A generalization of fluctuation theorems in stochastic processes is proposed. The new theorem is written in terms of posterior probabilities, which are introduced via Bayes’ theorem. In conventional fluctuation theorems, a forward path and its time reversal play an important role, so that a microscopically reversible condition is essential. In contrast, the microscopically reversible condition is not necessary in the new theorem. It is shown that the new theorem recovers various theorems and relations previously known, such as the Gallavotti–Cohen-type fluctuation theorem, the Jarzynski equality, and the Hatano–Sasa relation, when suitable assumptions are employed.
We study the time-dependent dynamical properties of two-component ultracold fermions in a one-dimensional optical superlattice by applying the adaptive time-dependent density matrix renormalization group to a repulsive Hubbard model with an alternating superlattice potential. We clarify how the time evolution of local quantities occurs when the superlattice potential is suddenly changed to a normal one. For a Mott-type insulating state at quarter filling, the time evolution exhibits a profile similar to that expected for bosonic atoms, where correlation effects are less important. On the other hand, for a band-type insulating state at half filling, the strong repulsive interaction induces an unusual pairing of fermions, resulting in some striking properties in time evolution, such as a paired fermion co-tunneling process and the suppression of local spin moments. We further address the effect of a confining potential, which causes spatial confinement of the paired fermions.
We have observed the circular-polarization-dependent intensities of forbidden (0,0,6n±2) reflections at the Cu K absorption edge in the chiral crystal CsCuCl3. The intensities for the right- and left-handed circularly-polarized incident beams interchange in CsCuCl3 crystals with opposite chiralities. The result is well explained by a model calculation based on the anisotropic tensor susceptibility of Cu2+ ions, taking only the electric-dipole (E1) process into account. This technique can determine crystallographic chirality by measuring the flipping ratio for only one reflection, in principle.
We report the first observation of the pressure effect on the first-order transition at Tp=7.5 K in the β-pyrochlore oxide superconductor KOs2O6 by specific-heat measurement. The peak in the specific heat at Tp disappeared at a low pressure of 0.02 GPa. With increasing pressure up to 0.02 GPa, the coefficient of the T5 dependence of the specific heat increases by 30%. This finding implies that low-energy excitations of phonons are enhanced by the suppression of the first-order transition. However, the specific-heat jump at Tc is unchanged with pressure up to 1 GPa, indicating that the strong coupling superconductivity is rather robust under pressure.
Spin susceptibility of superfluid 3He-B film with specular surfaces is calculated on the basis of the self consistent order parameter. It is shown that, when the magnetic field is applied in a direction perpendicular to the film, the suseptibility is significantly enhanced by the contribution from the surface bound states. No such enhancement is found for the magnetic field parallel to the film. A simplified model with spatially constant order parameter is used to elucidate the magnetic properties of the surface bound states. The Majorana nature of the zero energy bound state is also mentioned.
We investigate transport properties of the junctions in which the graphene nanoribbon with the zigzag shaped edges consisting of the N legs is sandwiched by the two normal metals by means of recursive Green’s function method. The conductance and the transmission probabilities are found to have the remarkable properties depending on the parity of N. The singular behaviors close to E=0 with E being the Fermi energy are demonstrated. The channel filtering is shown to occur in the case with N=even.
We investigate the in-plane transport properties of the electron-doped iron arsenide superconductor Ba(Fe1−xCox)2As2 (0≤x≤1). We observe a non-Fermi-liquid behavior characterized by an incoherent resistivity and a low-temperature increase in Hall coefficient (RH) at x≤0.14. We discuss this feature in terms of antiferromagnetic fluctuations. We find that the effective carrier density per Fe/Co atom neff=−V⁄(eRH) (V is the volume per Fe/Co atom and e is the elementary electric charge) at 250 K exhibits a nonmonotonic x dependence with a minimum value at x∼0.1, which is considered to reflect the disappearance of hole Fermi surfaces. We discuss the interrelationships among superconductivity, magnetism, and band nesting.
The electronic properties of an oxypnictide, LaFeAsO, pressurized in a diamond-anvil cell, were investigated by 57Fe Mössbauer spectroscopy and electrical resistance measurements at pressures up to 24 and 35 GPa, respectively. The Néel temperature gradually decreased from ∼140 K at 0.1 MPa to ∼50 K at 20 GPa, and fell below 8 K or disappeared at 24 GPa. The hyperfine field at 8 K decreased from 5.3 T at 0.1 MPa to 2.2 T at 20 GPa. On the other hand, the onset of superconductivity occurred at ∼9 K at 2 GPa. The superconductivity peaked at ∼21 K at 12 GPa, and then began a perceptible decline, disappearing at ∼35 GPa. This suggests that suppression of antiferromagnetic order plays an important role in the emergence of pressure-induced superconductivity.
The 63Cu nuclear quadrupole resonance (NQR) frequency νQ in the multilayered cuprates is calculated in the cluster model by the exact diagonalization method. The charge imbalance between the outer CuO2 plane (OP) with apical oxygen OA and the inner plane (IP) without OA in three-layered Tl2Ba2Ca2Cu3O10 is estimated by comparing our results with the experimental νQ. In Tl-based cuprates with more than three layers, we predict a large enhancement of the splitting of νQ between OP and IP by taking into account the reduction in bond length between Cu and OA and the resulting increase in charge transfer energy. Our results show that the NQR frequency is a useful quantity for estimating the charge imbalance in the multilayered cuprates.
We have succeeded in growing single crystals of orthorhombic CeT2Al10 (T=Fe, Ru, and Os) by the Al self-flux method for the first time, and measured their electrical resistivity ρ at pressures of up to 8 GPa, magnetic susceptibility χ, and specific heat C at ambient pressure. The results indicate that CeT2Al10 belongs to the heavy fermion compounds. CeRu2Al10 and CeOs2Al10 show similar phase transitions at T0=27.3 and 28.7 K, respectively. The temperature dependences of ρ, χ, and C in the ordered phases are well described by the thermally activated form, suggesting that a partial gap opens over the Fermi surfaces below T0. When pressure is applied to CeRu2Al10, T0 disappears suddenly between 3 and 4 GPa, and CeRu2Al10 turns into a Kondo semiconductor, and then into a metal. The similarity of CeT2Al10 compounds under respective pressures suggests a scaling relation by some parameter controlling the unusual physics of these compounds.
We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe2−xCoxAs2 (Ba122) (x=0,0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electron-doped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating a three-dimensional (3D) electronic structure. This observation suggests that antiferromagnetism and superconductivity in the pnictides have to be described in terms of an orbital-dependent 3D electronic structure, where FS nesting is not necessarily strong.
We report the resistivity measurements under pressure of two Fe-based superconductors with a thick perovskite oxide layer, Sr2VFeAsO3 and Sr2ScFePO3. The superconducting transition temperature Tc of Sr2VFeAsO3 markedly increases with increasing pressure. Its onset value, which was Tconset=36.4 K at ambient pressure, increases to Tconset=46.0 K at ∼4 GPa, ensuring the potential of the “21113” system as a high-Tc material. However, the superconductivity of Sr2ScFePO3 is strongly suppressed under pressure. The Tconset of ∼16 K decreases to ∼5 K at ∼4 GPa, and the zero-resistance state is almost lost. We discuss the factor that induces this contrasting pressure effect.
The temperature–magnetic field (T–H) phase diagram of YbRh2Si2 in the vicinity of its quantum critical point is investigated by low-T magnetization measurements. Our analysis reveals that the energy scale T\\star(H), previously related to the Kondo breakdown and terminating at 0.06 T for T→0, remains unchanged under pressure, whereas the antiferromagnetic critical field increases from 0.06 T (P=0) to 0.29 T (P=1.28 GPa), resulting in a crossing of TN(H) and T\\star(H). Our results are very similar to those on Yb(Rh1−xCox)2Si2, proving that the Co-induced disorder can not be the reason for the detachment of both scales under chemical pressure.
To investigate the relationship between superconductivity and low-energy spin fluctuations in the iron-based superconductor FeSe0.5Te0.5, we have conducted 125Te NMR measurements at ambient pressure and 2 GPa. As the superconducting transition temperature Tc is increased by applying pressure, the nuclear spin–lattice relaxation rate divided by temperature, 1⁄T1T, shows the development of antiferromagnetic fluctuations upon lowering temperature toward Tc. This supports the scenario that spin fluctuations promote superconducting pairing. The depressed Knight shift 125K and the absence of a coherence peak in 1⁄T1 below Tc are consistent with spin-singlet superconducting pairing with an anisotropic order parameter. In the normal metallic state, the comparison between the uniform and dynamic spin susceptibilities suggests the existence of a Fermi level located near the singularity of the band structure.
A double-exchange mechanism for the emergence of ferromagnetism in cubic uranium compounds is proposed on the basis of a j–j coupling scheme. The idea is orbital-dependent duality of 5f electrons concerning itinerant Γ8− and localized Γ7− states in the cubic structure. Since orbital degree of freedom is still active in the ferromagnetic phase, orbital-related quantum critical phenomenon is expected to appear. In fact, odd-parity p-wave pairing compatible with ferromagnetism is found in the vicinity of an orbital ordered phase. Furthermore, even-parity d-wave pairing with significant odd-frequency components is obtained. A possibility to observe such exotic superconductivity in manganites is also discussed briefly.
We measured the high-field magnetization and de Haas–van Alphen (dHvA) oscillations for YbIr2Zn20 with a cubic crystal structure, together with the electrical resistivity and magnetic susceptibility. The magnetic susceptibility χ with an effective magnetic moment of Yb3+ becomes temperature-independent at low temperatures, with a broad peak at Tχmax=7.4 K for H||‹110›. The corresponding magnetization indicates a metamagnetic transition at Hm=120 kOe, consistent with a Tχmax vs Hm relation in the Ce- and U-based heavy fermion compounds. The large cyclotron masses of 4–27 m0 are detected in the dHvA experiment, and are found to be reduced at magnetic fields higher than Hm=120 kOe. The resistivity follows the Fermi liquid relation ρ=ρ0+AT2 under magnetic field, and the \\sqrtA value is also found to have a maximum at Hm as a function of magnetic field. From the present experimental results, together with the results of 4f-itinerant energy band calculations, the 4f electrons are found to contribute to the heavy fermion state in YbIr2Zn20.
Performing the first-principles calculations, we investigate the anisotropy in the superconducting state of iron-based superconductors to gain an insight into their potential applications. The anisotropy ratio γλ of the c-axis penetration depth to the ab-plane one is relatively small in BaFe2As2 and LiFeAs, i.e., γλ∼3, indicating that the transport applications are promising in these superconductors. On the other hand, in those having perovskite type blocking layers such as Sr2ScFePO3 we find a very large value, γλ\\gtrsim200, comparable to that in strongly anisotropic high-Tc cuprate Bi2Sr2CaCu2O8−δ. Thus, the intrinsic Josephson junction stacks are expected to be formed along the c-axis, and novel Josephson effects due to the multi-gap nature are also suggested in these superconductors.
The 27Al-NQR/NMR measurements of CeRu2Al10 were carried out to clarify the unusual phase transition at T0=27 K. Distinct NQR peaks associated with five nonequivalent Al sites have been observed at T>T0, and each peak is successfully assigned to their respective Al sites. Below the transition temperature T<T0, each peak simply splits into two peaks except for one site. This indicates that the phase transition is not accompanied by magnetic order, but is presumably associated with the onset of structural transition with lowering symmetry. The nuclear spin–lattice relaxation rate 1⁄T1 suggests a local moment picture above T*∼60 K, and the development of Kondo coherence toward T0. Below T0, 1⁄T1 shows a gaplike decrease with a gap magnitude of 106 K, being consistent with the macroscopic measurements. The Korringa term below 10 K after the gaplike decrease suggests a gap opening over a portion of the Fermi surface.
We study the ground state properties of ultracold fermions trapped in a one-dimensional double-well optical lattice by using the density-matrix renormalization group method. The system is described by an extended Hubbard model with alternating hopping integrals and an external harmonic confining potential. We clarify the characteristic features of the metal–insulator phase transition originating from the double-well structure of the lattices. Several different types of insulating regions coexist when the number of atoms at each site is an integer or a half integer. We found that each insulating phase except for a band insulator exhibits rather large local density fluctuations, reflecting the strong dimerization of atoms within the unit cells of double-well lattices. The phase characteristics are elucidated in detail by investigating the profiles of the local density of atoms, the local density (spin) fluctuations, the double-occupancy probability and the spatially extended sp in correlations.
The generalized Gumbel distribution function (GGDF) has been conjectured to describe fluctuating order parameters of a large class of finite critical systems. We study probability distribution functions (PDFs) of rescaled order parameters in finite complex networks near their critical points to clarify whether this conjecture holds for any critical system. Our numerical results show that the PDF for fitness-model networks near the critical point, which has the scale-free property, cannot be described by the GGDF with the real parameter a equal to π⁄2 while the PDF for non-scale-free Erdos–Rényi random graphs obeys it. We also discuss the origin of the discrepancy with the GGDF in the scale-free network.
We calculate the maximal Lyapunov exponent of carbon tetra fluoride using the classical molecular dynamics (MD) simulation for different temperatures. The system is composed of 108 particles interacting either through an effective intermolecular potential energy in the vicinity of its liquid–gas phase transition point. As a main result, we find that at the transition temperature Tc, the Lyapunov exponent exhibits a power-law behavior with a functional form λ=|T−Tc|−ω, where the exponent ω is 0.155. This confirms that the maximal Lyapunov exponent behaves as an exact order parameter to signal the phase transition point for a system even with the infinite particles if the effective potential energy has been used in simulation. The applied effective intermolecular potential-energy function for carbon tetra fluoride has been obtained directly from the extended law of corresponding states for viscosity data using the inversion method. Then results were used to obtain a best Morse-Split-van der Waals (MSV) potential model.
We examine scattering properties of singular vertex of degree n=2 and 3, taking advantage of a new form of representing the vertex boundary condition, which has been devised to approximate a singular vertex with finite potentials. We show that proper identification of δ and δ′ components in the connection condition between outgoing lines enables the designing of quantum spectral branch-filters.
The Stark effect of the 5s2S1⁄2–5p2P3⁄2 transition (D2 line) of Rb has been studied by high-resolution atomic-beam laser spectroscopy. The splittings of magnetic sublevels of 5p2P3⁄2 have been resolved, and the Stark shifts and splittings of the D2 line have been measured at various electric fields. The scalar polarizability of the D2 line and the tensor polarizability of the 5p2P3⁄2 level have been determined to be αs(5p2P3⁄2) − αs(5s2S1⁄2) = 136.95(89) kHz/(kV/cm)2 and αt(5p2P3⁄2) = −40.91(38) kHz/(kV/cm)2, respectively, by considering the mixing between hyperfine structure levels of 5p2P3⁄2. The precision of αt(5p2P3⁄2) has been improved by a factor of 2 compared with the previously reported values.
We make the local stability analysis of a rotating flow with circular or elliptically strained streamlines, whose rotation axis executes constant precessional motion about an axis perpendicular to itself, based on the WKB method. In the frame rotating with the precessional angular velocity, the basic flow is a steady velocity field linear in coordinates in an unbounded domain. For the case of slow precession, without strain, the growth rate takes the same value as that of Kerswell (1993) though the basic flow is different. We find that, in the absence of strain, the growth rate approaches the angular velocity of the basic flow as the precessional angular velocity is increased. The cooperative action of the weak Coriolis force and the elliptical straining field is investigated both numerically and analytically. An analysis of using the Mathieu method reveals that the elliptical instability is weakened by the precession, while the precessional instability is either enhanced or weakened depending on the orientation of the strain.
A numerical model for describing both the transient and steady-state dynamics of viscous threads falling onto a plane is presented. The steady-state coiling frequency Ω is calculated as a function of fall height H. In the case of weak gravity, Ω∝H−1 and Ω∝H are obtained for smaller and larger fall heights, respectively. When the effect of gravity is significant, the relation Ω∝H2 is observed. These results agree with the scaling laws previously predicted. The critical Reynolds number for the coiling–uncoiling transition is discussed. When the gravity is weak, the transition occurs with hysteretic effects. If the plane moves horizontally at a constant speed, a variety of meandering oscillation modes can be observed experimentally. The present model can also describe this phenomenon. The numerically obtained state diagram for the meandering modes qualitatively agrees with the results of the experiment.
We systematically investigate the collisional heating process of space-charge-dominated coasting ion beams in a storage ring using the molecular dynamics simulation technique. To evaluate the heating rate over the whole temperature range, we start from an ultralow-emittance state where the beam is Coulomb crystallized, apply perturbation to it, and follow the emittance evolution until the beam comes to a regular high-temperature state. The dependence of the heating behavior on various machine and beam parameters, such as the line density, betatron tune, kinetic energy, mass number, and charge state of ions, is explored systematically. The parameter dependence of the heating behavior can be combined, in several cases, into the Coulomb coupling constant. An approximate formula is given for the magnitude of emittance at which the collisional heating is maximized.
The kinetic description of nonlinear plasma turbulence is discussed for Vlasov plasmas with multiple species. Kadomtsev’s formalism is reviewed, and the foundation from the Mori method of statistical physics is discussed. The coherent nonlinear term and nonlinear noise term are discussed. In addition to the diagonal term in memory function (eddy damping term), the off-diagonal term in the memory function and nonlinear noise are introduced simultaneously. The off-diagonal term in memory function affects the dynamics as the new nonlinear force, so that a nonlocal interaction in phase space occurs in wave-particle interactions. A set of special solutions for the eddy damping term and off-diagonal coherent term is obtained. The access to a nonlinear stationary state by using an incoherent emission term is explained. Quasi-modes, which are driven by the incoherent emission, are shown to accumulate at a frequency of ω(k)=2ω′(k⁄2), where ω′(k) indicates the dispersion relation of the mode. The formulation of spontaneous emission recovers the Balescu–Lenard collision operator in the limit of thermal equilibrium. In the multiple-species formulation, like-particle and unlike-particle collisions are clarified.
Coupling resonances can be used to control the phase–space configuration of a charged-particle beam. Here we study a compact storage ring, which enables one to achieve a wide variety of emittance manipulations. A simple analytic model and numerical examples are given to demonstrate the fundamental features of the coupled beam motion near resonance. As a possible application of the present idea, free-electron lasers (FELs) are studied. It is shown that, by employing a nonlinear coupling resonance, the phase–space distribution of an electron beam can be optimized for high FEL gain. A three-dimensional simulation code is used to confirm that the “conditioned” electron beam from the coupling storage ring improves the performance of the subsequent FEL system.
The lattice specific heat Clat of La-based filled skutterudites LaT4X12 (T=Fe, Ru and Os; X=P, As, and Sb) has been systematically studied, and both the Debye temperature ΘD and the Einstein temperature ΘE of LaT4X12 were carefully estimated. We confirmed that a correlation exists between ΘD and the reciprocal of the square root of average atomic mass for LaT4P12, LaT4As12, and LaT4Sb12. The ΘD of filled skutterudites was found to depend mainly on the nature of the species X forming the cage. The temperature dependence of Clat⁄T3 for LaT4X12 exhibited a large broad maximum at low temperatures (10–30 K), which suggests a nearly dispersionless low-energy optical mode characterized by Einstein specific heat. Since no such broad maximum exists for the unfilled skutterudite RhP3, the low-energy optical modes are associated with vibration involving La ions in the X12 cage (the so-called “guest ion modes”). The ΘE of filled skutterudites was found to roughly correspond to the energy of low-energy guest ion optical modes. Furthermore, a good correlation was shown to exist between ΘE and rR–X−rR3+, where rR–X is the R–X distance and rR3+ is the effective ionic radius of R3+. As rR–X−rR3+ increased, ΘE was found to decrease.
On a Si(111) vicinal face near the structural transition temperature (860 °C), the 7×7 structure and 1×1 structure coexist on a terrace. The 7×7 structure is on the upper side of steps and the 1×1 structure is on the lower side of steps. The diffusion coefficient on the 1×1 structure is larger than that on the 7×7 structure. In this paper, taking account of the difference in the diffusion coefficient, we study the possibility of step instabilities, step wandering and step bunching, occuring during sublimation.
We have studied the relaxation rate of vibrational modes in damped two-dimensional graded mass lattices. The relaxation rate spectrum in the weak damping limit can be obtained analytically through a perturbation theory based on the semiclassical quantum analogue envelope function. We found dip or peak structures on the relaxation rate spectrum. The dip or peak structures can be described quantitatively by the asymptotic behavior of relaxation rate at the transition frequencies. The frequency dependence of the relaxation rate is qualitatively explained by the mode patterns of gradon modes. The validity of the analytic results is confirmed by numerical solution with weak damping. In the strong damping case, we need to retain higher-order perturbations. These results can be applied to the energy relaxation in analogous systems.
We report the results of our magnetic susceptibility, magnetization, and multi-frequency electron spin resonance (ESR) measurements on single crystals of the two-leg spin-ladder material Na2Fe2(C2O4)3(H2O)2. Magnetization processes in magnetic fields up 53 T at 1.3 K paralell (H||a) and perpendicular (H⊥a) to the leg direction show a gradual increase compared with those of the isomorphous compound Na2Co2(C2O4)3(H2O)2, which was well explained by an isolated antiferromagnetic dimer model. Thus, we calculated the magnetic susceptibilities and magnetizations on the basis of a two-leg ladder model with a fictitious spin (S) one by a quantum Monte Carlo method and a density-matrix renormalization group method, respectively. These calculated results, however, did not agree well with the experimental ones. Therefore, we regarded Na2Fe2(C2O4)3(H2O)2 as an isolated S=1 antiferromagnetic dimer with not only the anisotropy of the exchange interactions due to the truncation of spin space but also single-ion anisotropy. Consequently, we obtained good agreement between the experimental and calculated results on the magnetic susceptibilities, magnetizations, ESR resonance modes, and specific heat.
A crystalline electric field (CEF) model of localized 4f5 electrons is proposed for understanding intriguing ordering phenomena in SmRu4P12. We take the CEF quartet and doublet (pseudo-sextet) of the Hund’s rule ground state with J=5⁄2, and include intersite interactions between multipoles by the mean-field theory. The model leads to a multipole order with representation Γ5u, and shows the following features: (i) increasing transition temperature TMI with increasing magnetic field; (ii) an anomaly in specific heat and other thermodynamic quantities at lower temperature T\\star, (iii) sharpening of the anomaly at T\\star as the magnetic field increases. These behaviors reproduce salient features observed experimentally in SmRu4P12. Possible mechanisms for spontaneous magnetic moment are discussed.
We have carried out a spin-polarized-neutron study on multiferroic CuCrO2 to clarify the origin of the ferroelectricity. The neutron results demonstrate that an incommensurate proper-screw magnetic structure of CuCrO2 induces electric polarization. Not only the magnetic structure but also the oxygen location contributes to the ferroelectricity of CuCrO2. The electric polarization of CuCrO2 can be explained not by a conventional spin-current model but by a theoretical prediction proposed by Arima. The spin helicities of CuCrO2 can be reversed by the reversal of the electric field E in the multiferroic phase.
We have investigated spin-related effects on the electronic transport in a single antidot system in the vicinity of the ν=2 quantum Hall state where two edge states with different spins of the lowest Landau level are formed around the antidot. The conductance exhibits the paired h⁄2e Aharonov–Bohm (AB) oscillations. Using a tilted magnetic field technique, we have observed the evolution of the oscillations with the total magnetic field and a concomitant change in the source–drain bias dependence of the differential conductance. From the observation, we extract the effect of the Zeeman energy on the AB oscillations.
We investigated the effects of hydrostatic pressure and uniaxial strain on the spin-Peierls (SP) transition of an organic radical magnet, benzo[1,2-d:4,5-d′]bis[1,3,2]dithiazole(BBDTA)·InCl4. It has a one-dimensional coordination polymer structure along its c-axis and its SP transition occurs at 108 K. The SP transition temperature TSP decreased to 99 K at a hydrostatic pressure of 10 kbar, while it increased to 132 K at a uniaxial strain along the c-axis of 8 kbar. The pressure dependences of TSP under these two conditions were discussed by evaluating two parameters, namely, the intrachain interaction 2J⁄kB and the effective spin–lattice coupling parameter η, that are related to TSP by the equation TSP=1.6ηJ⁄kB. Under ambient pressure, the a- and c-axes of this material shortened monotonically with decreasing temperature, while the b-axis elongated below TSP. In this study, we found the correlation between η and the change in the lattice constant b. 2J⁄kB increased with increasing hydrostatic pressure and uniaxial strain, suggesting that the contraction along the c-axis does not depend on the manner of pressurization. From the evaluation of η, the observed variation in TSP is explained by the difference between the changes in b under the two pressurization conditions.
We investigate the electronic states of a one-dimensional two-orbital Hubbard model with band splitting by the exact diagonalization method. The Luttinger liquid parameter Kρ is calculated to obtain superconducting (SC) phase diagram as a function of on-site interactions: the intra- and inter-orbital Coulomb U and U′, the Hund coupling J, and the pair transfer J′. In this model, electron and hole Fermi pockets are produced when the Fermi level crosses both the upper and lower orbital bands. We find that the system shows two types of SC phases, the SC I for U>U′ and the SC II for U<U′, in the wide parameter region including both weak and strong correlation regimes. Pairing correlation functions indicate that the most dominant pairing for the SC I (SC II) is the intersite (on-site) intraorbital spin-singlet with (without) sign reversal of the order parameters between two Fermi pockets. The result of the SC I is consistent with the sign-reversing s-wave pairing that has recently been proposed for iron oxypnictide superconductors.
The ground state of the Hubbard model is studied within the constrained Hilbert space where no order parameter exists. The self-energy of electrons is decomposed into the single-site and multisite self-energies. The calculation of the single-site self-energy is mapped to a problem of self-consistently determining and solving the Anderson model. When an electron reservoir is explicitly considered, it is proved that the single-site self-energy for the ground state is that of a normal Fermi liquid even if the multisite self-energy is so anomalous that the ground state itself is not a normal Fermi liquid. Thus, the ground state is a normal Fermi liquid in the supreme single-site approximation (S3A). In the strong-coupling regime, the Fermi liquid is stabilized by the Kondo effect in the S3A and further stabilized by the Fock-type term of the superexchange interaction or the resonating-valence-bond (RVB) mechanism beyond the S3A. The stabilized Fermi liquid is frustrated as much as an RVB spin liquid in the Heisenberg model, and is a relevant unperturbed state that can be used to study a normal or anomalous Fermi liquid and an ordered state in the entire Hilbert space by the Kondo lattice theory. Even if multisite terms of higher order than the Fock-type term are considered, the ground state cannot be an insulator with a complete gap nor a Mott insulator. It can only be a gapless semiconductor even if the multisite self-energy is so anomalous that it is divergent at the chemical potential. A Mott insulator is only possible as a high-temperature phase.
Detailed analysis of hyperfine interactions in γ-Mn60Fe37Cu3 is presented. Isotropic three-dimensional Gaussian shape was introduced for modeling the distribution of hyperfine magnetic field derived from Mössbauer measurements performed in a broad temperature range. Electrical quadrupole interactions resulting from broken local symmetry, which cannot be treated as small perturbation in the vicinity of transition temperature, were treated by exact method of invariants. Obtained values of quadrupolar interactions are small and cause only spectra broadening. Small values of measured electric field gradient are confirmed by empirically found correlation between known quadrupolar splitting in selected class of systems and an estimation based on postulated pseudopotential. The effects of anharmonicity were considered in the interpretation of the temperature dependence of shift of the spectrum centre.
The phase diagram of the Mn1−xAx (A=Ni, Ga, Rh, and Au) alloys, which are known as the first-kind antiferromagnets, is explained theoretically. Our Hamiltonian is composed of the polynomial of the variables describing multiple spin density wave (MSDW) states, the coupling between the variables and the symmetry strain and the elastic energy. The polynomial is derived from symmetry consideration. By calculating the partition function and the free energy, we show that the phase diagram is reproduced and elucidate the structure of the MSDW at each phase and the condition of the appearance of the orthorhombic phase.
A self-consistent projection operator method for single-particle excitations is developed. It describes the nonlocal correlations on the basis of a projection technique to the retarded Green function and the off-diagonal effective medium. The theory takes into account long-range intersite correlations making use of an incremental cluster expansion in the medium. A generalized self-consistent coherent potential is derived. It yields the momentum-dependent excitation spectra with high resolution. Numerical studies for the Hubbard model on a simple cubic lattice at half filling show that the theory is applicable in a wide range of Coulomb interaction strength. In particular, it is found that the long-range antiferromagnetic correlations in the strong interaction regime cause shadow bands in the low-energy region and sub-peaks of the Mott–Hubbard bands.
We microscopically study the effect of the magnetic field (Zeeman splitting) on the superconducting state in a model for quasi-one-dimensional organic superconductors (TMTSF)2X. We investigate the competition between spin singlet and spin triplet pairings and the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state by random phase approximation. While we studied the competition by comparison with the eigenvalue of the gap equation at a fixed temperature in our previous study [Phys. Rev. Lett. 102 (2009) 016403], here we obtain both the Tc for each pairing state and a phase diagram in the T (temperature)–hz (field)–Vy (strength of the charge fluctuation) space. The phase diagram shows that consecutive transitions from singlet pairing to the FFLO state and further to Sz=1 triplet pairing can occur upon increasing the magnetic field when 2kF charge fluctuations coexist with 2kF spin fluctuations. In the FFLO state, the singlet d-wave and Sz=0 triplet f-wave components are strongly mixed especially when the charge fluctuations are strong.
To investigate the material dependence of the electronic structure of arsenide superconductors, the chemical trend of the Kohn–Sham band structures of a series of compounds, i.e., NaFeAs, NaCoAs, and NaNiAs, is studied by first-principles calculation based on generalized gradient approximation. Hypothetical structures of NaCoAs and NaNiAs in P4⁄nmm are found to be stable by structural optimization simulation. Results on the electronic states suggest that a characteristic two-dimensional electronic structure appearing as rodlike Fermi pockets is clearly found only in NaFeAs, while three-dimensional electronic structures are found in NaCoAs and NaNiAs with larger density of states than NaFeAs at the Fermi level, when paramagnetic electronic states are assumed.
We grew single crystals of RCoGe3 (R: La, Ce, Pr, and Nd) by the Bi-flux method, and studied the magnetic properties of PrCoGe3 and NdCoGe3 and the pressure-induced superconducting property of CeCoGe3. These compounds crystallize in a non-centrosymmetric tetragonal structure. PrCoGe3 and NdCoGe3 were found to be a paramagnet and an antiferromagnet with the Néel temperature TN=3.8 K, respectively. Crystalline electric field (CEF) schemes were proposed for PrCoGe3 and NdCoGe3. The magnetic property of a pressure-induced superconductor CeCoGe3 is highly different from those of PrCoGe3 and NdCoGe3, or a similar pressure-induced superconductor CeIrSi3. This was discussed in connection with the superconductivity of CeCoGe3. We clarified the upper critical field Hc2 for H|| and  by measuring the electrical resistivity under pressure P in magnetic fields: Hc2(0)\\simeq24 T for H|| and Hc2(0)\\simeq3.1 T for H|| at 0 K at P=7.1 GPa. A huge Hc2(0) value for H|| is a combined phenomenon between the Rashba-type antisymmetric spin–orbit interaction and the electronic instability, as observed in CeIrSi3.
Ceramic composites [xNi0.15Cu0.30Zn0.55Fe2O4–(1−x)BaTiO3] were successfully prepared by a direct solid-state reaction of raw materials (BaCO3, CuO, α-Fe2O3, NiO, TiO2, and ZnO). The composites are so homogeneous that the ferrite and BaTiO3 grains do not react with each other. The x-dependent permeability and permittivity are found to obey Maxwell–Garnett effective medium theory, which suggests that the composites consist of ferrite particles with barium titanate medium. This model, however, starts to deviate from the experimental data at x=0.75, and the roles of medium and inclusions seem to be exchanged. It can be qualitatively explained by the fact that geometrical close-packing of spheres is limited up to about 74 vol % (Kepler conjecture).
The characterization of quaternary structures of proteins in solution remains challenging, especially for those undergoing dynamic changes. Small-angle neutron scattering (SANS) is a potentially powerful method for addressing this issue with little perturbation resulting from irradiation damage. However, it is usually difficult to determine the three-dimensional (3D) structure of protein complexes at the atomic level on the basis of only SANS data. To cope with this difficulty, we developed a novel approach combining 3D homology modeling with SANS profile simulation, in which whole simulated SANS profiles were examined together with experimental SANS data. We herein demonstrate the feasibilty of our strategy using proteasome activator 28 (PA28) as a model system. PA28 is a hetero-oligomeric protein composed of homologous α- and β-subunits. Although the crystal structure of the homoheptameric ring of α-subunits (PA28α7) has been reported, the physiologically relevant hetero-oligomeric structure remains to be elucidated. On the basis of the PA28α7 structure, we performed homology modeling to build hypothetical quaternary structures of the PA28 hetero-oligomer. By analyzing the SANS data of a PA28 mutant lacking a mobile loop in its α-subunit, we successfully revealed that α- and β-subunits form heteroheptameric rings, about half of which are stacked back to back to form a double-ring structure. Thus, our SANS approach provides in-depth information on the assembly states of protein subunits in aqueous solutions.
We introduce a linking force between two nodes in a complex network; the force considered to be proportional to the degree of each node and the inverse square of the shortest path length between them. The importance of a node can be inferred from the resultant linking force (RLF), which is the sum of all linking forces acting on the node. To characterize the statistical properties of the RLF, we measure the RLF F(k) as a function of the degree k and find that it follows the power law F(k)∼kα, with α≈1.3, for scale-free networks with the degree exponent ranging from 2 to 3 and the random network. It indicates that the exponent α is same irrespective of the structures of complex networks, which differ from the previous findings in which the most exponents depends on the structure of networks. We also find that the distribution of the RLF follows power law, with the exponent depending logarithmically on the degree exponent; additionally the mean RLF on all nodes also shows a power law relation with mean degree.