By measuring X-rays from a bent Si single crystal, we have observed the amplification of the reflected beams from the surface. The amplification is observed when the incident angle is adjusted to minimize the effective linear absorption coefficient due to the dynamical diffraction effect. When we increase the width of the incident X-rays along the incident azimuth, we observe on increase in reflected beam intensity. The amplification can be explained by the addition of the electric fields of the propagating beams along hyperbolic trajectories in the bent crystal to those of the reflected beams.

View full abstract

The sampling of Boltzmann distributions by stochastic Markov processes, can be strongly limited by the crossing time of high (free) energy barriers. As a result, the system may stay trapped in metastable states, and the relaxation time to the equilibrium Boltzmann distribution may be very large compared to the available computational time. In this paper, we show how, by a simple modification of the Hamiltonian, one can dramatically decrease the relaxation time of the system, while retaining the same equilibrium distribution. The method is illustrated on the case of a strongly corrugated one-dimensional potential.

View full abstract

We microscopically examine the intergrain Josephson current in iron-pnictide superconductors in order to solve the puzzle of why the intergrain current is much lower than the intragrain one. The theory predicts that the intergrain Josephson current is significantly reduced by the ±s-wave symmetry when the incoherent tunneling becomes predominant and the density of states and the gap amplitude between two bands are identical. We find in such a situation that the temperature dependence of the intergrain Josephson current shows an anomalously flat curve over a wide temperature range. Finally, we suggest important points for increasing the intergrain current.

View full abstract

We present ⁵⁷Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba_0.6K_0.4Fe₂As₂ (T_c=38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1⁄T₁) demonstrate the development of wave-number (q)-dependent spin fluctuations, except at q=0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the ⁵⁷Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The ⁵⁷Fe-1⁄T₁ results are totally consistent with a s^±-wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective 1⁄T₁ data for Ba_0.6K_0.4Fe₂As₂ and LaFeAsO_0.7, which seemingly follow a T⁵- and a T³-like behaviors below T_c, are consistently explained in terms of this model only by changing the size of the superconducting gap.

View full abstract

A microscopic theory for the spin triplet Cooper pairing in non-centrosymmetric superconductors like CePt₃Si and CeTSi₃ (T = Rh, Ir) is presented. The lack of inversion symmetry leads to new anomalous spin fluctuations which stabilize the triplet part in addition to the singlet part originating from the centrosymmetric spin fluctuations. It is shown that both parts have similar nontrivial momentum dependence of A₁ type. Therefore the mixed singlet-triplet gap function has accidental line nodes on both Fermi surface sheets which are stable as function of temperature. This gap function explains the salient features of CePt₃Si and CeTSi₃ superconductors.

View full abstract

Motivated by the discovery of superconductivity in F-doped LaFeAsO, we investigated the magnetic fluctuations in related compounds Fe(Se_1−xTe_x)_0.92 (x=0.75,1) using neutron scattering technique. Non-superconducting FeTe_0.92 shows a pronounced magnetic fluctuation around |Q|≈0.9 Å⁻¹, which is slightly smaller than the magnetic ordering vector |Q|≈0.97 Å⁻¹. In a superconductor Fe(Se_0.25Te_0.75)_0.92, we observed that a magnetic fluctuation is located at |Q|≈0.9 Å⁻¹ at low energies and shifts to a higher value of |Q|≈1.2 Å⁻¹ at higher energies. The latter value is close to a reciprocal lattice vector Q=(0.5,0.5,0.5) or (0.5,0.5,0), where the AF fluctuations are observed in other FeAs-based materials. The existence of this common characteristic in different Fe-based superconductors suggests that the AF fluctuations may play an important role in superconductivity.

View full abstract

While the superconducting transition temperature T_c of Sr₂RuO₄ is 1.5 K, its onset T_c is enhanced as high as 3 K in the Sr₂RuO₄–Ru eutectic system, which is often referred to as the 3-K phase. We have investigated effects of uniaxial pressure on the non-bulk superconductivity in the 3-K phase. While T_c of pure Sr₂RuO₄ is known to be suppressed by hydrostatic pressure, a large enhancement of the superconducting volume fraction of the 3-K phase was observed for both out-of-plane and in-plane uniaxial pressures. Especially, under the in-plane pressure, the shielding fraction at 1.8 K of only less than 0.5% at 0 GPa exceeds 40% at 0.4 GPa. Such a large shielding fraction suggests that under the uniaxial pressure interfacial 3-K phase superconductivity penetrates deep into the bulk of Sr₂RuO₄. The present finding provides a significant implication to the unresolved origin of the enhancement of T_c to 3 K in the Sr₂RuO₄–Ru eutectic system.

View full abstract

Excitations from a strongly frustrated system, the kagomé ice state of the spin ice Dy₂Ti₂O₇ under magnetic fields along a [111] direction, have been studied. They are theoretically proposed to be regarded as magnetic monopoles. Neutron scattering measurements of spin correlations show that close to the critical point the monopoles are fluctuating between high- and low-density states, supporting that the magnetic Coulomb force acts between them. Specific heat measurements show that monopole-pair creation obeys an Arrhenius law, indicating that the density of monopoles can be controlled by temperature and magnetic field.

View full abstract

Graph states are a special kind of multiparticle entangled state with great potential for applications in quantum information technologies, especially in measurement-based quantum computers. These states cause significant reductions of the number of qubits needed for a given computation, leading to shorter execution time. Here we propose a simple scheme for generating such graph states by using special gate operations, i.e., control-phase and swap gate operations, inherent in superconducting quantum nanocircuits.

View full abstract

To study the solution growth of crystals composed of chiral organic molecules, a spin-one Ising lattice gas model is proposed. The model turns out to be equivalent to the Blume–Emery–Griffiths model, which shows an equilibrium chiral symmetry breaking at low temperatures. The kinetic Monte Carlo simulation of crystal growth, however, demonstrates that Ostwald ripening is a very slow process with a characteristic time proportional to the system size: The dynamics is nonergodic. It is then argued that by incorporating grinding dynamics, homochirality is achieved in a short time, independent of the system size. Grinding limits cluster sizes to a certain range independent of system size and at the same time keeps the supersaturation so high that population numbers of average-sized clusters grow. If numbers of clusters for two types of enantiomers differ by chance, the difference is amplified exponentially and the system rapidly approaches the homochiral state. Relaxation time to the final homochiral state is determined by the average cluster size. We conclude that the system should be driven and kept in a nonequilibrium state to achieve homochirality.

View full abstract

We investigate the charge and spin susceptibilities in the vicinity of a charge-ordering transition point on the basis of a two-dimensional extended Hubbard model with nearest-neighbor Coulomb interaction on a square lattice. In order to evaluate the leading contribution of vertex corrections, we construct a new iterative method generating a series of non-skeleton diagrammatic conserving approximations, and utilize the first approximation for the calculation of response functions. This approximation can determine a charge-ordering transition point and static susceptibilities on an equal footing in a sense that the compressibility and spin-susceptibility sum rules are satisfied. As a prominent effect of vertex corrections, we find that the uniform spin susceptibility is enhanced owing to charge fluctuations that develop toward the charge-ordering transition. We also discuss the experimental relevance of our theory for organic conductors, together with its connection to Landau’s Fermi-liquid theory.

View full abstract

Using the time-dependent Hartree–Fock theory (TDHF), we study the ground states of dipolar fermion gases in a spherical trap. It is found that deformed configurations which have prolate shape both in real and momentum space are energetically favored and that when the dipolar interaction is not very strong, the deformation of the gases shows a prominent shell structure at closed oscillator-shell configurations. The quadrupole and breathing modes are also studied in TDHF and it is found that these modes are little affected by the dipolar interaction when the gases have the closed-shell configurations. The effects of the two-body correlations on the ground-state properties and also on the damping of the quadrupole mode are also investigated using a time-dependent density-matrix formalism. It is found that the effects of the two-body correlations are not significant for the closed-shell configurations.

View full abstract

We have specified the mechanism of suppressed electron emission from surfaces bombarded by fast cluster ions. From key information obtained from a comparison of the electron emissions for insulator KCl and conductor graphite, we concluded that the suppression is predominantly caused by the disturbance of the electron transport by the electric potential generated by moving cluster atoms. The possible shift from suppressed emission to enhanced emission of electrons as cluster speed increases is also discussed in relation to that in the case of cluster stopping power.

View full abstract

For dissipation-free photon–photon interaction at the single photon level, we analyze one-photon and two-photon transitions induced by photon pairs in three-level atoms using two-photon wave functions. We show that two-photon absorption can be substantially enhanced by adjusting the time correlation of photon pairs. We study two typical cases: a Gaussian wave function and a rectangular wave function. In the latter, we find that under special conditions one-photon transition is completely suppressed, while two-photon transition is maintained with a high probability.

View full abstract

A modified guiding-centre fundamental 1-form with strong E×B flow is derived by the phase space Lagrangian Lie perturbation method. Since the symplectic part of the derived 1-form is the same as the standard one without the strong E×B flow, it yields the standard Lagrange and Poisson brackets. Therefore the guiding-centre Hamilton equations keep their general form even when temporal evolution of the E×B flow is allowed. Compensation of keeping the standard symplectic structure is paid by complication of the guiding-centre Hamiltonian. However, it is possible to simplify the Hamiltonian in well localised transport barrier regions like a tokamak edge in a high confinement regime and an internal transport barrier in a reversed shear tokamak. The guiding-centre Vlasov and Poisson equations are derived from the variational principle. The conserved energy of the system is obtained from the Noether’s theorem. Correspondence to low-frequency fluid equations is shown.

View full abstract

In order to investigate the Li⁺ ionic diffusion and the electronic states in a mixed conductor β-LiGa with high Li⁺ ionic diffusibility and electron/hole conductivity, ⁷Li NMR linewidth and spin–lattice relaxation measurements have been performed in 44.0, 47.0, and 50.0 at. % Li β-LiGa samples at 10.03 MHz in the temperature range between 10 and 320 K. The onset temperature T_MN=70 K of the motional narrowing in 50.0 at. % sample has been determined from the temperature dependence of the linewidth. The Li⁺ ionic diffusion is found to contribute to the spin–lattice relaxation rate 1⁄T₁ down to ∼0.5 T_MN even below T_MN where the motional narrowing does not occur. The high diffusibility of Li⁺ ions has been proved from a microscopic point of view. At low temperatures, the relations 1⁄T₁T=3.5×10⁻⁴, 3.8×10⁻⁴, and 5.1×10⁻⁴ s⁻¹ K⁻¹ are observed in 44.0, 47.0, and 50.0 at. % Li samples, respectively. The density of states of conduction electrons at the Fermi level in these compounds becomes higher with increasing Li content, which is consistent with the predictions by band calculations.

View full abstract

We measured the thermal expansion of the valence fluctuating phase of SmS (golden SmS) to construct a pressure vs temperature phase diagram. The obtained phase diagram is characterized by three lines. One is a crossover line that divides the paramagnetic phase into two regions. The other two lines correspond to a second-order Néel transition and a first-order Néel transition. The crossover line appears to emerge from a tricritical point that separates the first-order Néel transition from the second-order one. We argue that a valence jump occurs at the border of antiferromagnetism.

View full abstract

A structural study of layered compounds Ni_xTiS₂ (x=0.30, 0.37, and 0.63) by X-ray and electron diffraction analyses was performed to investigate both the ordered atomic arrangement and disordering behavior of intercalated Ni atoms, and the nature of the chemical bond from the electron density distribution obtained by the maximum entropy method. The 2a×2a×2c and \\sqrt3a×\\sqrt3a×2c superstructures of the Ni atoms are observed for x=0.30 and 0.37 at room temperature, and they disappear at 420 and 425 K with second-order-like behavior. A partial 2a×2a×2c ordered arrangement of the Ni atoms is observed at room temperature and disappears at approximately 650 K for x=0.63. The electron density distribution reveals that the bonding between Ni and S atoms in the van der Waals gap layer has a covalent nature. Such characteristic behavior decreases the interlayer lattice spacing after intercalating Ni atoms into TiS₂.

View full abstract

The NMR spectrum and spin–lattice relaxation times, T₁, of the ¹H, ²H, and ⁷Li nuclei in LiNH₄SO₄ and LiND₄SO₄ crystals were obtained, and from these measurements, the rotational correlation times were calculated. In addition, the activation energies for the reorientation of ¹H, ²H, and ⁷Li were obtained for each phase. For both types of crystal, each curve of T₁ versus inverse temperature exhibited a minimum value, which is attributed to the effects of reorientational motion. From the ¹H, ²H, and ⁷Li T₁ curves, we conclude that the relaxation process is affected by molecular motion, as described by the Bloembergen–Purcell–Pound theory. The differences in activation energy among the various phases indicate that the ¹H, ²H, and ⁷Li ions are significantly affected by the transitions between phases. The present results indicate that the contributions from the Li–O–S of the skeleton and the dipole moments of the ammonium groups change discontinuously at each phase transition temperature. This study furthers our understanding of the roles of Li⁺, NH₄⁺, and ND₄⁺ ions in the phase transformation process in LiNH₄SO₄ and LiND₄SO₄ crystals.

View full abstract

Superconductivity of quasi-one-dimensional organic conductors with a quarter-filled band is investigated using the two-loop renormalization group approach to the extended Hubbard model for which both the single electron hopping t_⊥ and the repulsive interaction V_⊥ perpendicular to the chains are included. For a four-patches Fermi surface with deviations to perfect nesting, we calculate the response functions for the dominant fluctuations and possible superconducting states. By increasing V_⊥, it is shown that a d-wave (singlet) to f-wave (triplet) superconducting state crossover occurs, and is followed by a vanishing spin gap. Furthermore, we study the influence of a magnetic field through the Zeeman coupling, from which a triplet superconducting state is found to emerge.

View full abstract

Family behavior of excitons is studied in carbon nanotubes based on a k·p scheme. Effects responsible for family pattern are characterized by two parameters β representing higher-order terms in the effective Hamiltonian responsible for trigonal warping and p representing amount of effective flux due to curvature and lattice distortion. These parameters are determined through comparison with empirical formula of the lowest exciton and experiments of two-photon absorption giving excited exciton states. The observed family pattern can be reproduced much better for the first gap than for the second gap.

View full abstract

The absorption spectra of CdI₂ molecules adsorbed through a vapor phase into zeolite Na-FAU are investigated at liquid nitrogen temperature by changing the loading density L_d of CdI₂ per supercage from a dilute condition of 0.1 to a saturated condition of 5.5. The theoretical absorption spectrum of a CdI₂ molecule is also derived using the wave functions obtained from a relativistic self-consistent-field discrete-variational Xα calculation. A predominant band is observed at 5.8 eV together with sub-bands at 5.0 and 5.5 eV at L_d=0.1. The comparison between the experimental and theoretical spectra clarifies that these bands originate from the electronic transitions of an isolated CdI₂ molecule. Moreover, it suggests that the CdI₂ molecule interacts with zeolite. Another three bands appear at 4.8, 5.3, and 6.0 eV at L_d=2.0 when L_d increases, which are attributed to a CdI₂ two-molecule cluster. The lowest absorption band continuously shifts toward the lower-energy side with an increase in L_d between 2.0 and 5.5. This shift is explained by the quantum size effect on photoexcitation energy.

View full abstract

The first-principles band calculation of an itinerant cerium compound CeCoSi₂ is performed using a relativistic LAPW method with exchange and correlation potentials within a local-density approximation (LDA). The c–f hybridized bands with a large dispersion form two-dimensional Fermi surface (FS). The FS formation and the band dispersion including the contribution from each orbital obtained by the LDA calculation are in a qualitatively agreement with Ce 3d–4f resonant angle-resolved photoemission spectra. The LDA calculation is a good starting point for the investigation of the electronic structure of the heavy-fermion Ce compounds.

View full abstract

The mechanism of how critical end points of the first-order valence transition (FOVT) are controlled by a magnetic field is discussed. We demonstrate that critical temperature is suppressed to be a quantum critical point (QCP) by a magnetic field. This results explain the field dependence of the isostructural FOVT observed in Ce metal and YbInCu₄. Magnetic field scan can make the system reenter in a critical valence fluctuation region. Even in intermediate-valence materials, the QCP is induced by applying a magnetic field, at which magnetic susceptibility also diverges. The driving force of the field-induced QCP is shown to be a cooperative phenomenon of the Zeeman effect and the Kondo effect, which creates a distinct energy scale from the Kondo temperature. The key concept is that the closeness to the QCP of the FOVT is vital in understanding Ce- and Yb-based heavy-fermions. This explains the peculiar magnetic and transport responses in CeYIn₅ (Y=Ir, Rh) and metamagnetic transition in YbXCu₄ for X=In as well as the sharp contrast between X=Ag and Cd.

View full abstract

The third-order nonlinear susceptibility χ₃ in the paraelectric phase of finite ferroelectrics is theoretically studied on the basis of the Tilley–Zeks model. Using exact polarization profiles under an applied field, exact expressions of χ₃ can be derived. It has turned out that χ₃ in finite systems diverges as (α−α_c)⁻⁴ similarly to that in infinite systems, where α and α_c represent the temperature and transition temperature, respectively.

View full abstract

Single-layer and bilayer graphene are new classes of two-dimensional electron systems with unconventional band structures and valley degrees of freedom. The ground states and excitations in the integer and fractional quantum Hall regimes are investigated on torus and spherical geometries using the density matrix renormalization group (DMRG) method. At nonzero Landau level indices, the ground states at the effective filling factors 1, 1/3, 2/3, and 2/5 are valley polarized both in single-layer and bilayer graphene. We examine the elementary charge excitations that could couple with the valley degrees of freedom (so-called valley skyrmions). The excitation gaps are calculated and extrapolated to the thermodynamic limit. The largest excitation gap at the effective filling 1/3 is obtained in bilayer graphene, which is a good candidate for the experimental observation of the fractional quantum Hall effect.

View full abstract

The charge transport properties of a partially oxidized salt composed of Co(Pc)(CN)₂ units with a typical one-dimensional electronic system have been suggested to be determined by charge disproportionation. The current–voltage (I–V) characteristics show nonlinear behavior at low temperature, which is suppressed by applying pressure. The observed nonlinearity is considered to result from the electric-field-induced delocalization of carriers in the charge disproportionation state. In the isomorphous magnetic Fe(Pc)(CN)₂ system, the nonlinear behavior is observed at higher temperatures, suggesting that charge disproportionation is more developed in a system with local moments. The threshold voltage of negative differential resistance decreases when an external magnetic field is applied, confirming that the localization of charge carriers is released by magnetic field application.

View full abstract

We report on the normal-state properties of a superconducting type-III clathrate compound Ba₂₄Ge₁₀₀ investigated by ⁷³Ge-NMR at cage sites and by ^135⁄137Ba-NMR at guest sites at ambient pressure (P=0) and 2.7 GPa. The measurements of the nuclear spin-lattice relaxation rate 1⁄T₁ have revealed a significant increase in 1⁄T₁T of approximately 200 K at P=0, which may be relevant to the rattling motion of Ba atoms in Ge cages. This increase in 1⁄T₁T at high temperatures is markedly suppressed at P=2.7 GPa. As a result of the suppression of the successive structural transformations, the 1⁄T₁T=const. in a low-temperature regime is increased, proving the increase in the density of states at the Fermi level, N(E_F), at P. It is this increase in N(E_F) that increases the superconducting transition temperature T_c from 0.24 K at P=0 to 3.8 K at P∼2.7 GPa.

View full abstract

One of the most exciting discoveries in strongly correlated systems has been the existence of a superconducting dome on heavy fermions close to the quantum critical point where antiferromagnetic order disappears. It is hard even for the most skeptical not to admit that the excitations which bind the electrons in the Cooper pairs have a magnetic origin. As a system moves away from an antiferromagnetic quantum critical point, (AFQCP) the correlation length of the fluctuations decreases and the system goes into a local quantum critical regime. The attractive interaction mediated by the non-local part of these excitations vanishes and this allows to obtain an upper bound to the superconducting dome around an AFQCP.

View full abstract

Nonlinear bulk current conduction under a trap density gradient with ohmic contacts is mathematically formulated, and semianalytical and rigorous numerical solutions are presented. The conduction under shallow traps is characterized mainly by the magnitude of the trap density gradient (G), the energy level of the trap, and the ratio of the free to trapped carrier density (Θ). Unidirectionality, i.e., rectification appears for G>1 with not too large Θ. For G>>1 the relationship J∝V^m (1<m≤2, typically m≈1.5) is observed at a relatively low V (J: current, V: voltage). In the case of deep traps, the trap-filled-limit-to-trap-free transition voltage depends critically on the bias polarity, which yields a large rectification with J–V characteristics strikingly similar to those of typical diodes. The present theory excellently reproduces experimental diodelike J–V characteristics and demonstrates that bulk-limited conduction under a trap density gradient yields rectification without relying on surface potential barriers. In addition, a weak photovoltaic effect is predicted. The present results suggest that fractions of diodelike J–V characteristics, which have been considered due to the surface, can be reexamined on the basis of the nonlinear bulk conduction under trap density gradient.

View full abstract

The magnetic properties of organic conductor (TMTTF)₂X (X=Br, PF₆, and SbF₆), where TMTTF is tetramethyltetrathiafulvalene, were examined by electron spin resonance (ESR) spectroscopy, X-ray diffraction (XRD) of the single crystals, and quantum–chemical calculation of the g-tensor. In the case of salts with bulky counter anions such as the PF₆ and SbF₆, an anomalous temperature dependence of the g-tensor was observed in the temperature range from 20 to 296 K. This anomalous behavior of the g-tensor signifies the rotation of the principal axes as well as the shift of the principal values. The g-tensor of the Br salt is, however, temperature independent. No remarkable change in the intra-molecular structure as a function of temperature was observed for all salts. On the other hand, the distance between TMTTF and counter-anion molecules obviously decreases as the temperature decreases for the PF₆ and SbF₆ salts, while thermal contraction is not remarkable for the Br salt. In order to clarify the origin of the anomalous behavior of the g-tensor, we investigated the possibility of deformation of the wave-function by the counter-anion potentials using a quantum–chemical calculation for the actual crystal structures measured at low-temperatures. In this paper, we describe the first direct observation of the deformation of the frontier orbital by the counter anion potential for organic conductors. The intra-molecular spin-distribution as a function of temperature is also discussed from the microscopic point of view.

View full abstract

A semiclassical model of the intrinsic spin-Hall effect (SHE) is presented which is relevant for a wide class of systems, including the two-dimensional electron gas with Rashba SOC. We start with a fully quantum mechanical treatment, introducing a gauge field formulation of the effect. Using this formulation as a basis, we construct an intuitive picture of the SHE, and derive general expressions for the spin-Hall current and conductivity. Finally, we propose an analogous effect in bilayer graphene systems.

View full abstract

The ultrafast nonlinear optical responses of all-trans-β-carotene have been investigated by femtosecond pump–probe spectroscopy under a nonresonant excitation condition to the optically allowed S₂ (1¹B_u⁺) state. Instantaneous signals were ascribed to coherent nonlinear optical effects in a three-level electronic system in β-carotene without any convolution of the population of excited states. The temporal responses of the coherent signals were identified with an instrumental response function of the system. At a longer delay (>1 ps), transient signals due to the dark S₁ (2¹A_g⁻) state induced by two-photon excitation and to the lowest triplet T₁ state induced by four-photon excitation were well temporally and spectrally resolved. The S₁ decay time and T₁ formation time were determined to be 9.5 and 20 ps, respectively. We show the availability of multiphoton excitation spectroscopy for clarifying the ultrafast relaxation kinetics of a complex system.

View full abstract

The optical absorption spectrum for light with electric field perpendicular to a bilayer graphene is shown to be quite different from that for parallel polarization. This arises partly from the difference in the selection rule and from the important depolarization effect due to induced polarization. The depolarization effect is most prominent for the sharp transition between the lowest to the first excited conduction band, which are almost parallel to each other.

View full abstract

A new TMTTF (tetramethyl-tetrathia-fulvalene)-family salt, (TMTTF)₂TaF₆, which has the largest octahedral (O_h) symmetry counter anion among the various salts in the TMTTF family, was prepared. X-ray, static magnetic susceptibility, electron spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements were carried out in order to investigate the electronic state of (TMTTF)₂TaF₆. The unit-cell volume of (TMTTF)₂TaF₆ is larger than that of (TMTTF)₂MF₆ (M=P, As, and Sb). (TMTTF)₂TaF₆ shows the highest charge-ordering phase transition temperature (T_CO∼175 K) among TMTTF salts with the O_h-symmetry counter anion. These facts indicate that (TMTTF)₂TaF₆ is located on the most negative side in the generalized phase-diagram for TMTCF family salts. (TMTTF)₂TaF₆ undergoes an antiferromagnetic transition around 9 K. It turned out the phase diagram needs to be modified.

View full abstract

We study the influence of the anisotropic exchange interaction (AEI) on exciton coherence by using four-wave-mixing spectroscopy. The exciton fine structure (FS) of uniaxially-strained GaN exhibits linearly-polarized FS-splitting at low temperatures, which enables us to resolve the difference between the coherent dynamics of the FS; the FWM transient in each FS exhibits extended/contracted dephasing compared with that obtained in uniformly-strained GaN. The origin of this asymmetry is associated with the anisotropic pseudospin induced by AEI.

View full abstract

We investigate the temperature region in which a Tomonaga–Luttinger liquid (TLL) description of the charge sector of the one-dimensional Hubbard model is valid. By using the thermodynamic Bethe ansatz method, electron number is calculated at finite temperatures and fixed chemical potential. We observe maximum electron number as a function of temperature close to the chemical potential of the upper critical value that corresponds to half filling. As the chemical potential approaches the upper critical value from below, the temperature (T_M) at which the electron number shows its maximum asymptotically approaches a universal relation. We show that, below the energy corresponding to T_M, the charge excitation spectrum nearly obeys a linear dispersion relation. The results demonstrate that T_M marks the important temperature below which TLL is realized.

View full abstract

Magnetic susceptibility and transport properties of an itinerant helimagnet FeAs are studied in single-crystalline samples of this binary compound. A kink due to the magnetic transition is observed in the temperature dependences of both the susceptibility and the resistivity. The Hall coefficient shows a reentrant sign change with temperature, signifying a complicated competition between multiple bands. In the helical ordered state, we found an unexpected formation of some anisotropic antiferromagnetic domains, which is reflected in a spin-glass like behavior observed only in the c-axis magnetic susceptibility.

View full abstract

The magnetic and transport properties of single-crystal YbPtGe with the TiNiSi-type orthorhombic structure are presented. It is revealed that YbPtGe is a ferromagnet with T_C=5.4 K. Although both of its paramagnetic and ferromagnetic states show uniaxial anisotropy, its easy axis changes from the a-axis in the paramagnetic state to the c-axis below T_C. Magnetizations at 2 K along the a-, b-, and c-axes saturate to constant values of 1.5 μ_B for 6 kOe, 0.55 μ_B for 2 kOe, and 1.1 μ_B for 1 kOe, respectively. The magnetic contribution to the specific heat shows a Schottky anomaly centered at 90 K due to the crystalline electric field (CEF) effect that splits the eight-hold J-multiplet of the Yb³⁺ ion into four doublets. The Sommerfeld coefficient γ is estimated to be 209 mJ/(mol·K²) below T_C. The magnetic entropy released up to T_C is 52% of that of Rln2, expected for the doublet ground state. Therefore, YbPtGe is a middle-classed heavy-fermion compound with a ferromagnetic transition.

View full abstract

A theory of charge density wave (CDW) ordering and the metal–insulator transition in skutterudite PrRu₄P₁₂ is presented. We introduce a simplified model Hamiltonian which involves an interaction of conduction electrons with localized f electrons occupying singlet–triplet crystal field levels, and calculate the solutions of the coherent potential approximation. It is shown that the fluctuation of the f-electron occupancy plays very important roles in the mechanism of unusual transport properties. On the one hand, the fluctuation leads to a severe suppression of the quasi-particle gap in the CDW phase and resultingly produces a characteristic temperature dependence of the resistivity. On the other hand, the fluctuation induces breaking of the particle–hole symmetry, giving a drastic enhancement of the Hall conductivity in the CDW phase. The mechanism of sign reversal of the Hall conductivity is discussed in connection with the properties of the fluctuation. Comparisons of the theoretical results with the available experimental results are discussed.

View full abstract

We have developed an ab initio path integral molecular dynamics method based on the fragment molecular orbital method. This “FMO-PIMD” method can treat both nuclei and electrons quantum mechanically, and is useful to simulate large hydrogen-bonded systems with high accuracy. After a benchmark calculation for water monomer, water trimer and glycine pentamer have been studied using the FMO-PIMD method to investigate nuclear quantum effects on structure and molecular interactions. The applicability of the present approach is demonstrated through a number of test calculations.

View full abstract

For memory effect in drying pastes, reported by Nakahara and Matsuo and referred to as Nakahara effect, a possible clarification is given in terms of a macroscopic theory that predicts creation and retainment of tension in the direction of the external forcing. While the previous version of this theory was restricted to the two-dimensional stress field, now the theory is extended to the three-dimensional cases, which predicts not only residual tension in the direction of external forcing but also residual pressure in the perpendicular direction, thus confirming the presence of anisotropy in the horizontal plane.

View full abstract

The electric field around positively charged biological specimens is studied by electron holography. By the amplitude reconstruction process for holograms, the orbits of electron-induced secondary electrons are clarified on the nanometer scale. It is found that the stationary orbit of secondary electrons can be directly located without disturbing their motions under the condition that the current of secondary electrons in stationary orbit is considerably larger than that of incident electrons. The experimental conditions for the induction of the stationary orbit of secondary electrons are discussed, and furthermore the theoretical basis of the orbital location of secondary electrons through electric field visualization is discussed in the framework of quantum mechanics.

View full abstract