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Koichi Kindo

Publications and source records attributed to Koichi Kindo.

At least 19 recordsLinked to original sources

Extended strange metal regime in a pure nematic quantum critical superconductor

High-temperature superconductivity often emerges from a strange metallic state, where the electrical resistivity exhibits a linear-in-temperature dependence over an anomalously extended temperature range. The prevailing belief is that magnetic critical fluctuations gives rise to strange metallicity, enhancing the superconducting transition temperature. Here, using high pulsed magnetic fields, we have uncovered the strange metallic ground state hidden below the superconducting dome of nonmagnetic FeSe$_{1-x}$Te$_{x}$, which harbors a quantum critical point (QCP) of pure electronic nematicity, characterized by spontaneous rotational symmetry breaking. Unlike the conventional quantum criticality, this strange metallic state does not appear in a fan-shaped region above the singular QCP but spans a wide compositional range, where pairing interactions are strengthened by nonmagnetic nematic critical fluctuations. This stands in sharp contrast to the much cleaner system FeSe$_{1-x}$S$_{x}$, which displays a quantum critical fan above nematic QCP, indicating that disorder-induced spatial randomness of the nematic fluctuations likely enlarges the QCP of FeSe$_{1-x}$Te$_{x}$ into an extended region of criticality, as suggested by relevant hybrid Quantum Monte Carlo simulations. These observations highlight superconductivity promoted by a unique interplay between pure nematic critical fluctuations, strange metallicity and disorder, providing new insight into the emergence of non-Fermi-liquid transport in various correlated materials.

cond-mat.supr-con

Bridging ambient- and high-pressure superconductivity in La$_2$LnNi$_2$O$_7$ films

The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity near 80 K in bilayer nickelates under high pressure has sparked extensive studies. While superconductivity exceeding 40 K was subsequently discovered at ambient pressure in compressively strained films, the relationship between ambient- and high-pressure regimes remains an open question. Here we present a systematic investigation of superconductivity in compressively strained La$_2$LnNi$_2$O$_7$ films (Ln = lanthanides) at ambient and high pressures. The normal-state resistivity at ambient pressure, revealed by suppressing superconductivity with magnetic fields of 59 T, tends toward $T^2$ behaviour. Under high pressure in a cubic-anvil cell, $T_{\mathrm{c}}$ was enhanced from 41-42 K at ambient pressure to 67-73 K at 16 GPa. On the other hand, lattice compression induced by Ln substitution, which may mimic effects of pressure, lowers $T_{\mathrm{c}}$. In both cases, $T_{\mathrm{c}}$ correlates with the evolution of normal-state transport between $T^2$ and $T$-linear behaviour, offering insight into the interplay between lattice structure and superconductivity in bilayer nickelates.

cond-mat.supr-con

Discriminating superconducting fluctuations from the pseudogap in Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4+\delta} (n = 2,3)$: A magnetotransport study

Understanding the normal state is essential for uncovering the mechanism of high-$T_c$ superconductivity. We investigate magnetotransport in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ single crystals over a wide doping range. While the in-plane resistivity and Hall coefficient show strong pseudogap-induced temperature dependence, the $T^2$ Hall-angle behavior and the modified Kohler's rule remain robust across all dopings. The onset temperatures of the pseudogap are clearly distinct from superconducting fluctuations, although they scale with the pseudogap magnitudes with a factor consistent with a $d$-wave superconductor. These results demonstrate that the pseudogap does not arise from superconducting fluctuations and instead suggest that it may originate from preformed Cooper pairing in the BCS-BEC crossover regime.

cond-mat.supr-con

Weak Ferromagnetism in NiS$_2$ under Nanocrystallization

Structurally well-ordered NiS$_2$ nanocrystals with an average diameter of $27.0 \pm 6.5$ nm retain the bulk-like two-step antiferromagnetic transitions, as shown by magnetization and heat-capacity measurements. Below the lower transition, the nanocrystals exhibit a hysteretic ferromagnetic response with large coercivity, exchange bias, and a vertical loop shift after field cooling, whereas the $M$-$H$ response just above the transition is nearly linear. These features are best explained by uncompensated surface moments generated where the low-temperature antiferromagnetic order terminates at the nanocrystal surface. The absence of a clear additional bulk-like weak-ferromagnetic component constrains homogeneous-canting models and indirectly favors a domain-wall scenario for the weak ferromagnetism of bulk NiS$_2$.

cond-mat.str-el

Tetrahedrally ferromagnetic correlations and a glassy-freezing anomaly in the breathing pyrochlore magnet $\mathrm{AgInCr_4S_8}$ with partial $A$-site disorder

We investigate the chromium breathing pyrochlore sulfide $\mathrm{AgInCr_4S_8}$, a chromium-based thiospinel, by synchrotron x-ray and neutron powder diffraction, dc magnetization, and heat capacity. Diffraction confirms the $F\bar{4}3m$ breathing structure with alternating large and small $\mathrm{Cr_4}$ tetrahedra, a large breathing ratio ($d^\prime/d = 1.106$ at 300 K), and substantial Ag/In intermixing on the $A$ sublattice ($\sim 16\%$). No structural transition or magnetic Bragg peaks are detected down to 1.5 K. An enlarged low-angle difference plot between the 1.5 and 20 K neutron diffraction patterns shows a weak broad diffuse-like enhancement, consistent with short-range or frozen correlated moments within the sensitivity of the present data. Susceptibility yields a positive Weiss temperature $\theta_{\mathrm{W}} = +92$ K and a moment enhancement in 30--60 K, while the magnetic entropy released by $\sim 30$ K approaches a scale of order $R\ln 13$, together consistent with the development of short-range tetrahedral ferromagnetic correlations and an effective $S = 6$ cluster-moment picture. A broad susceptibility cusp with ZFC--FC bifurcation and a low-temperature specific heat anomaly near 9 K indicate a phenomenological glassy-freezing anomaly without long-range order. $\mathrm{AgInCr_4S_8}$ provides a benchmark for the interplay of strong breathing distortion and quenched $A$-site disorder in chromium breathing pyrochlores.

cond-mat.str-el

Magnetoelastic mapping of the high-field phase diagram in the topological cubic helimagnet SrFeO$_{3}$

The cubic perovskite SrFeO$_{3}$ is a prototypical centrosymmetric itinerant magnet that hosts a quadruple-${\mathbf Q}$ hedgehog-antihedgehog lattice and exhibits a complex magnetic-field-temperature phase diagram. Yet, the microscopic mechanism underlying the emergence of its versatile multiple-${\mathbf Q}$ phases remains unresolved. Here, we reveal the field-orientation dependence of the magnetic phase diagram and establish an effective spin Hamiltonian for SrFeO$_{3}$ that incorporates a cubic single-ion anisotropy together with bilinear and biquadratic interactions in momentum space, which originate from the spin-charge coupling. In addition, we observe magnetoelastic signatures of a redistribution of the ligand-hole density upon entering the forced ferromagnetic phase. These findings emphasize the pivotal importance of electronic itinerancy arising from the formation of a ligand-hole band in stabilizing multiple-${\mathbf Q}$ phases.

cond-mat.str-el

$5/9-$Magnetization Plateau and Spin Supersolidity in YCu$_3$(OD)$_{7-x}$Br$_{2+x}$ under Magnetic Fields up to 120~T

We performed high-precision magnetization measurements up to 120~T on three compositions of the newly discovered kagome antiferromagnet YCu$_3$(OD)$_{7-x}$Br$_{2+x}$ (YCOB), revealing a previously unobserved 5/9 fractional magnetization plateau. All YCOB samples with different Br$^-$ concentrations exhibit nearly identical magnetization curves below 60~T, whereas the 5/9 plateau appears at markedly different fields in the ultrahigh-field regime. By modeling the experimental data using tensor-network calculations, we derive the effective spin Hamiltonians for the YCOB family with three spatially anisotropic Heisenberg couplings (the 3$J$-type model), which quantitatively reproduces the measured magnetization processes and captures the composition-dependent evolution of the 5/9 plateau. Furthermore, our theoretical analysis suggests the emergence of a spin supersolid phase in the field window between the 1/3 and 5/9 plateaus, which is sensitive to spin exchange parameters and accounts for the significant variation in the critical fields of the 5/9 plateau observed among different YCOB compositions.

cond-mat.str-el

Magnetic-field switching of exciton-magnon coupling in LiNiPO$_4$

Exciton-magnon transitions provide a fundamental optical fingerprint of coupled excitonic and magnetic excitations in antiferromagnets. However, controlling such coupled excitations by external fields remains a key challenge. Here we report the temperature and magnetic-field evolution of exciton-magnon coupling in the magnetoelectric antiferromagnet LiNiPO$_4$ using pulsed magnetic fields up to 50 T. The magnon sideband intensity exhibits sharp switching across field-induced magnetic phases, with strong suppression in plateau phases and enhancement in canted spin states. This behavior is attributed to the interplay between the thermal magnon population and the spin-dependent optical transition matrix element. These results demonstrate that magnetic-field control of spin degrees of freedom enables selective switching of exciton-magnon coupling in antiferromagnets.

cond-mat.str-el

Unified Description of Spin-Lattice Coupling and Thermodynamics in the Pyrochlore Heisenberg Antiferromagnet

We study an extended model to describe the spin-lattice coupling, incorporating individual vibrations of bonds and atomic sites alongside distance-dependent exchange interactions. The proposed spin Hamiltonian can be effectively considered as an interpolation between two well-established minimum models, the bond-phonon model and the site-phonon model. The extended model, which treats bond phonons and site phonons on comparable footing, well reproduces successive field-induced phase transitions as well as the thermodynamic properties of a three-up-one-down state in the pyrochlore-lattice Heisenberg antiferromagnet, including negative thermal expansion, an enhanced magnetocaloric effect, and a sharp specific-heat peak. The present approach is broadly applicable to various spin models, providing a framework for identifying the primary phonon modes responsible for spin-lattice coupling and for understanding complex magnetic phase diagrams.

cond-mat.str-el

Dominant Kitaev Interaction and Field-induced Quantum Disordered Phase in the Cobaltate Na$_2$Co$_2$TeO$_6$

The identification of quantum spin liquid phases in Kitaev candidate materials remains a major experimental challenge. Since most Kitaev candidates develop antiferromagnetic (AFM) order at low temperatures, currently there are great interest on the field-induced magnetic disordered phase in these compounds, that are distinct from (partially) polarized states. Recently, a cobaltate Na$_2$Co$_2$TeO$_6$ has emerged as a promising Kitaev candidate with high-spin $t^{5}_{2g}e^2_g$ configuration and spin-orbit entangled $J_{\rm eff} = 1/2$ honeycomb lattice system. There are intensive studies on field-induced magnetic states and phase transitions under in-plane magnetic fields. In this study, we propose an intermediate disordered phase induced by an out-of-plane field along the $c$-axis, through high-field magnetization and magnetocaloric effect measurements. To explain the high-field behavior of Na$_2$Co$_2$TeO$_6$, we develop an effective $K$-$J$-$Γ$-$Γ^{\prime}$ spin model featuring a dominant AFM Kitaev interaction. This framework uncovers an intermediate quantum spin liquid phase, establishing the material as a unique platform for exploring Kitaev physics and field-induced quantum-disordered states.

cond-mat.str-el

Fate of Pomeranchuk effect in ultrahigh magnetic fields

The Pomeranchuk effect is a counterintuitive phenomenon where liquid helium-3 (3He) solidifies under specific pressures, not when cooled, but when heated. This behaviour originates from the magnetic entropy of nuclear spins, suggesting a magnetic field should influence it. However, its detailed response to magnetic fields remains elusive due to the small nuclear magneton of 3He and lack of analogous fermion systems. Here, we show that an electron system also exhibit the Pomeranchuk effect, where the Fermi liquid state solidifies in a high magnetic field, unlike conventional electron systems where a field melts an electron solid into a metal. Remarkably, the electron system displays a reentrant liquid state in ultrahigh fields. These responses are explained by changes in magnetic entropy and magnetisation, extending the underlying physics to 3He. Our findings clarify magnetic-field impact on the Pomeranchuk effect and open avenues for magnetic control of chemical interactions.

cond-mat.str-el

Non-linear transport in field-induced insulating states of graphite

Graphite exhibits multi-stage phase transitions in the quantum-limit states realized by magnetic fields applied along the c-axis. Despite extensive studies on this phenomenon, the origin remains a matter of debate to this day. We performed high-field magnetotransport measurements on single crystals of graphite, focusing on the non-linear conductivity in pulsed-magnetic fields of up to 75 T. The longitudinal magnetoresistance exhibits distinct non-linearity not only in the first but also in the second field-induced phases.

cond-mat.str-el

Experimental benchmark of the quantum-classical crossover in a spin ladder

We report a spin-(1/2, 5/2) three-leg ladder realized in a radical-Mn polymer, exhibiting an antiferromagnetic transition and magnetization curves accurately described by classical mean-field theory. Although the underlying spin model intrinsically supports strong quantum fluctuations, as confirmed by quantum Monte Carlo simulations, the real system shows an anomalously complete suppression of quantum behavior. These findings provide a key experimental benchmark for the quantum-classical crossover and suggest that lattice topology can play a crucial role in tuning the balance between quantum and classical physics in strongly correlated systems.

cond-mat.str-el

Systematic evolution of superconducting pairing strength and Seebeck coefficients in correlated infinite-layer La$_{1-x}$Sr$_x$NiO$_2$

The recently discovered superconducting infinite-layer nickelates offer a novel platform to explore an exotic pairing mechanism in multi-band systems towards high-temperature superconductivity and associated rich quantum phases, contrasting with cuprates. Here, we show that infinite-layer (La,Sr)NiO$_2$ exhibits strong-coupling superconductivity, resilient to in-plane magnetic fields exceeding 47 T at optimal doping - more than twice the Pauli limit for conventional BCS superconductors. This violation becomes pronounced towards the underdoped regime, implying an intriguing evolution of pairing glue. The unexpected observation of positive Seebeck coefficients in this regime indicates the presence of nontrivial electron correlations. Furthermore, our comprehensive investigation across the superconducting dome reveals that the evolution of (thermo)electric normal-state properties - specifically, the sign changes of the Hall and Seebeck coefficients - coincide with the evolution of superconducting anisotropy and pairing strength. This demonstrates a definitive link between electron correlations and strong-coupling superconductivity in (La,Sr)NiO$_2$, contributing to a unified framework for understanding unconventional superconductivity.

cond-mat.supr-con

Quantum phase with spontaneous translational symmetry breaking in an extended diamond chain

We report the experimental realization of a spin-1/2 extended diamond chain in a verdazyl-Cu complex, where competing interactions and lattice distortions give rise to exotic quantum phases. The magnetic properties exhibit a zero-field energy gap and 1/2 magnetization plateau, which is explained by a dimer-monomer model. Considering the effective interactions between the monomers, three types of dimer-dimer phases are expected to appear as the ground state, depending on the magnitude of the lattice distortions. By mapping to the nonlinear sigma model, three phases are distinguished topologically, and a symmetry-protected topological phase equivalent to the Haldane phase is identified. Furthermore, a nontrivial magnetization is observed above the 1/2 plateau region, suggesting a gapped dimer phase accompanied by a spontaneous breaking of translational symmetry. The discovery of this rare quantum state has broad implications for strongly correlated systems, topological matter, and quantum information science, where symmetry and topology play crucial roles.

cond-mat.str-el

Miniaturized and robust tunable monochromatic magneto-optical platform for pulsed magnetic fields

Tunable monochromatic magneto-transmission is one of the most established magneto-optical techniques, particularly well suited for pulsed magnetic fields. It employs fixed-wavelength monochromatic light as the probe, while the magnetic field is swept to bring the sample into resonance with the photon energy. The key component of this setup is a tunable laser system, typically consisting of a Ti:sapphire laser coupled with an optical parametric oscillator. However, such laser systems are often bulky, expensive, and inherently unstable, which significantly limits their widespread application in magneto-optical laboratories. In this work, we develop a high-accuracy, cost-effective, and compact tunable monochromatic magneto-transmission system based on a combination of a laser-driven white light source and a mini monochromator, and demonstrate its feasibility and performance in a millisecond-range pulsed magnetic field condition. To verify the accuracy of this new and simplified setup, we performed Faraday rotation measurements on the geometrically frustrated spin system CdCr2O4, as well as magneto-transmission experiments on the Shastry-Sutherland lattice antiferromagnet SrCu2(BO3)2. These results show excellent agreement with previous reports, confirming the reliability and precision of the new setup.

cond-mat.str-el

Frustrated $J_1-J_2$ Diamond Lattice Antiferromagnet Co$_2$Ti$_3$O$_8$ with a Vacancy-ordered Spinel Structure Synthesized via a Topochemical Reaction

Metastable Co$_2$Ti$_3$O$_8$ was synthesized through a topochemical reaction using Li$_2$CoTi$_3$O$_8$ as the precursor, resulting in a vacancy-ordered spinel structure. Crystal structure analysis confirmed that Co ions selectively occupy the A-site, giving rise to a frustrated diamond lattice. Magnetic susceptibility and heat capacity measurements revealed antiferromagnetic order at 4.4 K, which is markedly suppressed compared to the negative Weiss temperature of ${\sim}-27$ K, indicating a high degree of frustration effects. Pulsed high-field magnetization measurements revealed a four-step successive magnetic phase transition, demonstrating that Co$_2$Ti$_3$O$_8$ is a promising candidate for a frustrated $J_1-J_2$ diamond lattice. Additionally, the $J_2/J_1$ ration estimated from the molecular field approximation suggests the possibility of a spiral ordered ground state. These observations highlight the potential of frustrated magnetism in ordered spinel structures to expand the material search space for quantum magnetism, including magnetic skyrmions.

cond-mat.str-el

Electronic Structure of Kramers Nodal-Line Semimetal YAuGe and Anomalous Hall Effect Induced by Magnetic Rare-Earth Substitution

Nodal-line semimetals are a class of topological materials hosting one dimensional lines of band degeneracy. Kramers nodal-line (KNL) metals/semimetals have recently been theoretically recognized as a class of topological states inherent to all non-centrosymmetric achiral crystal lattices. We investigate the electronic structure of candidate KNL semimetal YAuGe by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillations as well as by density functional theory (DFT) calculations. DFT has revealed that YAuGe hosts KNLs on the G-A-L-M plane of the Brillouin zone, that are protected by the time reversal and mirror-inversion symmetries. Through ARPES and quantum oscillations we identify signatures of hole bands enclosing the G point, and the observed splitting of quantum oscillation frequency with angle is attributed to spin-orbit-coupling-induced band splitting away from the KNLs. Furthermore, we show that the degeneracy of the nodal lines along the G-A line is lifted by the time-reversal-symmetry breaking when the Y is substituted by magnetic R ions (R = rare earth). This becomes a source of Berry curvature and contributes to the anomalous Hall effect in magnetic RAuGe. These findings establish RAuGe as a new class of KNL semimetals offering significant potential for engineering of anomalous magnetotransport properties via magnetic rare-earth substitution.

cond-mat.mtrl-sci