arXiv subjects

Yasumasa Takano

Publications and source records attributed to Yasumasa Takano.

5 recordsLinked to original sources

arXiv2024

Coexistence of symmetry-protected topological order and Neel order in the spin-1/2 ladder antiferromagnet C9H18N2CuBr4

Topological phases of matter are beyond the paradigm of Landau's symmetry breaking and have challenged our understanding of condensed matter systems. Here we report a new type of symmetry-protected topological phase of matter in the spin-1/2 coupled two-leg ladder antiferromagnet C9H18N2CuBr4, DLCB for short. In this two-sublattice antiferromagnet with a weak easy-axis anisotropy, we find no evidence of a conventional spin-flop transition in the magnetization with the magnetic field applied parallel to the easy axis at T=0.4 K, well below TN=2.0 K. Moreover, the temperature dependence of the gapped transverse excitations across TN indicates that they are not the conventional S=1 magnons associated with explicit symmetry breaking. Instead, the thermal renormalization of the gap energy shows a remarkable agreement with a calculation for the three-dimensional O(3) nonlinear sigma model. Accordingly, the spin gap in DLCB is not due to the spin anisotropy but to the separation between a spin singlet state and a triplet excited state. Since an antiferromagnetic spin-1/2 ladder systems can be mapped onto the spin-1 chain, the notion of the Haldane gap is proposed to explain the opening of the spin gap in DLCB. Therefore, the ground state of DLCB is best described as a quantum superposition of a Haldane phase and a Neel-ordered phase, which resembles the quantum state of a qubit in quantum computing. Our results indicate the presence of a symmetry-protected topological order coexisting with an antiferromagnetic order in this material.

cond-mat.str-el↗

arXiv2023

Formation of random singlets in the nanocrystalline quasi-one-dimensional spin-1/2 antiferromagnet Sr21Bi8Cu2(CO3)2O41

Induced by quenched disorder, random-singlet states occur in a variety of low-dimensional spin-1/2 antiferromagnets, some of them candidates for quantum spin liquids. Here we report measurements of the specific heat, magnetization, and magnetic susceptibility of nanocrystalline Sr21Bi8Cu2(CO3)2O41, a quasi-one-dimensional spin-1/2 antiferromagnet with alternating bonds. The results uncover the predominant presence of random-singlet spin pairs in this material, with a logarithmic probability distribution, P(J), of the renormalized, emergent exchange interaction, J, in zero magnetic field and P(J) proportional to 1/J in magnetic fields. We postulate that these unexpected J dependences, in contrast to the usual P(J) \propto 1/$J^γ$ with 0 < $γ$ < 1, and possibly also the dichotomy, arise from the finite size of the nanocrystals. Scaling functions for the specific heat and magnetization reproduce our magnetocaloric-effect data, with no adjustable parameters.

cond-mat.str-el↗

arXiv2021

Fermi surfaces of the topological semimetal CaSn$_{3}$ probed through de Haas van Alphen oscillations

In the search of topological superconductors, nailing down the Fermiology of the normal state is as crucial a prerequisite as unraveling the superconducting pairing symmetry. In particular, the number of time-reversal-invariant momenta in the Brillouin zone enclosed by Fermi surfaces is closely linked to the topological class of time-reversal-invariant systems, and can experimentally be investigated. We report here a detailed study of de Haas van Alphen quantum oscillations in single crystals of the topological semimetal CaSn$_{3}$ with torque magnetometry in high magnetic fields up to 35 T. In conjunction with density functional theory based calculations, the observed quantum oscillations frequencies indicate that the Fermi surfaces of CaSn$_{3}$ enclose an odd number of time-reversal-invariant momenta, satisfying one of the proposed criteria to realize topological superconductivity. Nonzero Berry phases extracted from the magnetic oscillations also support the nontrivial topological nature of CaSn$_{3}$.

cond-mat.supr-con↗

arXiv2021

Unusual upper critical fields of the topological nodal-line semimetal candidate Sn$_{x}$NbSe$_{2-δ}$

We report superconductivity in Sn$_{x}$NbSe$_{2-δ}$, a topological nodal-line semimetal candidate with a noncentrosymmetric crystal structure. The superconducting transition temperature $T_{c}$ of this compound is extremely sensitive to Sn concentration $x$ and Se deficiency $δ$, 5.0 K for Sn$_{0.13}$NbSe$_{1.70}$ and 8.6 K for Sn$_{0.14}$NbSe$_{1.71}$ and Sn$_{0.15}$NbSe$_{1.69}$. In all samples, the temperature dependence of the upper critical field $H_{c2}(T)$ differs from the prediction of the Werthamer-Helfand-Hohenberg theory. While the zero-temperature value of the in-plane upper critical field of Sn$_{x}$NbSe$_{2-δ}$ with the higher $T_{c}$ is lower than the Pauli paramagnetic limit $H_{P}$, that of the lower $T_{c}$ sample exceeds $H_{P}$ by a factor of $\sim$2. Our observations suggest that odd-parity contribution dominates the superconducting gap function of Sn$_{x}$NbSe$_{2-δ}$, and it can be fine-tuned by the Sn concentration and Se deficiency.

cond-mat.supr-con↗

arXiv2021

Thermal and magnetoelastic properties of α-RuCl3 in the field-induced low temperature states

We discuss the implications that new magnetocaloric, thermal expansion and magnetostriction data in $α$-RuCl$_{3}$ single crystals have on its temperature-field phase diagram and uncover the magnetic-field dependence of an apparent energy gap structure $Δ(H)$ that evolves when the low temperature antiferromagnetic order is suppressed. We show that, depending on how the thermal expansion data is modeled, $Δ(H)$ can show a cubic field dependence and remain finite at zero field, consistent with the pure Kitaev model hosting itinerant Majorana fermions and localized $\mathbb{Z}_{2}$ fluxes. Our magnetocaloric effect data provides, below $1\,\mathrm{K}$, unambiguous evidence for dissipative phenomena at $H_{\mathrm{c}}$, smoking gun for a first order phase transition. Our results, on the other hand, show little support for a phase transition from a QSL to a polarized paramagnetic state above $H_{\mathrm{c}}$.

cond-mat.str-el↗