We report a ^139 La-NMR study of Ba_2La_2CoTe_2O_12 , S=1/2 equilateral triangular-lattice antiferromagnet with easy-plane anisotropy at low temperatures. This compound undergoes a magnetic phase transition at T_N = 3.26 K into an ordered state with the 120^∘ spin structure. Under magnetic fields above 3T, T_N splits into T_N1 and T_N2 , which correspond to the transitions from the paramagnetic phase to the up-up-down (uud) phase and from the uud phase to the triangular coplanar phase, respectively. The NMR spin-lattice relaxation rate 1/T_1 exhibits a critical divergence at T_N1 , indicating the onset of long-range magnetic order. At T_N2 , the NMR-linewidth measured at 5.4 T exhibits an anomalous decrease, which we attribute to a change in the spin structure from the uud to the triangular coplanar phase.

We have measured the magneto-optical Kerr effect (MOKE) in an orthorhombic organic antiferromagnet kappa-(BEDT-TTF)2Cu[N(CN)2]Cl (kappa-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 kappa-Cl. The MOKE of kappa-Cl appeared at around the N & eacute;el temperature and exhibited a nonlinear field dependence in the antiferromagnetic phase. This nonlinear field dependence eliminates a simple origin due to the net canted magnetization. The obtained off-diagonal optical conductivity spectra for the entire pi-electron band clearly show 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 kappa-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 kappa-Cl. We also discuss the analogy between the magnetization in ferromagnets and the net magnetization and N & eacute;el vector with respect to the magneto-optical configurations.

(BEDT-TTF)Cu[N(CN) 2 ] 2 (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene) is a monomer Mott insulator, in which the BEDT-TTF molecules form a three-dimensional (3D) distorted diamond structure; however, its physical properties resemble those of a quasi -one-dimensional (Q1D) zigzag BEDT-TTF chain structure. We investigated the magnetic ground state of this compound using electron spin resonance (ESR) spectroscopy in the X -band region (9.12 GHz) to confirm the occurrence of a transition to a spin -singlet state. The ESR spectra showed an asymmetric spectral line comprising two Lorentzian components, suggesting different correlations of the two independent 1D zigzag chains of the BEDT-TTF molecules. The spin susceptibility derived from the ESR spectra showed an opening of the spin gap below the transition temperature T c ti 25 K possibly due to alternating intrachain interactions in both zigzag chains with different spin correlations.

Existence of 1D spin diffusion in the electrochemical sugar recognition system consisting of a nano-sized gold particle (GNP), a ruthenium complex and a phenylboronic acid was investigated by NMR and mu SR. When sugar molecules are recognized by the phenylboronic site, the response of electrochemical voltammetry of the Ru site changes, enabling the system to work as a sensitive sugar-sensor. In this recognition process, the change in the electronic state at the boron site caused by sugar must be transferred to the Ru site via alkyl chains. We have utilized the muon-labelled electrons method and the proton NMR to find out a channel of the electron transfer from the phenylboronic acid site to the gold nano particle via the one dimensional alkyl chain. If this transfer is driven by diffusive spin channel, characteristic field dependence is expected in the longitudinal spin relaxation rate of mu SR and 1H-NMR. We have observed significant decrease in the spin relaxation rates with increasing applied field. The result is discussed in terms of low dimensional spin diffusion.

We report the band structure calculations and the experimental results of resistivity and magnetic susceptibility in a spin-1/2 (BEDT-TTF)$^{\bullet +}$ monomer Mott insulator (BEDT-TTF)Cu[N(CN)$_2$]$_2$. The band calculations indicate a Dirac semimetal state with nodal lines at the Fermi level. The resistivity and the magnetic susceptibility as functions of temperature are well interpreted in terms of the monomer Mott insulating state instead of the expected semimetal state probably owing to strong electron correlation. In addition, we find that an Arrhenius-type steep reduction of the paramagnetic susceptibility appears below approximately 25 K, which indicates a spin-singlet ground state.

We have investigated the temperature- and field-variation of electronic state for the Dirac semimetal of ${\mathrm{EuMnBi}}_{2}$ by means of optical spectroscopy and theoretical calculation. The optical conductivity spectra show a clear Drude peak in the paramagnetic phase, which gradually diminishes in the Mn-$3d$ antiferromagnetic phase with decreasing temperature. Meanwhile, the absorption peaks due to the interband transition grow at low temperatures, resulting in a pseudogap feature with an energy scale of 0.07 eV. The analysis of Drude weight shows that the Drude response is nearly governed by the Dirac electrons at low temperatures. On the contrary, both the antiferromagnetic transition and spin reorientation of Eu-$4f$ moment do not significantly change the spectra except the moderate variation of Drude weight. As a comparison, we have also investigated the charge dynamics for ${\mathrm{EuZnBi}}_{2}$, which is an analog without the Mn-$3d$ antiferromagnetic ordering. In ${\mathrm{EuZnBi}}_{2}$, the optical conductivity spectra do not show the pseudogap structure, but show an intense Drude peak at all temperatures. Combined with the results of ab initio calculation, in ${\mathrm{EuMnBi}}_{2}$, it is likely that the reconstruction of electronic state driven by the Mn-$3d$ antiferromagnetic ordering causes the Dirac semimetallic state with tiny hole pockets, wherein electronic states other than the Dirac band are nearly gapped-out from the Fermi level.

We have investigated a newly found alpha ''-phase bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecular arrangement system, namely, alpha ''-(BEDT-TTF)(2)Rb2xCo(SCN)(4) (alpha ''-Rb2xCo), with localized S = 3/2 Co2+ spins. From X-ray structural analyses, we found that owing to the nonstoichiometric ratio of Rb ions (x = 0.6), this compound takes an intermediate value of pi-electron band filling between those of alpha ''-(BEDT-TTF)(2)CsHg(SCN)(4) and alpha ''-(BEDT-TTF)(2)K1.4Co(SCN)(4). alpha ''-Rb2xCo (x = 0.6) is a paramagnetic metal at room temperature and exhibits a first-order transition to an insulating state at 100 K, which is the lowest among the three compounds. The results of the two-dimensional optical conductivity spectra and peak splitting of charge-sensitive nu(27) molecular vibrational modes indicate the phase transition to a state with a gap of 600 cm(-1), which is attributed to charge ordering consisting of at least four differently charged BEDT-TTF molecules. The pi-spin susceptibility suddenly decreases at the transition temperature, and another anomaly can be seen at about 40 K. Given that pi-spin disappears at 100K (i.e., formation of a spin-singlet state), it is difficult to systematically explain other physical properties as a whole at low temperatures. However, by simply assuming the presence of pi spins, we can account for all the observed results without contradiction. We also discuss anomalies in the dielectricity of pi electrons under magnetic fields mediated by pi-d interactions and the spin-charge coupling.

Magnetic susceptibility and torque measurements have been performed for neutral BEDT-TTF crystals to investigate the diamagnetic properties. The diamagnetic susceptibility is found to be anisotropic at room temperature, -1.9 x 10(-4) emu/mol for the magnetic field H parallel to [011] and -2.5 x 10(-4) emu/mol for H parallel to [100], comparable to the Langevin diamagnetism estimated from the Pascal constants, -2.04 x 10(-4) emu/mol. The diamagnetism for both directions is gradually enhanced with decreasing temperature. Sinusoidal torque curves are clearly observed for two field rotations in the (0 (1) over bar1) and (100) planes, reflecting the anisotropic diamagnetism of the neutral BEDT-TTF molecule. The results show that careful attention must be paid to quantitative discussion of the torque signals in various organic materials.

Spontaneous symmetry breaking in crystalline solid often produces exotic nonreciprocal phenomena. As one such example, the unconventional optical rotation with nonreciprocity, which is termed gyrotropic birefringence, is expected to emerge from the magnetoelectric coupling. However, the fundamental nature of gyrotropic birefringence remains to be examined. Here w`e demonstrate the gyrotropic birefringence enhanced by the dynamical magnetoelectric coupling on the electrically active magnon resonance, i.e. electromagnon, in a multiferroic helimagnet. The helical spin order having both polarity and chirality is found to cause the giant gyrotropic birefringence in addition to the conventional gyrotropy, i.e. natural optical activity. It is demonstrated that the optical rotation of gyrotropic birefringence can be viewed as the nonreciprocal rotation of the optical principal axes, while the crystallographic and magnetic anisotropies are intact. The independent control of the nonreciprocal linear (gyrotropic birefringence) and circular (natural optical activity) birefringence/dichroism paves a way for the optically active devices.

This corrects the article DOI: 10.1103/PhysRevLett.123.027601.

We investigate the low-frequency charge-carrier dynamics of a molecular dimer-Mott insulator beta'-(BEDT-TTF)(2)ICl2, where the freezing of charge fluctuations on the dimers gives rise to electronic ferroelectricity. We show that conductance fluctuation (noise) spectroscopy allows one to probe changes in the dielectric properties at elevated temperatures, where samples are even still in the conductive regime. Our results explain the formation of electric polarization states leading to glassy and relaxor-type ferroelectric behavior that is frequently observed in these systems. The onset of distinct two-level fluctuations and changes of the underlying 1/f-type noise indicate the formation of nanoscale polar regions, the dynamics of which depends on the applied electric fields. Conductance noise spectroscopy therefore is a suitable tool for investigating the onset of electric-polarization dynamics in molecular and other, inorganic charge-driven ferroelectrics.

This overview describes the progressive results of the superconducting critical temperature in bulk nanostructured metals (niobium, vanadium and tantalum) processed by high-pressure torsion (HPT). Bulk nanostructured superconductors provide a new route to control superconducting property, because ultrafine-grain structures with a high density of grain boundaries, dislocations, and other crystalline defects modify the superconducting order parameter. The critical temperature T-c in Nb increases with the evolution of grain refinement owing to the quantum confinement of electrons in ultrafine grains. In Vand Ta, however, T-c decreases at a certain HPT revolution number (i.e. at certain strain levels). The different behaviour of T-c in the three materials is explained by the competition effect between the quantum size effect and disorder effect; these effects are characterized by the parameters of grain size, electron mean free path, and superconducting coherence length.

Low-temperature magnetic properties were investigated on the gold nano particles (GNPs) with an average size of 11.5 nm, assembled with molecules of ruthenium complex (Ru0), and phenylboronic acid (B0) by the proton nuclear magnetic resonance (1H-NMR) and susceptibility measurements. The temperature dependence of the NMR shift and the uniform susceptibility was described as the sum of the Curie–Weiss term and a positive constant term. From the former, the average number of Ru0 on each GNP was estimated to be 118, which is 23% of the calculation. The finite positive constant term shows a clear contrast with the well-known fact that the bulk gold is diamagnetic. Finally, a disappearance of the motional narrowing effect in the proton NMR spectra below 60 K indicates that the wavering motion of Ru0 complexes on GNP at room temperature is frozen at low temperatures.

Inelastic neutron scattering measurements on the molecular dimer-Mott insulator κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Cl reveal a phonon anomaly in a wide temperature range. Starting from T_{ins}∼50-60  K where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature T_{N}=27  K, where also a ferroelectric ordering at T_{FE}≈T_{N} occurs. Conversely, the phonon damping becomes small again when spins and charges are ordered below T_{N}, while no change of the lattice symmetry is observed across T_{N} in neutron diffraction measurements. We assign the phonon anomalies to structural fluctuations coupled to charge and spin degrees of freedom in the BEDT-TTF molecules.