arXiv subjects

Kumpei Imamura

Publications and source records attributed to Kumpei Imamura.

4 recordsLinked to original sources

arXiv2026

A continuous confinement-deconfinement transition in a triangular quantum magnet

A continuous transition between phases hosting distinct excitations---bosonic magnons versus fermionic spinons---is a long-sought phenomenon in quantum magnetism, analogous to the confinement--deconfinement transition in quantum chromodynamics. We report evidence for such a transition in the triangular-lattice antiferromagnet TlYbS$_2$. Antiferromagnetic order develops below $T_\mathrm{N} \approx 0.53\,\mathrm{K}$. A $c$-axis field suppresses this order, driving the system into a gapless quantum spin liquid with a spinon Fermi surface above $\mu_0 H_\mathrm{c} \approx 3\,\mathrm{T}$, evidenced by a finite residual linear term in thermal conductivity, a Pauli-like susceptibility, and a temperature-independent NMR Knight shift. Approaching $H_\mathrm{c}$ from above, the scattering rate of itinerant excitations is strongly enhanced while their density of states shows no critical enhancement, atypical of conventional magnetic quantum criticality. These results point to a continuous confinement--deconfinement transition governed by fractionalized excitations beyond the Ginzburg--Landau paradigm.

cond-mat.str-el↗

arXiv2026

Revealing nonvolatile behaviors in magneto-thermal switching using microstructure-controlled superconducting composites

Thermal conductivity in a conductor changes by the application of an external magnetic field, which functions as a magneto-thermal switch. For superconductors, a large magneto-thermal switching can occur through a superconducting-to-normal conducting phase transition due to the change in the electron contribution in thermal conductivity. Arima et al. recently reported a nonvolatile nature of the magneto-thermal switching for superconducting solders, which consist of phase-separated Sn and Pb domains. Although they clarified that magnetic flux trapping is required to induce the nonvolatile magneto-thermal switching, a rule for such material design is still unclear. Here, we investigate the microstructure dependence of magneto-thermal switching in superconducting Sn/Pb multilayered composites, which are created by an accumulative roll bonding method. The thickness of each layer, that is the scale of microstructure, can be systematically controlled by the repetition number of roll bonding while the whole sample size and average composition are unchanged. We find that, as the formation of micro-scaled Sn domains proceeds by increasing the repetition number, a nonvolatility in the magneto-thermal conductivity gradually appears in correlation with the remanent magnetization. This study directly confirms that the inclusions with a size comparable to or less than the magnetic vortex in superconducting matrix is essential for magnetic flux trapping, enabling the nonvolatile magneto-thermal switching in superconducting composites.

cond-mat.supr-con↗

arXiv2025

Bulk excitations in ultraclean $α$-RuCl$_3$: Quantitative evidence for Majorana dispersions in a Kitaev quantum spin liquid

The spin-orbit coupled Mott insulator $α$-RuCl$_3$ has emerged as a prime candidate for realizing the Kitaev quantum spin liquid (KQSL), characterized by Majorana quasiparticles, whose edge states exhibit a distinctive half-integer quantized thermal Hall conductivity. However, its van der Waals nature makes its thermal Hall response highly sensitive to structural disorder, leading to sample-dependent variations. Here, we investigate low-energy bulk excitations in the field-induced quantum disordered (FIQD) state of newly available ultraclean single crystals of $α$-RuCl$_3$. High-resolution specific heat measurements under in-plane magnetic field rotation reveal an anisotropic excitation gap, whose field dependence is consistent with the Majorana gap in the KQSL state. Remarkably, when the field aligns with Ru-Ru bond directions, we observe gapless excitations with Dirac-like dispersions that quantitatively match theoretical predictions of Majorana bands based on the reported Kitaev interactions. Our findings in these ultraclean crystals provide strong evidence that the FIQD state of $α$-RuCl$_3$ is a robust KQSL, resilient against small disorder perturbations.

cond-mat.str-el↗

arXiv2025

Field-Angle-Resolved Specific Heat in Na$_2$Co$_2$TeO$_6$: Evidence against Kitaev Quantum Spin Liquid

Kitaev quantum spin liquids (KSLs) in layered honeycomb magnets are known to host Majorana quasiparticles, whose excitations depend strongly on the direction of the applied magnetic field. In the high-field phase of $α$-RuCl$_3$, specific heat measurements have revealed characteristic field-angle dependence of low-energy excitations consistent with the Kitaev model, providing bulk evidence for the KSL state. Here we present low-temperature measurements of specific heat $C(T)$ for another KSL candidate $\textrm{Na}_2\textrm{Co}_2\textrm{Te}\textrm{O}_6$ (NCTO) under field rotation within the honeycomb plane. Above the critical field of antiferromagnetic order, the field-angle dependence of $C/T$ exhibits minima along the bond directions, contrasting with the maxima observed in the KSL state of $α$-RuCl$_3$. Our analysis indicates nodeless, fully-gapped excitations, which are inconsistent with the angle-dependent Majorana excitations with gapless nodes predicted by the Kitaev model. These findings suggest that low-energy excitations in NCTO are governed by gapped magnon excitations rather than Majorana quasiparticles, providing thermodynamic evidence against a KSL state.

cond-mat.str-el↗