.str-el↗

arXiv2024

Connection between f-electron correlations and magnetic excitations in UTe2

The detailed anisotropy of the low-temperature, low-energy magnetic excitations of the candidate spin-triplet superconductor UTe$_2$ is revealed using inelastic neutron scattering. The magnetic excitations emerge from the Brillouin zone boundary at the high symmetry $Y$ and $T$ points and disperse along the crystallographic $\hat{b}$-axis. In applied magnetic fields to at least $μ_0 H=11$~T along the $\hat{c}-$axis, the magnetism is found to be field-independent in the $(hk0)$ plane. The scattering intensity is consistent with that expected from U$^{3+}$/U$^{4+}$ $f$-electron spins with preferential orientation along the crystallographic $\hat{a}$-axis, and a fluctuating magnetic moment of 2.3(7) $μ_B$. These characteristics indicate that the excitations are due to intraband spin excitons arising from $f$-electron hybridization.

cond-mat.str-el↗

arXiv2023

Orphan High Field Superconductivity in Non-Superconducting Uranium Ditelluride

Reentrant superconductivity is a phenomenon in which the destructive effects of magnetic field on superconductivity are mitigated, allowing a zero-resistance state to survive under conditions that would otherwise destroy it. Typically, the reentrant superconducting region derives from a zero-field parent superconductor. Here, we show that in specifically-prepared UTe$_2$ crystals, extremely large magnetic field gives rise to an unprecedented high field superconductor that lacks a zero-field parent phase. This orphan superconductivity exists at fields between 37 T and 52 T, over a smaller angular range than observed in superconducting UTe$_2$. The stability of field-induced orphan superconductivity is a challenge to existing theoretical explanations, and underscores the likelihood of a field-induced modification of the electronic structure of UTe$_2$.

cond-mat.supr-con↗

arXiv2022

The planar thermal Hall conductivity in the Kitaev magnet α-RuCl3

We report detailed measurements of the Onsager-like planar thermal Hall conductivity $κ_{xy}$ in $α$-RuCl$_3$, a spin-liquid candidate of topical interest. With the thermal current ${\bf J}_{\rm Q}$ and magnetic field $\bf B\parallel a$ (zigzag axis), the observed $κ_{xy}/T$ varies strongly with temperature $T$ (1-10 K). The results are well-described by bosonic edge excitations which evolve to topological magnons at large $B$. Fits to $κ_{xy}/T$ yield a Chern number $\sim 1$ and a band energy $ω_1\sim$1 meV, in agreement with sharp modes seen in electron spin-resonance experiments. The bosonic character is incompatible with half-quantization of $κ_{xy}/T$.

cond-mat.str-el↗

arXiv2021

Oscillations of the thermal conductivity observed in the spin-liquid state of $α$-RuCl$_3$

In the class of materials called spin liquids, a magnetically ordered state cannot be attained even at milliKelvin temperatures because of conflicting constraints on each spin (for e.g. from geometric or exchange frustration). The resulting quantum spin-liquid (QSL) state is currently of intense interest because it exhibits novel excitations as well as wave-function entanglement. The layered insulator $α$-RuCl$_3$ orders as a zigzag antiferromagnet below $\sim$7 K in zero magnetic field. The zigzag order is destroyed when a magnetic field $\bf H$ is applied parallel to the zigzag axis a. Within the field interval (7.3, 11) Tesla, there is growing evidence that a QSL state exists. Here we report the observation of oscillations in its thermal conductivity below 4 K. The oscillation amplitude is very large within the interval (7.3, 11) T and strongly suppressed on either side. Paradoxically, the oscillations are periodic in 1/\emph{H}, analogous to quantum oscillations in metals, even though $α$-RuCl$_3$ is an excellent insulator with a gap of 1.9 eV. By tilting $\bf H$ out of the plane, we find that the oscillation period is determined by the in-plane component $H_a$. As the temperature is raised above 0.5 K, the oscillation amplitude decreases exponentially. The decrease anticorrelates with the emergence above $\sim$2 K of an anomalous planar thermal Hall conductivity measured with $\bf H\parallel a$. To exclude extrinsic artifacts, we carried out several tests. The implications of the oscillations are discussed.

cond-mat.str-el↗

arXiv2019

Enhanced thermal Hall conductivity below 1 Kelvin in the pyrochlore magnet Yb$_2$Ti$_2$O$_7$

In this letter, we report the gigantic thermal Hall effect $κ_{xy}$ in the low-field correlated-paramagnetic state of the frustrated pyrochlore Yb$_2$Ti$_2$O$_7$. We observed a record magnitude for the thermal Hall angle in an insulator, $\left|κ_{xy}\right|/κ_{xx}\sim 2\,\%$. The signal onsets at $T\sim 3\,$K and is severely weakened around the transition to canted ferromagnetic order at $T_\text{CFM}=0.275\,$K. Besides the large $κ_{xy}>0$ of the fluctuating regime, a sign change towards negative $κ_{xy}$ occurs at the lowest temperatures and in moderate field, where sharp magnon excitations appear in the inelastic neutron scattering spectra. We analyze the magnon-Hall signal and its suppression with field semi-quantitatively. A contribution of phonon skew scattering to $κ_{xy}$ is ruled out by a comparison of $κ_{xx}$ for Tb-, Yb-, and Y-based rare earth pyrochlore titanates. These results represent the first report of non-vanishing $κ_{xy}$ measured in a dilution refrigerator ($T < 0.29\,$K).

cond-mat.str-el↗