arXiv subjects

Tetsuya Furukawa

Publications and source records attributed to Tetsuya Furukawa.

7 recordsLinked to original sources

arXiv2026

Single-orbital tight-binding model for chiral one-dimensional hybrid organic-inorganic lead halide perovskites

We present a single-orbital tight-binding model for the low-energy electronic states of the chiral one-dimensional hybrid organic-inorganic lead halide perovskite $\mathrm{(}R/S\mathrm{-PEA)PbI}_3$. The model is constructed from a single effective orbital on each of the four symmetry-related sites in the primitive unit cell and incorporates layer, in-plane sublattice, and spin degrees of freedom. Using separate parameter sets for the conduction and valence bands, the effective Hamiltonian reproduces the overall band dispersions obtained from density-functional-theory calculations and quantitatively captures the spin splittings near the band edges. It also captures the leading spin-polarization patterns of the Bloch states, showing that the band-edge spin splitting and spin polarization are encoded in a small number of symmetry-adapted spin-dependent hopping terms. We further analyze the accidental degeneracies of the effective Hamiltonian using screw eigenvalues and antiunitary operators. This analysis separates accidental degeneracies originating from the restricted term content of the effective Hamiltonian from degeneracies enforced by nonsymmorphic screw symmetries and time-reversal symmetry. The present model provides a symmetry-transparent starting point for understanding the band-edge electronic structure of chiral lead halide perovskites and for analyzing optical, spin, and transport responses in this class of materials.

cond-mat.mtrl-sci↗

arXiv2026

First-principles calculations of spin-split bands in chiral hybrid organic-inorganic perovskites ($R$/$S$-PEA)PbI$_3$ and ($R$/$S$-NEA)PbI$_3$

Chiral hybrid organic-inorganic perovskites provide a promising platform for investigating the physics of chirality-driven spin-split bands because they combine robust molecular chirality with strong spin-orbit coupling from heavy inorganic ions. First-principles calculations including spin-orbit coupling are performed for the one-dimensional chiral perovskites ($R$/$S$-PEA)PbI$_3$ and ($R$/$S$-NEA)PbI$_3$ to compare their spin-split band structures and to identify the factors controlling their differences. In ($R$/$S$-PEA)PbI$_3$, the lowest conduction bands predominantly consist of Pb orbitals, whereas in ($R$/$S$-NEA)PbI$_3$, they are formed by hybridization between Pb orbitals and the lowest unoccupied molecular orbital of NEA. Both compounds exhibit pronounced spin splitting near the valence-band maximum and conduction-band minimum. The effective spin splitting of the edges of the valence bands is stronger in ($R$/$S$-NEA)PbI$_3$, despite similar linear-in-$k$ splitting coefficients near the relevant high-symmetry points. This enhancement originates from larger gaps induced by spin-orbit coupling at high-symmetry points and band (anti)crossings in the multiband structure. For a given molecular handedness, the PEA- and NEA-based compounds exhibit opposite spin textures, consistent with the opposite chiral distortions of the [PbI$_6$]$^{4-}$ octahedra and with the previously observed opposite signs of circular dichroism. Group-theoretical analysis for the nonsymmorphic space group $P2_12_12_1$ further accounts for band sticking, symmetry-enforced degeneracies, and the disappearance of spin polarization at specific Brillouin-zone-boundary points. These results provide a solid foundation for future studies of chirality-dependent electromagnetic responses, including circular dichroism, in chiral hybrid organic-inorganic perovskites.

cond-mat.mtrl-sci↗

arXiv2026

Intimate relationship between spin configuration in the triplet pair and superconductivity in UTe$_2$

Spin-triplet superconductivity is an intriguing quantum coherent state with both spin and orbital degrees of freedom, which holds significant potential for future applications in quantum technology. However, how the spin of the triplet pairs responds to an external magnetic field remains poorly understood. This is mainly due to the absence of suitable spin-triplet superconductors. Here, we report results of Knight-shift and ac-susceptibility measurements on UTe$_2$. We demonstrate that the spin susceptibility, which slightly decreases compared to the normal-state value below the superconducting (SC) transition temperature $T_{\rm c}$, is rapidly restored and nearly recovers to the normal-state values around 5 T, well below the SC upper critical field $H_{c2}$ when the magnetic field is applied along the $c$ axis ($H \parallel c$). In addition, we found that $H_{\rm c2}$ of superconductivity becomes larger when the SC spin aligns with the magnetic field. By considering the results on $H \parallel b$, our results suggest the presence of a close relationship between the spin configuration of the triplet pair and $H_{\rm c2}$, as well as the anisotropic pinning interaction acting on the triplet pairs. These phenomena, which have never been observed in spin-singlet superconductors, represent characteristic features unique to spin-triplet superconductors. We discuss the similarities between superconductivity in UTe$_2$ and superfluid $^3$He, focusing on their spin-triplet pairing states.

cond-mat.supr-con↗

arXiv2025

Magneto-optical spectra of an organic antiferromagnet as a candidate for an altermagnet

We have measured the magneto-optical Kerr effect (MOKE) in an orthorhombic organic antiferromagnet $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl, which is a candidate for an altermagnet. From the Maxwell equations, we derived matrix-type general formulae describing the optical propagation and reflection for arbitrary crystals. These formulae enabled us to correctly measure and obtain the off-diagonal optical responses of $κ$-Cl. The MOKE of $κ$-Cl appeared at around the Néel temperature and exhibited a nonlinear field dependence in the antiferromagnetic phase. These nonlinear field dependence eliminates a simple origin due to the net canted magnetization. The obtained off-diagonal optical conductivity spectra for almost entire $π$-electron band clearly shows three features. One is the large peaks at the spectral ends due to the magnetic origin with the large energy scale compared to the very small spin-orbit interaction in $κ$-Cl. The others are the middle region spectra proportional to the diagonal conductivities, possibly related to the symmetric piezomagnetic effect and the standard antisymmetric origins. These results suggest the altermagnetic response of $κ$-Cl. We also discuss the analogy between the magnetization in ferromagnets and the net magnetization and Néel vector with respect to the magneto-optical configurations.

cond-mat.str-el↗

arXiv2022

Microscopic Evidence for Preformed Cooper Pairs in Pressure-Tuned Organic Superconductors near Mott Transition

A weird electronic state accompanied with an anomalous superconducting precursor and/or exotic orders, called the pseudogap state, arises prior to a superconducting condensate in underdoped cuprates that are situated near Mott transition. Another way to make the system approach the Mott transition is the variation of bandwidth or correlation strength, which gives a new dimension to exploring this exotic state. Here we report nuclear magnetic resonance (NMR) studies on layered organic superconductors with half-filled bands whose widths are pressure-tuned near the Mott transition. The system situated on the verge of the Mott transition shows a pseudogap-like anomalous suppression of spin excitations on cooling from well above the superconducting critical temperature $T_{\mathrm{c}}$. The pressure variation of the NMR relaxation rate shows that the pseudogap-like behavior is rapidly suppressed by applying pressure. The NMR experiments under various magnetic fields varied up to 18 T proves the absence of symmetry breaking orders that compete with superconductivity, such as charge orders, in the metallic phase. Remarkably, the pseudogap-like behavior above $T_{\mathrm{c}}$ and the superconducting condensate fade out in parallel under ascending magnetic fields with similar field-orientation dependence, indicating a superconducting precursor is the predominant origin of the pseudogap. Our further investigation of different materials, which take different "distances" from the Mott transition by chemical pressure, confirms that the superconducting precursor is not the conventional amplitude fluctuations arising from low dimensionality but unconventional preformation of Cooper pairs enhanced near the Mott transition. These findings conclude that preformed Cooper pairs persist up to twice as high as $T_{\mathrm{c}}$ on the verge of the bandwidth-controlled Mott transition.

cond-mat.str-el↗

arXiv2020

Chirality-Induced Electrical Generation of Magnetism in Nonmagnetic Elemental Tellurium

Chiral matter has a structure that lacks inversion, mirror, and rotoreflection symmetry; thus, a given chiral material has either a right- or left-handed structure. In chiral matter, electricity and magnetism can be coupled in an exotic manner beyond the classical electromagnetism (e.g., magneto chiral effect in chiral magnets). In this paper, we give a firm experimental proof of the linear electric-current-induced magnetization effect in bulk nonmagnetic chiral matter elemental trigonal tellurium. We measured a $^{125} $Te nuclear magnetic resonance (NMR) spectral shift under a pulsed electric current for trigonal tellurium single crystals. We provide general symmetry considerations to discuss the electrically (electric-field- and electric-current-) induced magnetization and clarify that the NMR shift observed in trigonal tellurium is caused by the linear current-induced magnetization effect, not by a higher-order magnetoelectric effect. We also show that the current-induced NMR shift is reversed by a chirality reversal of the tellurium crystal structure. This result is the first direct evidence of crystal-chirality-induced spin polarization, which is an inorganic-bulk-crystal analogue of the chirality-induced spin selectivity in chiral organic molecules. The present findings also show that nonmagnetic chiral crystals may be applied to spintronics and coil-free devices to generate magnetization beyond the classical electromagnetism.

cond-mat.mtrl-sci↗

arXiv2017

Quasi-continuous transition from a Fermi liquid to a spin liquid

The Mott metal-insulator transition-a drastic manifestations of Coulomb interactions among electrons-is the first-order transition of clear discontinuity, as shown by various experiments and the celebrated dynamical mean-field theory. Recent theoretical works, however, suggest that the transition is continuous if the Mott insulator carries an exotic spin liquid with a spinon Fermi surface. Here, we demonstrate the case of a quasi-continuous Mott transition from a Fermi liquid to a spin liquid in an organic triangular-lattice system k-(ET)2Cu2(CN)3. Transport experiments performed under fine pressure tuning find that, as the Mott transition is approached, the Fermi-liquid coherence temperature continuously falls to the scale of kelvins with divergent quasi-particle decay rate in the metal side and the charge gap gradually closes in the insulator side. The Clausius-Clapeyron analysis of the pressure-temperature phase diagram provides thermodynamic evidence for the extremely weak first-order nature of the Mott transition. These results suggest that the spin liquid hosts a spinon Fermi surface, which turns into an electron Fermi surface when charges are Mott delocalized.

cond-mat.str-el↗