Why the Hidden Order in URu_{2}Si_{2} Is Still Hidden—One Simple Answer

Hisatomo Harima, Kazumasa Miyake, and Jacques Flouquet — 2010-03-10T09:00:00+09:00

Authors: Hisatomo Harima, Kazumasa Miyake, and Jacques Flouquet
For more than two decades, the nonmagnetic anomaly observed around 17.5 K in URu_{2}Si_{2}, has been investigated intensively. However, any kind of fingerprint for the lattice anomaly has not been observed in the low-temperature ordered phase. Therefore, the order has been called “the hidden order”. One simple answer to why the hidden order is still hidden is presented from the space group analysis. The second-order phase transition from I4/mmm (No. 139) to P4_{2}/mnm (No. 136) does not require any kind of lattice distortion in this system and allows the NQR frequency at a Ru site unchanged. It is compatible with O_{xy}-type antiferro-quadrupole ordering with Q=(0, 0, 1). The characteristics of the hidden order are discussed based on the local 5 f^{2} electron picture.©2010 The Physical Society of Japan

Current-Induced Exchange Length and Geometrically Constrained Magnetic Wall

Katsuyoshi Matsushita, Jun Sato, and Hiroshi Imamura — 2010-03-10T09:00:00+09:00

Authors: Katsuyoshi Matsushita, Jun Sato, and Hiroshi Imamura
We analyzed stable magnetic structures of a geometrically constrained magnetic wall under direct current (dc) and found that there exists a characteristic length induced by the competition between the spin transfer torque and the torque due to exchange interaction. This current-induced exchange length is inversely proportional to the applied dc, and the magnetic ripple structure appears when the current-induced exchange length is smaller than the nanoscale constriction. We also found that the amplitude of the magnetic ripple structure oscillates as a function of the applied dc, which can be observed as an oscillation of the resistance against the applied dc.©2010 The Physical Society of Japan

Origin of Drastic Change of Fermi Surface and Transport Anomalies in CeRhIn_{5} under Pressure

Shinji Watanabe and Kazumasa Miyake — 2010-03-10T09:00:00+09:00

Authors: Shinji Watanabe and Kazumasa Miyake
The mechanism of drastic change of Fermi surfaces as well as transport anomalies near P=P_{c}≈2.35 GPa in CeRhIn_{5} is explained theoretically. The key mechanism is pointed out to be the interplay of magnetic order and Ce-valence fluctuations. It is shown that the antiferromagnetic state with “small” Fermi surfaces changes to the paramagnetic state with “large” Fermi surfaces with huge enhancement of effective mass of electrons with keeping finite c–f hybridization. This explains the drastic change of the de Haas–van Alphen signals. Furthermore, it is also consistent with the emergence of T-linear resistivity simultaneous with the residual resistivity peak at P=P_{c} in CeRhIn_{5}.©2010 The Physical Society of Japan

Roles of Bond Alternation in Magnetic Phase Diagram of RMnO_{3}

Nobuo Furukawa and Masahito Mochizuki — 2010-03-10T09:00:00+09:00

Authors: Nobuo Furukawa and Masahito Mochizuki
In order to investigate the nature of the antiferromagnetic structures in perovskite RMnO_{3}, we study a Heisenberg J_{1}–J_{2} model with bond alternation using analytical and numerical approaches. The magnetic phase diagram which includes incommensurate spiral states and commensurate collinear states is reproduced. We discuss that the magnetic structure with spin ↑↑↓↓ configuration (E-type structure) and the ferroelectricity emerge cooperatively to stabilize this phase. Magnetoelastic couplings are crucial to understand the magnetic and electric phase diagram of RMnO_{3}.©2010 The Physical Society of Japan

Semiclassical Interpretation of Electron Correlation in Helium

Mitsusada M. Sano — 2010-03-10T09:00:00+09:00

Authors: Mitsusada M. Sano
Electron correlation in helium atom is analyzed by a semiclassical theory. The case of zero angular momentum is considered. A semiclassical expression for the formula <cos(α) > appeared in the Herrick's theory is derived, where α is the angle between the two vectors pointing electrons from the nucleus. Examining the reliability of the Herrick's result from numerical results of quantum mechanics in the literature, it is found that the Herrick's result holds in the case close to the collinear eZe configuration. Using the semiclassical expression, the Herrick's result is analyzed. In particular, a formula for periodic orbits is predicted in the case that the Herrick's result holds.©2010 The Physical Society of Japan

Dynamics of Quadruply Quantized Vortices in ^{87}Rb Bose–Einstein Condensates Confined in Magnetic and Optical Traps

Takeshi Kuwamoto, Hayato Usuda, Satoshi Tojo, and Takuya Hirano — 2010-03-10T09:00:00+09:00

Authors: Takeshi Kuwamoto, Hayato Usuda, Satoshi Tojo, and Takuya Hirano
We studied the dynamics of quadruply quantized vortices in ^{87}Rb Bose–Einstein condensates. Vortices were created in magnetically trapped condensates with hyperfine spin F = 2 by employing topological phase imprinting technique, wherein the direction of atomic spin is adiabatically reversed by applying a bias magnetic field. Vortices were observed for a holding time of up to 10 ms. Disappearance of the vortices was attributed to considerable expansion and excitation of the condensates, which were caused by the distortion of the magnetic potential. In order to observe the long-term behavior of the vortices, we transferred the condensates to a crossed-type optical dipole force trap after creating the vortices. In this case, the vortices were observed for a holding time of up to 22 ms. We also observed density profiles, which indicated the presence of split vortices.©2010 The Physical Society of Japan

Stability of the Singular Vortex and Associated Majorana Zero Modes in Trapped p-Wave Resonant Superfluids of Neutral Cold Atoms

Yasumasa Tsutsumi and Kazushige Machida — 2010-03-10T09:00:00+09:00

Authors: Yasumasa Tsutsumi and Kazushige Machida
The stability conditions for the singular vortex that accompanies Majorana zero modes at the core are investigated for p-wave resonant superfluids of atomic Fermi gases. Within the Ginzburg–Landau framework, we determine the stable conditions in the parameter space for the external rotation frequency and the harmonic trap frequency. There exists a narrow stable region in this parameter space for quasi-two-dimensional condensates. We also describe the detailed characterizations of the spatial structure of the order parameter in the chiral p-wave superfluids under rotation.©2010 The Physical Society of Japan

Real-Time Phase Difference Control of Optical Beams Using a Mach–Zehnder Interferometer

Bor-Wen Shiau, Tzu-Ping Ku, and Dian-Jiun Han — 2010-03-10T09:00:00+09:00

Authors: Bor-Wen Shiau, Tzu-Ping Ku, and Dian-Jiun Han
We present an experimental scheme for fast phase difference control of optical beams based on a simple, however robust setup of Mach–Zehnder interferometer. We demonstrate to smoothly tune the relative phase by 140° in 275 ms, with an average peak-to-peak phase difference jitter less than 0.9°. The overall achievable tuning range, both for continuous and stepwise scans, can be more than 320°. This scheme is totally immune to intensity fluctuation and allows to engage the conventional phase-shifting imaging. It is especially suitable for real-time configuration control of 2D and 3D optical lattice potentials to study the tunneling and transport effects on cold atomic samples.©2010 The Physical Society of Japan

Calorimetric Study of Kinetic Glass Transition in Various Glasses

Yosio Hiki and Haruyuki Takahashi — 2010-03-10T09:00:00+09:00

Authors: Yosio Hiki and Haruyuki Takahashi
Differential scanning calorimetry (DSC) experiments were carried out for metallic, ionic, and polymer glasses below and above the glass transition temperature T_{g}. The values of T_{g} were determined from the DSC curves. A wide range of heating rate, q=dT/dt=0.1–100 K/min, was adopted and the q dependence of the apparent value of T_{g} was investigated. As q was decreased, the value of T_{g} decreased rapidly, then more slowly, and approached a constant value at low q. The experimental result of this kinetic glass transition was analyzed, where the relaxation time of the transition was considered to be equal to a characteristic observation time of the process of the experiment. The Arrhenius and Vogel–Tammann–Fulcher relaxation-time forms were adopted. The latter form was found to be adequate for explaining the experimental results, and the parameters in the form were determined for various glasses. On the basis of these experimental results the characteristics of the relaxation near the glass transition were precisely discussed.©2010 The Physical Society of Japan

Theory of Photoinduced Changes of the Superfluid Weight in Superconductors

Takanobu Jujo — 2010-03-10T09:00:00+09:00

Authors: Takanobu Jujo
We investigate changes in superfluid weight in photo-excited states. This quantity is obtained by a microscopic calculation of the third-order nonlinear response function on the assumption that the pumping intensity is low. The resultant expression includes the energy dependence of self-energy explicitly. The importance of this dependence is shown by performing numerical calculations with the electron–boson interaction included. Moreover, it is found that the vertex correction makes a predominant contribution to the photoinduced change in superfluid weight. The similarity between the linear and photoinduced responses in its temperature dependence is discussed, and a quantitative estimation of the calculated results is made in comparison with that of the experiments.©2010 The Physical Society of Japan

Edge States and Flat Bands of Graphene Nanoribbons with Edge Modification

2010-03-10T09:00:00+09:00

Authors: Katsunori Wakabayashi, Susumu Okada, Ryutaro Tomita, Shigeo Fujimoto, and Yuhei Natsume
We study the electronic states of graphene nanoribbons with modified edge structures by attaching Klein's bearded bonds as a minimal model of edge modification. The partial attachment of Klein's bearded bonds to graphene nanoribbons gives rise to the partial flat bands at zero-energy even under the condition of |N_{A}-N_{B}| = 0, where N_{A} (N_{B}) is the number of A (B)-sublattice sites. Using transfer matrix method, we successfully derive the analytic representation of edge states for modified zigzag edge. The modification of armchair edges causes the complete flat bands, where the wavefunction has the character of valley polarization. We also applied the density functional theory to optimize the lattice structure and estimate the spin density. Our results indicate that the chemical and structural modification of graphene edge will serve to design and stabilize the spin polarized edge states.©2010 The Physical Society of Japan

Magnetic Excitations in Infinite-Layer Antiferromagnetic Insulator

2010-03-10T09:00:00+09:00

Authors: Keisuke Tomiyasu, Hiroshi Kageyama, Changhoon Lee, Mike H. Whangbo, Yoshihiro Tsujimoto, Kazuyoshi Yoshimura, Jon W. Taylor, Anna Llobet, Frans Trouw, Kazuhisa Kakurai, and Kazuyoshi Yamada
The magnetic excitations of an infinite-layer antiferromagnetic insulator SrFeO_{2} were examined by powder inelastic neutron scattering to find dispersive magnetic excitations from ∼15 up to 63 meV. The scattering intensity distribution is well described by a spin wave model, confirming that the out-of-plane direct Fe…Fe exchange is comparable in strength to the in-plane Fe–O–Fe superexchange. SrFeO_{2} shows an additional magnetic excitation mode around 30 meV, which is suggestive of an orbital magnon arising from small orbital moment on Fe^{2+} brought about by spin–orbit coupling.©2010 The Physical Society of Japan

Charge, Lattice, and Spin Dynamics in Photoinduced Phase Transitions from Charge-Ordered Insulator to Metal in Quasi-Two-Dimensional Organic Conductors

Satoshi Miyashita, Yasuhiro Tanaka, Shinichiro Iwai, and Kenji Yonemitsu — 2010-03-10T09:00:00+09:00

Authors: Satoshi Miyashita, Yasuhiro Tanaka, Shinichiro Iwai, and Kenji Yonemitsu
To elucidate the different photoinduced melting dynamics of charge orders observed in the quasi-two-dimensional organic conductors θ-(BEDT-TTF)_{2}RbZn(SCN)_{4} and α-(BEDT-TTF)_{2}I_{3} [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene], we theoretically study the photoinduced time evolution of charge and spin correlation functions on the basis of exact many-electron wave functions coupled with classical phonons in extended Peierls–Hubbard models on anisotropic triangular lattices. In both salts, the so-called horizontal-stripe charge order is stabilized by nearest-neighbor repulsive interactions and electron–lattice interactions. In θ-(BEDT-TTF)_{2}RbZn(SCN)_{4} (abbreviated as θ-RbZn), the stabilization energy due to lattice distortion is larger, so that a larger quantity of energy needs to be absorbed to melt charge and lattice orders. The photoinduced charge dynamics shows a complex behavior owing to a substantial number of nearly degenerate eigenstates involved. This is related to the high structural symmetry when the lattice is undistorted. In α-(BEDT-TTF)_{2}I_{3} (abbreviated as α-I_{3}), the lattice stabilization energy is smaller, and a small quantity of energy is sufficient to melt charge and lattice orders leading to a metallic phase. The photoinduced charge dynamics shows a sinusoidal oscillation. In α-I_{3}, the low structural symmetry ensures nearly spin-singlet bonds between hole-rich sites, where the spin correlation survives even after photoexcitation.©2010 The Physical Society of Japan

Photoelectron Spectroscopic Study of Electronic Structures of L-Cysteine

2010-03-10T09:00:00+09:00

Authors: Masao Kamada, Harue Sugiyama, Kazutoshi Takahashi, Junpei Azuma, Souichi Kitajima, Koji Ogawa, Michinori Sumimoto, Kenji Hori, and Hitoshi Fujimoto
The valence band structure of L-cysteine films was investigated by ultraviolet photoelectron spectroscopy (UPS) in the photon energy range of 40–100 eV. Smooth thin films were produced by vacuum evaporation and characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and Raman scattering spectroscopy. The photon energy dependence of the UPS spectra indicates that electronic states close to the valence band maximum may be attributed to sulfur, while the other states in the higher-binding-energy region may be attributed to carbon, nitrogen, and oxygen. Molecular orbital calculation was performed on seven possible geometries of L-cysteine. The observed UPS spectrum is in good agreement with the simulated one in terms of the C3 geometry of L-cysteine. It is concluded that the sulfur-originated electronic state is located at the valence-band maximum, indicating that cysteine is one of the useful and promising materials in future bioelectronics.©2010 The Physical Society of Japan

Simultaneous Metal–Insulator and Spin-State Transition in (Pr_{1-y}RE_{y})_{1-x}Ca_{x}CoO_{3} (RE=Nd, Sm, Gd, and Y)

Tomoyuki Naito, Hiroko Sasaki, and Hiroyuki Fujishiro — 2010-03-10T09:00:00+09:00

Authors: Tomoyuki Naito, Hiroko Sasaki, and Hiroyuki Fujishiro
We have studied a Pr-site substitution effect using various RE ions (RE=Nd, Sm, Gd, and Y) on a simultaneous metal–insulator (MI) and spin-state (SS) transition in (Pr_{1-y}RE_{y})_{1-x}Ca_{x}CoO_{3} using measurements of electrical resistivity, magnetization, and thermal dilatation. The MI–SS transition took place at the appropriate combination of x and y for samples of RE=Sm, Gd, and Y. The MI–SS transition temperatures T_{MI–SS} can be scaled universally by the average ionic radius <r_{A} > of the A-site in the perovskite ACoO_{3}, which is independent of x, y, and the RE ion species. The atomic randomness of the A-site, which is defined as the mean square deviation σ^{2}, larger than the critical value σ_{cr}^{2} is also necessary for the occurrence of the MI–SS transition and T_{MI–SS} increases with increasing σ^{2}. In contrast, no MI–SS transition was observed in the RE=Nd samples (x=0.2–0.4), which can be inferred from the small σ^{2} value because of the small difference in ionic radius between Pr^{3+} and Nd^{3+}. The atomic randomness of the A-site might be an important parameter that dominates the MI–SS transition through the difference in electronic energy δE between the spin states of Co^{3+} ions.©2010 The Physical Society of Japan

Pressure-Induced Antiferromagnetic Order in Filled Skutterudite PrFe_{4}P_{12} Studied by Single-Crystal High-Pressure Neutron Diffraction

2010-03-10T09:00:00+09:00

Authors: Toyotaka Osakabe, Keitaro Kuwahara, Daichi Kawana, Kazuaki Iwasa, Daisuke Kikuchi, Yuji Aoki, Masahumi Kohgi, and Hideyuki Sato
The order parameter in the pressure-induced insulating phase of the filled skutterudite compound PrFe_{4}P_{12} has been investigated using a high-pressure single-crystal neutron diffraction technique. Clear evidence of the antiferromagnetic order with the propagation vector q = (1, 0, 0) was observed in the insulating phase above 2.7 GPa. The q = (1, 0, 0) structure is identical with that in the nonmagnetic ordered phase below 2.5 GPa and with the nesting property of the Fermi surface. The observed magnetic moment of the Pr ion is 2 µ_{B} and almost pressure-independent up to at least 4.2 GPa. The 2 µ_{B} magnetic moment originates from the dipole in the low-lying quasi-quartet crystal field state. The pressure-induced antiferromagnetic structure at 3.2 GPa is suppressed by applying a magnetic field of about 1.5 T, which is interpreted as a flop of magnetic moments into the alignment of the ferromagnetic state. This brings about a steep decrease in the electrical resistivity due to the disappearance of the antiferromagnetic superstructure.©2010 The Physical Society of Japan

Theory of Room Temperature Ferromagnet V(TCNE)_{x} (1.5 < x < 2): Role of Hidden Flat Bands

Hiroyasu Matsuura, Kazumasa Miyake, and Hidetoshi Fukuyama — 2010-03-10T09:00:00+09:00

Authors: Hiroyasu Matsuura, Kazumasa Miyake, and Hidetoshi Fukuyama
Theoretical studies on the possible origin of room temperature ferromagnetism (ferromagnetic once crystallized) in the molecular transition metal complex, V(TCNE)_{x} (1.5 < x < 2) have been carried out. For this family, there have been no definite understanding of crystal structure so far because of sample quality, though the effective valence of V is known to be close to +2. Proposing a new crystal structure for the stoichiometric case of x = 2, where the valence of each TCNE molecule is -1 and resistivity shows insulating behavior, exchange interaction among d-electrons on adjacent V atoms has been estimated based on the cluster with 3 vanadium atoms and one TCNE molecule. It turns out that Hund's coupling among d orbitals within the same V atoms and antiferromagnetic coupling between d oribitals and LUMO of TCNE (bridging V atoms) due to hybridization result in overall ferromagnetism (to be precise, ferrimagnetism). This view based on localized electrons is supplemented by the band picture, which indicates the existence of a flat band expected to lead to ferromagnetism as well consistent with the localized view. The off-stoichiometric cases (x < 2), which still show ferromagnetism but semiconducting transport properties, have been analyzed as due to Anderson localization.©2010 The Physical Society of Japan

Proper Permittivity for Depolarization Field and Its Implication to Universal Instability of Insulating Ferroelectric: A Note

Yukio Watanabe — 2010-03-10T09:00:00+09:00

Authors: Yukio Watanabe
Because the permittivity for the depolarization field E_{d} depends on the definition of the spontaneous polarization P_{S}, improper permittivity is often used in the literature. Therefore, we define proper combinations of the permittivity and P_{S} for E_{d} that are consistent with experimental conditions. We show that the permittivity for E_{d} should be unity in most cases, especially for nonideal capacitor structures, which are often discussed on the basis of the Mehta formula. This choice of permittivity is shown to be consistent with the experimental linear permittivity in response to the external field. This permittivity indicates the problem with assuming a ferroelectric as an insulator under E_{d}: A strain-free two-dimensional single-domain ferroelectric phase with a P_{S} ⊥ surface cannot exist regardless of the thickness of the ferroelectric, when at least one of the surfaces is unscreened. Alternatively, the agreement between experimental and theoretical results obtained at a permittivity ≫1 for E_{d} suggests the existence of nondielectric screening mechanisms. Simple criteria for the stability of the insulating ferroelectric phase in capacitor structures are presented.©2010 The Physical Society of Japan

Lateral Diffusion of Magnetic Colloidal Chains Confined in Thin Films and Monolayer Colloids

Yayoi Terada and Michio Tokuyama — 2010-03-10T09:00:00+09:00

Authors: Yayoi Terada and Michio Tokuyama
Extensive Brownian dynamics simulations are performed on dilute polydisperse magnetic colloidal chains confined in thin films and on a monolayer of polydisperse magnetic colloids. Simulation results show that the magnetic field dependences of the mean-square displacements and the radial distribution functions of the chains are qualitatively similar to those of the monolayer colloids, even if the field dependence of the hexatic bond-orientational order function of the nearest chains is different from that of the monolayer colloids. It is also found that these long-time self-diffusion coefficients collapse on a single master curve recently proposed by Tokuyama.©2010 The Physical Society of Japan

Erratum: “Complete Assignment of Spin Domains in Antiferromagnetic NiO(100) by Photoemission Electron Microscopy and Cluster Model Calculation”

2010-03-10T09:00:00+09:00

Authors: Kuniaki Arai, Taichi Okuda, Arata Tanaka, Masato Kotsugi, Keiki Fukumoto, Masaki Oura, Yasunori Senba, Haruhiko Ohashi, Tetsuya Nakamura, Tomohiro Matsushita, Takayuki Muro, Akito Kakizaki, and Toyohiko Kinoshita

Recent articles in J. Phys. Soc. Jpn.

Why the Hidden Order in URu_{2}Si_{2} Is Still Hidden—One Simple Answer

Current-Induced Exchange Length and Geometrically Constrained Magnetic Wall

Origin of Drastic Change of Fermi Surface and Transport Anomalies in CeRhIn_{5} under Pressure

Roles of Bond Alternation in Magnetic Phase Diagram of RMnO_{3}

Semiclassical Interpretation of Electron Correlation in Helium

Dynamics of Quadruply Quantized Vortices in ^{87}Rb Bose–Einstein Condensates Confined in Magnetic and Optical Traps

Stability of the Singular Vortex and Associated Majorana Zero Modes in Trapped p-Wave Resonant Superfluids of Neutral Cold Atoms

Real-Time Phase Difference Control of Optical Beams Using a Mach–Zehnder Interferometer

Calorimetric Study of Kinetic Glass Transition in Various Glasses

Theory of Photoinduced Changes of the Superfluid Weight in Superconductors

Edge States and Flat Bands of Graphene Nanoribbons with Edge Modification

Magnetic Excitations in Infinite-Layer Antiferromagnetic Insulator

Charge, Lattice, and Spin Dynamics in Photoinduced Phase Transitions from Charge-Ordered Insulator to Metal in Quasi-Two-Dimensional Organic Conductors

Photoelectron Spectroscopic Study of Electronic Structures of L-Cysteine

Simultaneous Metal–Insulator and Spin-State Transition in (Pr_{1-y}RE_{y})_{1-x}Ca_{x}CoO_{3} (RE=Nd, Sm, Gd, and Y)

Pressure-Induced Antiferromagnetic Order in Filled Skutterudite PrFe_{4}P_{12} Studied by Single-Crystal High-Pressure Neutron Diffraction

Theory of Room Temperature Ferromagnet V(TCNE)_{x} (1.5 < x < 2): Role of Hidden Flat Bands

Proper Permittivity for Depolarization Field and Its Implication to Universal Instability of Insulating Ferroelectric: A Note

Lateral Diffusion of Magnetic Colloidal Chains Confined in Thin Films and Monolayer Colloids

Erratum: “Complete Assignment of Spin Domains in Antiferromagnetic NiO(100) by Photoemission Electron Microscopy and Cluster Model Calculation”