arXiv subjects

N. Kurita

Publications and source records attributed to N. Kurita.

At least 19 recordsLinked to original sources

arXiv2026

High-field fate of the Kitaev quantum spin liquid in $α$-RuCl$_3$

Kitaev quantum spin liquids (KQSLs) host fractionalized excitations described by itinerant Majorana quasiparticles and gapped $Z_2$ fluxes (visons), providing a platform for emergent topological matter. Whether such a state survives under strong magnetic fields, however, remains an open question. The layered honeycomb magnet $α$-RuCl$_3$ is a leading candidate material: an in-plane field of $\sim$ 7 T suppresses antiferromagnetic order and induces a quantum-disordered phase exhibiting signatures consistent with Majorana excitations, including an anomalous thermal Hall effect and field-angle-dependent specific heat. At higher fields, the magnetization approaches saturation, suggesting a transition to a spin-polarized state, yet the microscopic evolution between these limits remains unresolved. Here we report high-field specific heat measurements up to 24 T that reveal a distinct crossover at $μ_0H^*\approx$15 T, beyond which the perturbative Kitaev description breaks down. Above $H^*$, the characteristic six-fold angular modulation of the specific heat collapses and the excitation gap deviates from the predicted $H^3$ scaling. Meanwhile, the gap decreases with increasing field and the in-plane magnetization anisotropy persists up to $\sim$ 24 T, both in sharp contrast to a trivial spin-polarized state, indicating that KQSL signatures are preserved even at $\sim$ 90 % of magnetization saturation. These results reveal that the KQSL in $α$-RuCl$_3$ extends well beyond the perturbative window, persisting as a nonperturbative regime in which the Majorana and vison energy scales merge, before eventually giving way to spin polarization. This thermodynamic roadmap provides a basis for understanding how fractionalized phases evolve under strong magnetic fields.

cond-mat.str-el↗

arXiv2026

DMRadio-Core: A new approach for GUT-scale axion searches

Searches for QCD axions with masses in the neV/$c^2$ mass range are strongly motivated by new physics at the GUT scale and by well-motivated pre-inflationary axion symmetry breaking scales. This parameter space is challenging to probe due to the small axion-photon couplings, which typically require large, high-field magnets with substantial stored energy. In this paper, we propose a new experimental geometry based on a narrow-bore, segmented solenoid that optimizes the collection of the axion-induced signal using LC resonators outside the high-field region of the magnet bore. This alternative optimization significantly reduces the required stored magnetic energy while preserving sensitivity, enabling a near-term experiment in the 30-200 MHz (120-830 neV/$c^2$) range, with a cost-effective, staged scaling to a GUT-scale experiment in the 100 kHz-30 MHz (0.4-120 neV/$c^2$) range.

hep-ex↗

arXiv2025

Pressure-Tuned Metamagnetism and Emergent Three-Body Interactions in CsFeCl$_3$

We present a combined experimental and theoretical study of the triangular-lattice quantum antiferromagnet CsFeCl$_3$ under high magnetic fields and high pressure. Pulsed-field magnetization for the magnetic field along the symmetric $c$ direction at ambient pressure reveals a magnetization process from a nonmagnetic singlet ground state with a nearly linear increase between 3.7 and 10.7 T, a plateau-like region, and then a sharp stepwise metamagnetic transition near 32 T. Wide frequency--field range electron spin resonance indicates that the low-field regime originates from the $J = 1$ manifold, while the high-field metamagnetic transition suggests a level crossing between the $J = 1$ and $J = 2$ lowest states. Pulsed-field magnetic susceptibilities measured with a proximity detector oscillator under high pressure show that the low-field nonmagnetic singlet phase is gradually suppressed, while the high-field metamagnetic transition evolves into an increasingly rich pattern of fractional steps. While the observations at low to intermediate fields can be understood within the established spin-1 description, the high-field regime requires a new perspective, which we provide through a projected spin-1/2 framework built from Zeeman-selected crystal-field states not related by time reversal. This construction naturally allows emergent three-body interactions on triangular plaquettes and explains the asymmetric evolution of the fractional steps in the magnetization. Our findings reveal that high-field effective spin models in quantum magnets with separated yet accessible crystal-field multiplets are not constrained to even-body couplings, but can naturally host odd-body terms, opening a broader avenue for realizing field-asymmetric magnetization processes and exotic phases beyond conventional even-body physics.

cond-mat.mtrl-sci↗

arXiv2025

Electromagnetic modeling and science reach of DMRadio-m$^3$

DMRadio-m$^3$ is an experiment that is designed to be sensitive to KSVZ and DFSZ QCD axion models in the 10--200\,MHz (41 neV$/c^2$ -- 0.83 $μ$eV/$c^2$) range. The experiment uses a solenoidal dc magnetic field to convert an axion dark-matter signal to an ac electromagnetic response in a coaxial copper pickup. The current induced by this axion signal is measured by dc SQUIDs. In this work, we present the electromagnetic modeling of the response of the experiment to an axion signal over the full frequency range of DMRadio-m$^3$, which extends from the low-frequency, lumped-element limit to a regime where the axion Compton wavelength is only a factor of two larger than the detector size. With these results, we determine the live time and sensitivity of the experiment. The primary science goal of sensitivity to DFSZ axions across 30--200 MHz can be achieved with a $3σ$ live scan time of 2.9 years.

hep-ex↗

arXiv2024

Defect-Induced Low-Energy Majorana Excitations in the Kitaev Magnet $α$-RuCl$_3$

The excitations in the Kitaev spin liquid (KSL) can be described by Majorana fermions, which have characteristic field dependence of bulk gap and topological edge modes. In the high-field state of layered honeycomb magnet $α$-RuCl$_3$, experimental results supporting these Majorana features have been reported recently. However, there are challenges due to sample dependence and the impact of inevitable disorder on the KSL is poorly understood. Here we study how low-energy excitations are modified by introducing point defects in $α$-RuCl$_3$ using electron irradiation, which induces site vacancies and exchange randomness. High-resolution measurements of the temperature dependence of specific heat $C(T)$ under in-plane fields $H$ reveal that while the field-dependent Majorana gap is almost intact, additional low-energy states with $C/T=A(H)T$ are induced by introduced defects. At low temperatures, we obtain the data collapse of $C/T\sim H^{-γ}(T/H)$ expected for a disordered quantum spin system, but with an anomalously large exponent $γ$. This leads us to find a power-law relationship between the coefficient $A(H)$ and the field-sensitive Majorana gap. These results are consistent with the picture that the disorder induces low-energy linear Majorana excitations, which may be considered as a weak localization effect of Majorana fermions in the KSL.

cond-mat.str-el↗

arXiv2023

Majorana-fermion origin of the planar thermal Hall effect in the Kitaev magnet $α$-RuCl$_3$

The field-induced quantum disordered state of layered honeycomb magnet $α$-RuCl$_3$ is a prime candidate for Kitaev spin liquids hosting Majorana fermions and non-Abelian anyons. Recent observations of anomalous planar thermal Hall effect demonstrate a topological edge mode, but whether it originates from Majorana fermions or bosonic magnons remains controversial. Here we distinguish these origins from low-temperature measurements of high-resolution specific heat and thermal Hall conductivity with rotating in-plane fields. In the honeycomb bond direction, a distinct closure of the low-energy bulk gap is observed concomitantly with the sign reversal of the Hall effect. General discussions of topological bands show that this is the hallmark of an angle-rotation-induced topological transition of fermions, providing conclusive evidence for the Majorana-fermion origin of the thermal Hall effect in $α$-RuCl$_3$.

cond-mat.str-el↗

arXiv2022

Quantized and unquantized thermal Hall conductance of Kitaev spin-liquid candidate $α$-RuCl$_3$

Despite extensive investigations, a topological state that hosts Majorana edge modes in the magnetic field-induced quantum disordered state of the Kitaev candidate material $α$-RuCl$_3$ has been hotly debated. To gain more insight into this issue, we measured the thermal Hall conductivity $κ_{xy}$ of various samples grown by the Bridgman method. The results show that the half-integer quantum thermal Hall effect is intimately related to the magnitude of longitudinal thermal conductivity and the Néel temperature at zero field, both of which are sample dependent. Samples exhibiting the half-integer quantum thermal Hall effect have larger zero-field thermal conductivity values than a threshold value, implying that a long mean free path of heat carriers is an important prerequisite. In addition, we find that samples with a higher Néel temperature exhibit a higher magnetic field at which quantization starts to occur. These results indicate that the quantization phenomenon is significantly affected by the impurity scatterings and the non-Kitaev interactions.

cond-mat.str-el↗

arXiv2022

Origin of oscillatory structures in the magnetothermal conductivity of the putative Kitaev magnet $α$-RuCl$_3$

The layered honeycomb magnet $α$-RuCl$_3$ has been suggested to exhibit a field-induced quantum spin liquid state, in which the reported large thermal Hall effect close to the half-quantized value still remains a subject of debate. Recently, oscillatory structures of the magnetothermal conductivity were reported and interpreted as quantum oscillations of charge-neutral particles. To investigate the origin of these oscillatory structures, we performed a comprehensive measurement of the in-plane magnetothermal conductivity $κ(H)$ down to low temperature (100 mK), as well as magnetization $M$, for single crystals grown by two different techniques: Bridgman and chemical vapor transport. The results show a series of dips in $κ(H)$ and peaks in the field derivative of $M$ located at the same fields independent of the growth method. We argue that these structures originate from field-induced phase transitions rather than quantum oscillations. The positions of several of these features are temperature-dependent and connected to the magnetic phase transitions in zero field: the main transition at 7 K and weaker additional transitions which likely arise from secondary phases at 10 K and 13 K. In contrast to what is expected for quantum oscillations, the magnitude of the structure in $κ(H)$ is smaller for the higher conductivity crystal and decreases rapidly upon cooling below 1 K.

cond-mat.str-el↗

arXiv2022

Evidence for the first-order topological phase transition in a Kitaev spin liquid candidate $α$-RuCl$ _3$

The Kitaev quantum spin liquid (QSL) on the two-dimensional honeycomb lattice epitomizes an entangled topological state, where the spins fractionalize into Majorana fermions. This state has aroused tremendous interest because it harbors non-Abelian anyon excitations. The half-integer quantized thermal Hall (HIQTH) conductance observed in $α$-RuCl$_3$ is a key signature of these excitations. However, the fate of this topologically nontrivial state at intense fields remains largely elusive. Here, we report the thermal conductivity $κ$ and specific heat $C$ of $α$-RuCl$_3$ with in-plane magnetic fields $H$. For the field direction perpendicular to the Ru-Ru bond, where the HIQTH effect is observed, we find a discontinuous jump in $κ(H)$ and a peak anomaly in $C(H)$ at $μ_0H^*\approx11$\,T, evidencing a weak first-order phase transition. Remarkably, the HIQTH effect vanishes close to $H^*$. Furthermore, we find that the spin-fractionalization feature is retained well above $H^\ast$. These imply the emergence of the phase transition that separates two QSL phases with distinct topological properties.

cond-mat.str-el↗

arXiv2021

Thermodynamic evidence for field-angle dependent Majorana gap in a Kitaev spin liquid

The exactly-solvable Kitaev model of two-dimensional honeycomb magnet leads to a quantum spin liquid (QSL) characterized by Majorana fermions, relevant for fault-tolerant topological quantum computations. In the high-field paramagnetic state of $α$-RuCl3, half-integer quantization of thermal Hall conductivity has been reported as a signature of edge current, but the bulk nature of this state remains elusive. Here, from high-resolution heat capacity measurements under in-plane field rotation, we find strongly angle-dependent low-energy excitations in bulk $α$-RuCl3. The excitation gap has a sextuple node structure, and the gap amplitude increases with field, exactly as expected for itinerant Majorana fermions in the Kitaev model. Our thermodynamic results are fully linked with the transport quantization properties, providing the first demonstration of the bulk-edge correspondence in a Kitaev QSL. Moreover, at higher fields where the quantum thermal Hall effect vanishes, we find the possible emergence of a novel nematic QSL state with two-fold rotational symmetry.

cond-mat.str-el↗

arXiv2021

Robustness of the thermal Hall effect close to half-quantization in a field-induced spin liquid state

Thermal signatures of fractionalized excitations are a fingerprint of quantum spin liquids (QSLs). In the $J_{eff}=1/2$ honeycomb magnet $α$-RuCl$_3$, a QSL state emerges upon applying an in-plane magnetic field $H_{||}$ greater than the critical field $H_{C2} \approx$ 7 T along the a-axis, where the thermal Hall conductivity ($k_{XY}/T$) was reported to take on the half-quantized value $k_{HQ}/T$. This finding was discussed as a signature of an emergent Majorana edge mode predicted for the Kitaev QSL. The $H_{||}$- and $T$-range of the half-quantized signal and its relevance to a Majorana edge mode are, however, still under debate. Here we present a comprehensive study of $k_{XY}/T$ in $α$-RuCl$_3$ with $H_{||}$ up to 13 T and $T$ down to 250 mK, which reveals the presence of an extended region of the phase diagram with $k_{XY}/T \approx k_{HQ}/T$ above $H_{C2}$, in particular across a plateau-like plane for $H_{||}$ > 10 T and $T$ < 6.5 K. From 7 T up to $\sim$10 T, $k_{XY}/T$ is suppressed to zero upon cooling to lowest temperature without any plateau-like behavior and exhibits correlations with complex anomalies in the longitudinal thermal conductivity ($k_{XX}$) and magnetization around 10 T. The results are in support of a topological state with a half-quantized $k_{XY}/T$ and suggest an interplay with crossovers or weak phase transitions beyond $H_{C2}$ in RuCl$_3$.

cond-mat.str-el↗

arXiv2020

Sample dependence of the half-integer quantized thermal Hall effect in the Kitaev candidate $α$-RuCl$_3$

We have investigated the sample dependence of the half-integer thermal Hall effect in $α$-RuCl$_3$ under a magnetic field tilted 45 degree from the $c$ axis to the $a$ axis. We find that the sample with the largest longitudinal thermal conductivity ($κ_{xx}$) shows the half-integer quantized thermal Hall effect expected in the Kitaev model. On the other hand, the quantized thermal Hall effect was not observed in the samples with smaller $κ_{xx}$. We suggest that suppressing the magnetic scattering effects on the phonon thermal conduction, which broaden the field-induced gap protecting the chiral edge current of the Majorana fermions, is important to observe the quantized thermal Hall effect.

cond-mat.str-el↗

arXiv2020

Half-integer quantized anomalous thermal Hall effect in the Kitaev material $α$-RuCl$_3$

Heat transport mediated by Majorana edge modes in a magnetic insulator leads to a half-integer thermal quantum Hall conductance, which has recently been reported for the two-dimensional honeycomb material $α$-RuCl$_3$. While the conventional electronic Hall effect requires a perpendicular magnetic field, we find that this is not the case in $α$-RuCl$_3$. Strikingly, the thermal Hall plateau appears even for a magnetic field with no out-of-plane components. The field-angular variation of the quantized thermal Hall conductance has the same sign structure of the topological Chern number, which is either $\pm$1, as the Majorana band structure of the pure Kitaev spin liquid. This observation of a half-integer anomalous thermal Hall effect firmly establishes that the Kitaev interaction is primarily responsible and that the non-Abelian topological order associated with fractionalization of the local magnetic moments persists even in the presence of non-Kitaev interactions in $α$-RuCl$_3$.

cond-mat.str-el↗

arXiv2018

Majorana quantization and half-integer thermal quantum Hall effect in a Kitaev spin liquid

The quantum Hall effect (QHE) in two-dimensional (2D) electron gases, which is one of the most striking phenomena in condensed matter physics, involves the topologically protected dissipationless charge current flow along the edges of the sample. Integer or fractional electrical conductance are measured in units of $e^2/2π\hbar$, which is associated with edge currents of electrons or quasiparticles with fractional charges, respectively. Here we discover a novel type of quantization of the Hall effect in an insulating 2D quantum magnet. In $α$-RuCl$_3$ with dominant Kitaev interaction on 2D honeycomb lattice, the application of a parallel magnetic field destroys the long-range magnetic order, leading to a field-induced quantum spin liquid (QSL) ground state with massive entanglement of local spins. In the low-temperature regime of the QSL state, we report that the 2D thermal Hall conductance $κ_{xy}^{2D}$ reaches a quantum plateau as a function of applied magnetic field. $κ_{xy}^{2D}/T$ attains a quantization value of $(π/12)(k_B^2/\hbar)$, which is exactly half of $κ_{xy}^{2D}/T$ in the integer QHE. This half-integer thermal Hall conductance observed in a bulk material is a direct signature of topologically protected chiral edge currents of charge neutral Majorana fermions, particles that are their own antiparticles, which possess half degrees of freedom of conventional fermions. These signatures demonstrate the fractionalization of spins into itinerant Majorana fermions and $Z_2$ fluxes predicted in a Kitaev QSL. Above a critical magnetic field, the quantization disappears and $κ_{xy}^{2D}/T$ goes to zero rapidly, indicating a topological quantum phase transition between the states with and without chiral Majorana edge modes. Emergent Majorana fermions in a quantum magnet are expected to have a major impact on strongly correlated topological quantum matter.

cond-mat.str-el↗

arXiv2018

Unusual thermal Hall effect in a Kitaev spin liquid candidate $α$-RuCl$_3$

The Kitaev quantum spin liquid displays the fractionalization of quantum spins into Majorana fermions. The emergent Majorana edge current is predicted to manifest itself in the form of a finite thermal Hall effect, a feature commonly discussed in topological superconductors. Here we report on thermal Hall conductivity $κ_{xy}$ measurements in $α$-RuCl$_3$, a candidate Kitaev magnet with the two-dimensional honeycomb lattice. In a spin-liquid (Kitaev paramagnetic) state below the temperature characterized by the Kitaev interaction $J_K/k_B \sim 80$ K, positive $κ_{xy}$ develops gradually upon cooling, demonstrating the presence of highly unusual itinerant excitations. Although the zero-temperature property is masked by the magnetic ordering at $T_N=7$ K, the sign, magnitude, and $T$-dependence of $κ_{xy}/T$ at intermediate temperatures follows the predicted trend of the itinerant Majorana excitations.

cond-mat.str-el↗

arXiv2017

High-field magnetization and magnetic phase diagram of $α$-Cu$_{2}$V$_{2}$O$_{7}$

High-field magnetization of the spin-$1/2$ antiferromagnet $α$-Cu$_2$V$_2$O$_7$ was measured in pulsed magnetic fields of up to 56 T in order to study its magnetic phase diagram. When the field was applied along the easy axis (the $a$-axis), two distinct transitions were observed at $H_{c1}=6.5$~T and $H_{c2}=18.0$~T. The former is a spin-flop transition typical for a collinear antiferromagnet and the latter is believed to be a spin-flip transition of canted moments. The canted moments, which are induced by the Dzyaloshinskii-Moriya interactions, anti-align for $H_{c1}<H<H_{c2}$ due to the anisotropic exchange interaction that favors the antiferromagnetic arrangement along the $a$-axis. Above $H_{c2}$, the Zeeman energy of the applied field overcomes the antiferromagnetic anisotropic interaction and the canted moments are aligned along the field direction. Density functional theory was employed to compute the exchange interactions, which were used as inputs for quantum Monte Carlo calculations and then further refined by fitting to the magnetic susceptibility data. Contrary to our previous report in Phys. Rev. B {\bf 92}, 024423, the dominant exchange interaction is between the third nearest-neighbor spins, which form zigzag spin-chains that are coupled with one another through an intertwining network of the nonnegligible nearest and second nearest-neighbor interactions. In addition, elastic neutron scattering under the applied magnetic fields of up to 10 T reveals the incommensurate helical spin structure in the spin-flop state.

cond-mat.str-el↗

arXiv2011

Crystal fields, disorder, and antiferromagnetic short-range order in Yb0.24Sn0.76Ru

We report extensive measurements on a new compound (Yb0.24Sn0.76)Ru that crystallizes in the cubic CsCl structure. Valence band photoemission and L3 x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb3+ is split by the crystalline electric field (CEF) into a Γ7 doublet ground state and a Γ8 quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a Γ6 excited state resides at 32 meV; however, the Γ8/Γ6 transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch- Grüneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K, there is a peak in specific heat that is too broad to represent a magnetic phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand, this peak also is too narrow to represent the Kondo effect in the Γ7 ground state doublet. On the basis of the field-dependence of the specific heat, we argue that antiferromagnetic shortrange order (possibly co-existing with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.

cond-mat.str-el↗

arXiv2011

Fully gapped superconductivity in Ni-pnictide superconductors BaNi2As2 and SrNi2P2

We have performed low-temperature specific heat $C$ and thermal conductivity $κ$ measurements on the Ni-pnictide superconductors BaNi$_2$As$_2$ ($T_\mathrm{c}$=0.7 K and SrNi$_2$P$_2$ ($T_\mathrm{c}$=1.4 K). The temperature dependences $C(T)$ and $κ(T)$ of the two compounds are similar to the results of a number of s-wave superconductors. Furthermore, the concave field responses of the residual $κ$ for BaNi$_2$As$_2$ rules out the presence of nodes on the Fermi surfaces. We postulate that fully gapped superconductivity could be universal for Ni-pnictide superconductors. Specific heat data on Ba$_{0.6}$La$_{0.4}$Ni$_2$As$_2$ shows a mild suppression of $T_\mathrm{c}$ and $H_\mathrm{c2}$ relative to BaNi$_2$As$_2$.

cond-mat.supr-con↗