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Metal-insulator transition was microscopically investigated by orbital-resolved nuclear magnetic resonance (OR-NMR) spectroscopy in a single crystal of vanadium dioxide VO$_2$. Observations of the anisotropic $^{51}$V Knight shift and the nuclear quadrupole frequency allow us to evaluate orbital-dependent spin susceptibility and $d$ orbital occupations. The result is consistent with the degenerated $t_{2g}$ orbitals in a correlated metallic phase and the $d$ orbital ordering in a nonmagnetic insulating phase. The predominant orbital pointing along the chain facilitates a spin-singlet formation triggering metal-insulator transition. The asymmetry of magnetic and electric hyperfine tensors suggests the $d$ orbital reformation favored by a low-symmetry crystal field, forming a localized molecular orbital. The result highlights the cooperative electron correlation and electron-phonon coupling in Mott transition with orbital degrees of freedom., Comment: 8 figures, 9 pages

All the iron-based superconductors identified to date share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square lattice structures but also in ladder structures. Yet iron-based superconductors without a square lattice motif have not been found despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below ~120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity., Comment: in Nature Materials, 20 July 2015

The microscopic mechanism of the metal-insulator transition is studied by orbital-resolved 51V NMR spectroscopy in a prototype of the quasi-one-dimensional system V6O13. We uncover that the transition involves a site-selective d orbital order lifting twofold orbital degeneracy in one of the two VO6 chains. The other chain leaves paramagnetic moments on the singly occupied dxy orbital across the transition. The two chains respectively stabilize an orbital-assisted spin-Peierls state and an antiferromagnetic long-range order in the ground state. The site-selective Mott transition may be a source of the anomalous metal and the Mott-Peierls duality., Comment: 9 pages, 8 figures

We investigated the iron-based ladder compounds (Ba,Cs)Fe$_2$Se$_3$. Their parent compounds, BaFe$_2$Se$_3$ and CsFe$_2$Se$_3$, have different space groups, formal valences of Fe and magnetic structures. Electrical resistivity, specific heat, magnetic susceptibility, X-ray diffraction and powder neutron diffraction measurements were conducted to obtain temperature and composition phase diagram of this system. Block magnetism observed in BaFe$_2$Se$_3$ is drastically suppressed with Cs doping. In contrast, stripe magnetism observed in CsFe$_2$Se$_3$ is not so fragile against Ba doping. New type of magnetic structure appears in intermediate compositions, which is similar to stripe magnetism of CsFe$_2$Se$_3$, but inter-ladder spin configuration is different. Intermediate compounds show insulating behavior, nevertheless finite $T$-linear contribution in specific heat was obtained at low temperatures., Comment: 12 pages, 13 figures

Spinodal decomposition is a ubiquitous phenomenon leading to phase separation from a uniform solution. We show that a spinodal decomposition occurs in a unique combination of two rutile compounds of TiO2 and VO2, which are chemically and physically distinguished from each other: TiO2 is a wide-gap insulator with photo catalytic activities and VO2 is assumed to be a strongly correlated electron system which exhibits a dramatic metal-insulator transition at 342 K. The spinodal decomposition takes place below 830 K at a critical composition of 34 mol% Ti, generates a unidirectional composition modulation along the c axis with a wavelength of approximately 6 nm, and finally results in the formation of self-assembled lamella structures made up of Ti-rich and V-rich layers stacked alternately with 30-50 nm wavelengths. A metal-insulator transition is not observed in quenched solid solutions with intermediate compositions but emerges in the thin V-rich layers as the result of phase separation. Interestingly, the metal-insulator transition remains as sharp as in pure VO2 even in such thin layers and takes place at significantly reduced temperatures of 310-340 K, which is probably due to a large misfit strain induced by lattice matching at the coherent interface., Comment: 18 pages, 1 figures

We report on our 51V-NMR study of static and dynamical magnetic properties in the quasi-one-dimensional antiferromagnet BaCo2V2O8. Although the NMR spectrum shows well-defined antiferromagnetic (AF) order in the N\'eel ground state, the AF characteristic from the NMR spectrum is incomplete between 3.5 K and TN= 5.4 K, which could be affected by quantum spin fluctuations. The AF NMR spectrum indicates two V sites experiencing different magnetic field magnitudes, HA1=2.1 kOe and HA2=3.8 kOe. These internal fields could be explained by accounting for the classical and the pseudo-dipolar fields from Co2+ spins with a proposed magnetic structure based on the neutron diffraction measurements. In the paramagnetic state, the nuclear spin relaxation is dominated by AF spin fluctuations through the dipolar-type coupling between V and surrounding Co2+ ions. The linear relation between the nuclear spin-lattice relaxation rate 1/T1T and the magnetic susceptibility chi indicates that the Q component of magnetic susceptibility chi(Q) is roughly proportional to chi, where Q is the AF wave number. A change in slope of 1/T1T with respect to \chi around 150 K suggests a change in the AF spin fluctuation spectrum.

Magnetism in the insulating BaFe$_2$Se$_3$ was examined through susceptibility, specific heat, resistivity and neutron diffraction measurements. After formation of a short-range magnetic correlation, a long-range ordering was observed below $T_{\rm N}\sim 255$ K. The transition is obscured by bulk properties. Magnetic moments ($\parallel a$) are arranged to form a Fe$_4$ ferromagnetic unit, and each Fe$_4$ stacks antiferromagnetically. This block magnetism is of the third type among magnetic structures of ferrous materials. The magnetic ordering drives unusually large distortion via magnetoelastic coupling., Comment: 5 pages, 4 figures

The Faraday rotation and magneto-optical absorption spectral measurements were conducted to reveal the full-magnetization process and map out a magnetic phase diagram of a typical geometrical frustrated magnet, ZnCr2O4, by using the electromagnetic flux compression method in ultra-magnetic fields up to 600 T. A fully polarized ferromagnetic phase is observed in which the absorption spectra associated with an exciton-magnon-phonon transition disappears. Furthermore, prior to the fully polarized ferromagnetic phase above 410 T, we found a novel magnetic phase above 350 T followed by a canted 3:1 phase., Comment: 4 pages, 4 figures

The Faraday rotation technique is used to map out the finite-temperature phase diagram of the prototypical frustrated magnet ZnCr2O4, in magnetic fields of up to 190 T generated by the single-turn coil method. We find evidence for a number of magnetic phase transitions, which are well-described by the theory based on spin-lattice coupling. In addition to the 1/2 plateau and a 3:1 canted phase, a 2:1:1 canted phase is found for the first time in chromium spinel oxides, which has been predicted by a theory of Penc et al. to realize in a small spin-lattice coupling limit. Both the new 2:1:1 and the 3:1 phase are regarded as the supersolid phases according to a magnetic analogy of Matsuda and Tsuneto, and Liu and Fisher., Comment: 13 pages, 6 figures, accepted for publication in JPSJ

We report a neutron diffraction and muon spin relaxation muSR study of static and dynamical magnetic properties of BaCo2V2O8, a quasi-one-dimensional spin-chain system. A proposed model for the antiferromagnetic structure includes: a propagation vector k_AF = (0, 0, 1), independent of external magnetic fields for fields below a critical value H_c(T). The ordered moments, of 2.18 \mu_B per Co ion, are aligned along the crystallographic c-axis. Within the screw chains, along the c axis, the moments are arranged antiferromagnetically. In the basal planes the spins are arranged ferromagnetically (forming zig-zags paths) along one of the axis and antiferromagnetically along the other. The temperature dependence of the sub-lattice magnetization is consistent with the expectations of the 3D Ising model. A similar behavior is observed for the internal static fields at different muon stopping sites. Muon time spectra measured at weak longitudinal fields and temperatures much higher than T_N can be well described using a single muon site with an exponential muon spin relaxation that gradually changes into an stretched exponential on approaching T_N. The temperature-induced changes of the relaxation suggest that the Co fluctuations dramatically slow down and the system becomes less homogeneous as it approaches the antiferromagnetic state., Comment: 7 pages, 9 figures

We investigated the magnetic phase of the perovskite CaCrO$_3$ by using the muon spin relaxation technique accompanied by susceptibility measurements. A thermal hysteresis loop is identified with a width of about 1 K at the transition temperature. Within the time scale of the muon lifetime, a static antiferromagnetic order is revealed with distinct multiple internal fields which are experienced in the muon interstitial sites below the phase-transition temperature, $T_N=90 K$. Above $T_N$, lattice deformations are indicated by transverse-field muon-spin rotation and relaxation suggesting a magneto-elastic mechanism., Comment: 5 pages, 4 figures. Accepted for publication in PRB

Coherent spin oscillations were non-thermally induced by circularly polarized pulses in fully compensated antiferromagnetic NiO. This effect is attributed to an entirely new mechanism of the action, on the spins, of the effective magnetic field generated by an inverse Faraday effect. The novelty of this mechanism is that spin oscillations are driven by the time derivative of the effective magnetic field acting even on "pure" antiferromagnets with zero net magnetic moment in the ground state. The measured frequencies (1.07 THz and 140 GHz) of the spin oscillations correspond to the out-of-plane and in-plane modes of antiferromagnetic magnons., Comment: 4 pages, 2 figures. Submitted to Physical Review Letters

The two-dimensional (2D) quantum spin system (CuCl)LaNb2O7 has a spin-singlet ground state with a gap of 2.3 meV, while the isostructural material (CuBr)LaNb2O7 displays collinear antiferromagnetic order at TN = 32 K. Here we report on the synthesis of solid solution (CuCl1-xBrx)LaNb2O7 (0 < x < 1), and its structural and magnetic properties by means of magnetic susceptibility, high-field magnetization, and neutron diffraction studies. The x-dependence of cell parameters follows Vegards law, verifying the uniform distribution of Cl and Br atoms at the halide site, though a more complex structural evolution is inferred from an opposing correlation between the intra- and interlayer cell distances (vs. x). 5%-Br substitution is found to induce antiferromagnetic order with TN = 7 K, consistent with a recent mSR study, and the magnetic structure is collinear, having a significantly reduced moment. Further Br substitution leads to a linear increase in TN up to x = 1. These results indicate that (CuCl)LaNb2O7 is located in the vicinity of the quantum phase boundary., Comment: 14 pages, 5 figures

We report results of 11B NMR and susceptibility measurements on the quasi 2D frustrated dimer spin system SrCu2(BO3)2 under high pressure. At 2.4 GPa and in a magnetic field of 7 T, NMR lines split with decreasing temperature in two steps. A gradual splitting below T=30 K breaking the four-fold symmetry of magnetic response is followed by a further sudden splitting below 3.6 K. The latter indicates a magnetic phase transition, which is also marked by a kink in the susceptibility at 1.44 GPa. From the magnetic hyperfine shift data, we conclude that the low-T phase has a doubled unit cell containing two types of dimers, one in a nearly singlet state and the other with a finite magnetization down to T=0., Comment: 5 pages, 4 figures, to appear in J. Phys. Soc. Jpn. (July 2007)

The pressure-induced changes in the optical properties of $\beta$-Na$_{0.33}$V$_2$O$_5$ single crystals at room temperature were studied by polarization-dependent Raman and far-infrared reflectivity measurements under high pressure. From the changes in the Raman- and infrared-active phonon modes in the pressure range 9 - 12 GPa a transfer of charge between the different V sites can be inferred. The importance of electron-phonon coupling in the low-pressure regime is discussed., Comment: 7 pages, 5 figures

The present resonant x-ray scattering from each of monoclinically-split single domains of NaV2O5 has critically enhanced contrast between V4+ and V5+ ions strong enough to lead to unambiguous conclusion of the charge-order pattern of its low-temperature phase below Tc = 35 K. The zig-zag type charge-order patterns in the $ab$-plane previously confirmed have four kinds of configurations (A, A', B and B') and the stacking sequence along the c-axis is determined as the AAA'A' type by comparison with model calculations. By assigning the A and A' configurations to Ising spins, one can reasonably understand the previously discovered "devil's staircase"-type behavior with respect to the modulation of the layer-stacking sequences at high pressures and low temperatures, which very well resembles the global phase diagram theoretically predicted by the ANNNI model., Comment: 4 pages, 3 figures

The low-temperature (LT) superstructure of $\alpha'$-NaV$_2$O$_5$ was determined by synchrotron radiation x-ray diffraction. Below the phase transition temperature associated with atomic displacement and charge ordering at 34K, we observed the Bragg peak splittings, which evidence that the LT structure is monoclinic. It was determined that the LT structure is $(a-b)\times 2b \times 4c$ with the space group $A112$ where $a, b$ and $c$ represent the high temperature orthorhombic unit cell. The valence estimation of V ions according to the bond valence sum method shows that the V sites are clearly separated into two groups of V$^{4+}$ and V$^{5+}$ with a $zigzag$ charge ordering pattern. This LT structure is consistent with resonant x-ray and NMR measurements, and strikingly contrasts to the LT structure previously reported, which includes V$^{4.5+}$ sites., Comment: 4 pages, 3 figures, 1 table

We report the observation of nuclear quadrupole resonance (NQR) of Cu from the sites near the doping center in the spin-Peierls system CuGe$_{1-x}$Si$_{x}$O$_3$. The signal appears as the satellites in the Cu NQR spectrum, and has a suppressed nuclear spin-lattice relaxation rate indicative of a singlet correlation rather than an enhanced magnetic correlation near the doping center. Signal loss of Cu nuclei with no neighboring Si is also observed. We conclude from these observations that the doping-induced moments are not in the vicinity of the doping center but rather away from it., Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Lett

Low-frequency spin dynamics in the S=1/2 antiferromagnetic spin-chain compound \alpha-VO(PO_3)_2 has been studied by means of 31P NMR. The nuclear spin-lattice relaxation rate 1/T_1 at the P site exhibits H^{-1/2} dependence on the applied magnetic field (H) at temperatures (T) well above the intrachain coupling strength J/k_{B} = 3.50 K indicating one-dimensional diffusive spin dynamics. The diffusive contribution to 1/T_1 decreases on cooling as electronic spins acquire short-range antiferromagnetic correlations within the chain, and vanishes almost entirely around T \approx J/k_{B}. This is accompanied by an apparent increase of the spin-diffusion constant from the value expected in the classical limit. On the other hand, the field-independent part of 1/T_1 increases with decreasing temperature, which may be a precursor for the true long-range antiferromagnetic ordering found below T_{N} = 1.93 \pm 0.01 K., Comment: 9 pages, 11 embedded eps figures uses JPSJ.sty, revised for publication in J. Phys. Soc. Jpn

The nuclear spin-lattice relaxation rate (1/T1) at the V sites in CaV2O5 was measured. The result was compared with the calculation for the two-leg ladder with the strong-coupling expansion. We found a good agreement on the temperature and magnetic field dependence but a discrepancy between the exchange parameters deduced from 1/T1 and the magnetic susceptibility., Comment: 7 pages, 7 figures, submitted to J. Phys. Soc. Jpn

23Na NMR spectrum measurements in alpha'-NaV2O5 with a single- crystalline sample are reported. In the charge-ordered phase, the number of inequivalent Na sites observed is more than that expected from the low-temperature structures of space group Fmm2 reported so far. This disagreement indicates that the real structure including both atomic displacement and charge disproportionation is of lower symmetry. It is suggested that zigzag ordering is the most probable. The temperature variation of the NMR spectra near the transition temperature is incompatible with that of second-order transitions. It is thus concluded that the charge ordering transition is first-order., Comment: 4 pages, 5 eps figures, submitted to J. Phys. Soc. Jpn

Nuclear magnetic resonance (NMR) of 31P and 51V nuclei has been measured in a spin-1/2 alternating-chain compound (VO)_2P_2O_7. By analyzing the temperature variation of the 31P NMR spectra, we have found that (VO)_2P_2O_7 has two independent spin components with different spin-gap energies. The spin gaps are determined from the temperature dependence of the shifts at 31P and 51V sites to be 35 K and 68 K, which are in excellent agreement with those observed in the recent inelastic neutron scattering experiments [A.W. Garrett et al., Phys. Rev. Lett. 79, 745 (1997)]. This suggests that (VO)_2P_2O_7 is composed of two magnetic subsystems showing distinct magnetic excitations, which are associated with the two crystallographically-inequivalent V chains running along the b axis. The difference of the spin-gap energies between the chains is attributed to the small differences in the V-V distances, which may result in the different exchange alternation in each magnetic chain. The exchange interactions in each alternating chain are estimated and are discussed based on the empirical relation between the exchange interaction and the interatomic distance., Comment: 10 pages, 11 embedded eps figures, REVTeX, Submitted to Phys. Rev. B