M. Tachibana - Kobe Univ.

M. Tachibana
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Name
M. Tachibana
Affiliation
Kobe Univ.
City
Kōbe-shi
Country
Japan

Pubs By Year

Pub Categories

 
High Energy Physics - Phenomenology (20)
 
High Energy Physics - Theory (18)
 
Physics - Strongly Correlated Electrons (9)
 
Nuclear Theory (6)
 
Physics - Materials Science (4)
 
Physics - Superconductivity (3)
 
High Energy Physics - Lattice (2)
 
Astrophysics (2)
 
Physics - Other (1)
 
Quantum Physics (1)
 
Mathematical Physics (1)
 
High Energy Astrophysical Phenomena (1)
 
Solar and Stellar Astrophysics (1)
 
Mathematics - Mathematical Physics (1)

Publications Authored By M. Tachibana

We report neutron scattering measurements on Er$_2$Pt$_2$O$_7$, a new addition to the XY family of frustrated pyrochlore magnets. Symmetry analysis of our elastic scattering data shows that Er$_2$Pt$_2$O$_7$ is the first XY pyrochlore to order into the $k=0$, $\Gamma_7$ magnetic structure (the Palmer-Chalker state), at $T_N = 0.38$ K. Read More

We consider multi-flavor massless $(1+1)$-dimensional QED with chemical potentials at finite spatial length and the zero-temperature limit. Its sign problem is solved using the mean-field calculation with complex saddle points. Read More

The gadolinium pyrochlores, Gd$_2B_2$O$_7$, are amongst the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of Gd$_2$Pt$_2$O$_7$, a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that Gd$_2$Pt$_2$O$_7$ undergoes an antiferromagnetic ordering transition at $T_N = 1. Read More

The ytterbium pyrochlore magnets, Yb2B2O7 (B = Sn, Ti, Ge) are well described by S_eff = 1/2 quantum spins decorating a network of corner-sharing tetrahedra and interacting via anisotropic exchange. Structurally, only the non-magnetic B-site cation, and hence, primarily the lattice parameter, is changing across the series. Nonetheless, a range of magnetic behaviors are observed: the low temperature magnetism in Yb2Ti2O7 and Yb2Sn2O7 has ferromagnetic character, while Yb2Ge2O7 displays an antiferromagnetically ordered Neel state at low temperatures. Read More

We report neutron scattering and muon spin relaxation measurements (muSR) on the pyrochlore antiferromagnet Yb2Ge2O7. Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the J=7/2 Yb3+ ion in this environment. The crystal electric field ground state doublet in Yb2Ge2O7 corresponds primarily to m_J = +/- 1/2 with local XY anisotropy, consistent with an S_eff = 1/2 description for the Yb moments. Read More

We discuss the issue on dark matter capture by neutron stars, in particular the process of dark matter thermalization, by which the scattering cross section and the mass of dark matter can be constrained. At first, we evaluate the thermalization time of self-interacting dark matter and find the effect of the self-interaction is small compared with that of the interaction with nucleons. Then we generalize the thermalization time by introducing a set of new parameters. Read More

We study a holographic gauge theory living in the AdS$_4$ space-time at finite temperature. The gravity dual is obtained as a solution of the type IIB superstring theory with two free parameters, which correspond to four dimensional (4D) cosmological constant ($\lambda$) and the dark radiation ($C$) respectively. The theory studied here is in confining and chiral symmetry broken phase for $\lambda <0$ and small $C$. Read More

Structural phase transitions in Pr$_{1-x}$La$_x$AlO$_3$ (0 $\leq x\leq$ 1) single crystals have been studied through heat capacity and high-resolution x-ray scattering measurements. For PrAlO$_3$, the heat capacity shows a sharp first-order peak at the rhombohedral to orthorhombic transition, while a classical mean-field anomaly is observed at the orthorhombic to monoclinic transition at lower temperatures. The transition temperature and the heat capacity anomaly of the two transitions diminish with increasing $x$, and only a single rhombohedral to monoclinic transition is observed for $x = 0. Read More

We take advantage of the site-selective nature of the $^{75}$As and $^{63}$Cu NMR techniques to probe the Cu substitution effects on the local magnetic properties of the FeAs planes in Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$. We show that the suppression of antiferromagnetic Fe spin fluctuations induced by Cu substitution is weaker than a naive expectation based on a simple rigid band picture, in which each Cu atom would donate 3 electrons to the FeAs planes. Comparison between $^{63}$Cu and $^{75}$As NMR data indicates that spin fluctuations are suppressed at the Cu and their neighboring Fe sites in the tetragonal phase, suggesting the strongly local nature of the Cu substitution effects. Read More

In this short paper, we argue the issue on dark matter capture in neutron stars. After summarizing the whole scenario and the introduction of previous studies along this line, we propose some potentially important effects due to the appearance of exotic phases such as neutron superfluidity, meson condensation and quark superconductivity. Those effects might be sizable and alter the previous results. Read More

We have previously found a new phase of cold nuclear matter based on a holographic gauge theory, where baryons are introduced as instanton gas in the probe D8/$\overline{\rm D8}$ branes. In our model, we could obtain the equation of state (EOS) of our nuclear matter by introducing fermi momentum. Then, here we apply this model to the neutron star and study its mass and radius by solving the Tolman-Oppenheimer-Volkoff (TOV) equations in terms of the EOS given here. Read More

We study analytically the chiral phase transition for hot quark matter in presence of a strong magnetic background, focusing on the existence of a critical point at zero baryon chemical potential and nonzero magnetic field. We build up a Ginzburg-Landau effective potential for the chiral condensate at finite temperature, computing the coefficients of the expansion within a chiral quark-meson model. Our conclusion is that the existence of the critical point at finite $\bm B$ is very sensitive to the way the ultraviolet divergences of the model are treated. Read More

We study cold nuclear matter based on the holographic gauge theory, where baryons are introduced as the instantons in the probe D8/D8 branes according to the Sakai-Sugimoto model. Within a dilute gas approximation of instantons, we search for the stable states via the variational method and fix the instanton size. We find the first order phase transition from the vacuum to the nuclear matter phase as we increase the chemical potential. Read More

We employ high-resolution total neutron scattering in conjunction with reverse Monte Carlo simulations to examine, in a detailed and unbiased manner, the crystal structure of the vacancy-ordered oxide pyrochlore Pb2Ru2O6.5 in light of its structural analogy with proton-ordering in the structures of ice. We find that the vacancy and the O' ion are completely ordered, and that the average structure in the F43m space group describes the vacancy ordering precisely. Read More

The cubic, stoichiometric oxide compounds Bi2Ti2O6O' (also written Bi2Ti2O7) and Bi2Ru2O6O' (also written Bi2Ru2O7) have in common lone-pair active Bi3+ cations on the pyrochlore A-site with a propensity to off-center. Unlike Bi2Ti2O6O', Bi2Ru2O6O' is a metal, so it is of interest to ask whether conduction electrons and/or involvement of Bi 6s states at the Fermi energy influence Bi3+ displacements. The Bi3+ off-centering in Bi2Ti2O6O' is incoherent as revealed in detail by reverse Monte Carlo analysis of total neutron scattering. Read More

We found that the ZrCuSiAs-type crystal CeNi0.8Bi2 with a layered structure composed of alternate stacking of [CeNixBi(1)]{\delta}+ and Bi(2){\delta}- exhibits a superconductive transition at ~4 K, which was confirmed by zero resistance and the Meissner effect (shielding volume faction ~100% at 2 K). Heat capacity measurements revealed that the electron mass at the normal state (>5 K) is heavy ({\gamma} = 0. Read More

We discuss the phase structure of dense matter, in particular the nature of the transition between hadronic and quark matter. Calculations within a Ginzburg-Landau approach show that the axial anomaly can induce a critical point in this transition region. This is possible because in three-flavor quark matter with instanton effects a chiral condensate can be added to the color-flavor locked (CFL) phase without changing the symmetries of the ground state. Read More

In this paper we argue that boundary condition may run with energy scale. As an illustrative example, we consider one-dimensional quantum mechanics for a spinless particle that freely propagates in the bulk yet interacts only at the origin. In this setting we find the renormalization group flow of U(2) family of boundary conditions exactly. Read More

We consider a process of the Andreev reflection between a normal metal and the s-wave superconductor in the FFLO state. It is shown that the process takes place if the energy of the incoming electron is bound within the finite interval called the Andreev window. The position of the window determines the value of the non-zero total momentum of Cooper pairs and the value of the gap. Read More

In this manuscript, a unified approach to hadron physics from holographic point of view is described. After introduction of a general setup for meson-nucleon system based on the bottom-up approach of QCD (AdS/QCD), as an illustration, we specifically examine meson-nucleon couplings. This is an example of the notion we call "holographic unification" in hadron physics. Read More

There has been tremendous research activity in the field of magneto-electric (ME) multiferroics after Kimura et al. [1] showed that antiferromagnetic and ferroelectric order coexist in orthorhom- bically distorted perovskite TbMnO3 and are strongly coupled. It is now generally accepted that ferroelectricity in TbMnO3 is induced by magnetic long range order that breaks the symmetry of the crystal and creates a polar axis [2]. Read More

Na-based osmium oxide pyrochlore was synthesized for the first time by an ion-exchange method. KOs2O6 was used as a host compound. Elelectron probe micro-analysis, synchrotron x-ray diffraction analysis, and thermo-gravimetric analysis confirmed its structure not as the beta-type but as the defect-type pyrochlore. Read More

The QCD axial anomaly, by coupling the chiral condensate and BCS pairing fields of quarks in dense matter, leads to a new critical point in the QCD phase diagram \cite{HTYB,chiral2}, which at sufficiently low temperature should terminate the line of phase transitions between chirally broken hadronic matter and color superconducting quark matter. The critical point indicates that matter at low temperature should cross over smoothly from the hadronic to the quark phase, as suggested earlier on the basis of symmetry. We review here the arguments, based on a general Ginzburg-Landau effective Lagrangian, for the existence of the new critical point, as well as discuss possible connections between the QCD phase structure and the BEC-BCS crossover in ultracold trapped atomic fermion systems at unitarity. Read More

The vector mesons in three-flavor quark matter with chiral and diquark condensates are studied using the in-medium QCD sum rules. The diquark condensate leads to a mass splitting between the flavor-octet and flavor-singlet channels. At high density, the singlet vector meson disappears from the low-energy spectrum, while the octet vector mesons survive as light excitations with a mass comparable to the fermion gap. Read More

Magnetic entropy and adiabatic temperature changes in and above the room-temperature region has been measured for La0.7Sr0.3Mn1-xM'xO3 (M' = Al, Ti) by means of magnetization and heat capacity measurements in magnetic fields up to 6 T. Read More

Using a general Ginzburg-Landau effective Lagrangian, we study the topological structure and low-lying collective modes of dense QCD having both chiral and diquark condensates, for two and three massless flavors. As we found earlier, the QCD axial anomaly acts as an external field applied to the chiral condensate in a color superconductor and, as a new critical point emerges, leads to a crossover between the broken chiral symmetry and color superconducting phases. At intermediate densities where both chiral and diquark condensates are present, we derive a generalized Gell-Mann- Oakes-Renner relation between the masses of pseudoscalar bosons and the magnitude of the chiral and diquark-condensates. Read More

We investigate the interplay between the chiral and diquark condensates on the basis of the Ginzburg-Landau potential with QCD symmetry. We demonstrate that the axial anomaly drives a new critical point at low temperature in the QCD phase diagram and leads to a smooth crossover between the hadronic and color superconducting phases. Read More

We study the interplay between chiral and diquark condensates within the framework of the Ginzburg-Landau free energy, and classify possible phase structures of two and three-flavor massless QCD. The QCD axial anomaly acts as an external field applied to the chiral condensate in a color superconductor and leads to a crossover between the broken chiral symmetry and the color superconducting phase, and, in particular, to a new critical point in the QCD phase diagram. Read More

Several physical quantities of light hadrons are examined by a new holographic model of QCD, which is the modified version of the one proposed by Erlich et al. defined on AdS${}_5$. In our model, AdS${}_5$ is deformed by a non-trivial bulk scalar, this is corresponding to adding the mass term of the adjoint fermions to the 4d SYM theory dual to the gravity on AdS${}_5$. Read More

In this talk, a supersymmetric (SUSY) composite model of color superconductivity is discussed. In this model, quark and diquark supermultiplets are dynamically generated as massless composites by a newly introduced confining gauge dynamics. It is analytically shown that the scalar component of diquark supermultiplets develops vacuum expectation value (VEV) at a certain critical chemical potential. Read More

Thermal color superconducting phase transitions in three-flavor quark matter at high baryon density are investigated in the Ginzburg-Landau (GL) approach. We constructed the GL potential near the boundary with a normal phase by taking into account nonzero quark masses, electric charge neutrality, and color charge neutrality. We found that the density of states averaged over paired quarks plays a crucial role in determining the phases near the boundary. Read More

A supersymmetric composite model of color superconductivity is proposed. Quarks and diquarks are dynamically generated as composite fields by a newly introduced strong gauge dynamics. It is shown that the condensation of the scalar component of the diquark supermultiplet occurs when the chemical potential becomes larger than some critical value. Read More

2003Dec
Affiliations: 1RIKEN, 2Univ. Tokyo, 3RIKEN, 4Univ. Tokyo

Thermal color superconducting phase transitions in high density three-flavor quark matter are investigated in the Ginzburg-Landau approach. Effects of nonzero strange quark mass, electric and color charge neutrality, and direct instantons are considered. Weak coupling calculations show that an interplay between the mass and electric neutrality effects near the critical temperature gives rise to three successive second-order phase transitions as the temperature increases: a modified color-flavor locked (mCFL) phase (ud, ds, and us pairings) -> a ``dSC'' phase (ud and ds pairings) -> an isoscalar pairing phase (ud pairing) -> a normal phase (no pairing). Read More

We study weak interactions involving Goldstone bosons in the neutral kaon condensed phase of color flavor locked quark matter. We calculate the rates for the dominant processes that contribute to the neutrino mean free p ath and to neutrino production. A light $K^+$ state, with a mass $\tilde{m}_{K^+} \propto (\Delta/\mu) (\Delta/m_s)(m_d-m_u)$, where $\mu$ and $\Delta$ are the quark chemical potential and superconducting gap respectively, is shown to play an important role. Read More

We address solutions of brane-world with cosmological constant $\lambda$ by introducing the dilaton in 5d bulk, and we examine the localization of graviton, gauge bosons and dilaton. For those solutions, we find that both graviton and gauge bosons can be trapped for either sign, positive or negative, and wide range of $\lambda$ due to the non-trivial dilaton. While the dilaton can not be trapped on the brane. Read More

In this paper we discuss the phenomenon of the Andreev reflection of quarks at the interface between the 2SC and the Color-Flavor-Locked (CFL) superconductors appeared in QCD at asymptotically high densities. We also give the general introduction to the Andreev reflection in the condensed matter systems as well as the review of this subject in high density QCD. Read More

We study the weak interaction rates involving Goldstone bosons in the Color Flavor Locked (CFL) quark matter. Neutrino mean free path and the rate of energy loss due to neutrino emission in a thermal plasma of CFL pions and kaons is calculated. We find that in addition to neutrino scattering off thermal mesons, novel Cherenkov like processes wherein mesons are either emitted or absorbed contribute to the neutrino opacity. Read More

2002Jan
Affiliations: 1MIT, LNS & Cracow, INP, 2MIT, LNS & Kyoto, YITP

In this letter we address the question of the phenomena of Andreev reflection between the cold quark-gluon plasma phase and CFL color superconductor. We show that there are two different types of reflections connected to the structure of the CFL phase. We also calculate the probability current at the interface and we show that it vanishes for energy of scattering quarks below the superconducting gap. Read More

We discuss the relation between two different models which are recently proposed as the model of localizing bulk gauge fields on a brane. In the former model, the localization of gauge field is achieved by adding both bulk and boundary mass terms while in the latter, it is done by taking into consideration the coupling between the gauge field and the dilaton field (this model is also regarded as the gauge theory with nontrivial dielectric ``constant''). We make a certain transformation for the gauge field in the latter Lagrangian. Read More

We study behavior of bulk gauge field in the bigravity model in which two positive tension $AdS_4$ branes in $AdS_5$ bulk are included. We solve the equations of motions for Kaluza-Klein modes and determine the mass spectrum. It is shown that unlike the case of graviton, we find no ultralight Kaluza-Klein modes in the spectrum. Read More

Motivated by a recently proposed "bigravity" model with two positive tension $AdS_4$ branes in $AdS_5$ by Kogan et al.[hep-th/0011141], we study behavior of bulk gauge field in the model. In this case, the zero mode of the gauge field is not constant but depends on the fifth dimensional coordinate transverse to the brane. Read More

2000Nov

We propose a new mechanism of spontaneous supersymmetry breaking. The existence of extra dimensions with nontrivial topology plays an important role. We investigate new features resulting from this mechanism. Read More

We propose a new mechanism of spontaneous supersymmetry breaking. The existence of extra dimensions with nontrivial topology plays an important role. We investigate new features resulted from the mechanism in two simple supersymmetric Z_2 and U(1) models. Read More

Recently, Randall and Sundrum proposed a static solution to Einstein's equations in five spacetime dimensions with two 3-branes located at the fixed points of $S^1/Z_2$ to solve the hierarchy problem. We extend the solution and construct static and also inflationary solutions to Einstein's equations in five spacetime dimensions, one of which is compactified on $S^1$, with any number of 3-branes whose locations are taken to be arbitrary. We discuss how the hierarchy problem can be explained in our model. Read More

1999Feb

We propose a mechanism to break the translational invariance of compactified space spontaneously. As a simple model, we study a real $\phi^4$ model compactified on $M^{D-1}\otimes S^1$ in detail, where we impose a nontrivial boundary condition on $\phi$ for the $S^1$-direction. It is shown that the translational invariance for the $S^1$-direction is spontaneously broken when the radius $R$ of $S^1$ becomes larger than a critical radius $R^*$ and also that the model behaves like a $\phi^4$ model on a single kink background for $R \to \infty$. Read More

1999Feb
Affiliations: 1Kobe Univ., 2Kobe Univ., 3I.N.F.N, Pisa

We propose a new spontaneous supersymmetry breaking mechanism, in which extra compact dimensions play an important role. To illustrate our mechanism, we study a simple model consisting of two chiral superfields, where one spatial dimension is compactified on a circle $S^1$. It is shown that supersymmetry is spontaneously broken irrespective of the radius of the circle, and also that the translational invariance for the $S^1$-direction and a global symmetry are spontaneously broken when the radius becomes larger than a critical radius. Read More

1998Jun
Affiliations: 1YITP, Kyoto, 2Tokyo Nat. Coll., 3Kobe Univ.

We investigate canonical structure of the Abelian Higgs model within the framework of DLCQ. Careful boundary analysis of differential equations, such as the Euler-Lagrange equations, leads us to a novel situation where the canonical structure changes in a drastic manner depending on whether the (light-front) spatial Wilson line is periodic or not. In the former case, the gauge-field ZM takes discrete values and we obtain the so-called ``Zero-Mode Constraints'' (ZMCs), whose semiclassical solutions give a nonzero vev to the scalar fields. Read More

We investigate the phase structures of various N=1 supersymmetric gauge theories including even the exceptional gauge group from the viewpoint of superconvergence of the gauge field propagator. Especially we analyze in detail whether a new type of duality recently discovered by Oehme in $SU(N_c)$ gauge theory coupled to fundamental matter fields can be found in more general gauge theories with more general matter representations or not. The result is that in the cases of theories including matter fields in only the fundamental representation, Oehme's duality holds but otherwise it does not. Read More

Motivated by the work of Kalloniatis, Pauli and Pinsky, we consider the theory of light-cone quantized $QCD_{1+1}$ on a spatial circle with periodic and anti-periodic boundary conditions on the gluon and quark fields respectively. This approach is based on Discretized Light-Cone Quantization (DLCQ). We investigate the canonical structures of the theory. Read More

We study toroidal orbifold models with topologically invariant terms in the path integral formalism and give physical interpretations of the terms from an operator formalism point of view. We briefly discuss a possibility of a new class of modular invariant orbifold models. Read More