arXiv subjects

Hidekazu Tanaka

Publications and source records attributed to Hidekazu Tanaka.

159 records · Page 9Linked to original sources

arXiv2009

Elementary excitations of S=1/2 one-dimensional antiferromagnet KCuGaF6 in magnetic field and quantum sine-Gordon model

Elementary excitations of the S=1/2 one-dimensional Heisenberg antiferromagnet KCuGaF$_6$ with exchange constant $J/k_B$=103 K were investigated by high-frequency ESR measurements combined with a pulsed high magnetic field. When an external magnetic field H is applied in KCuGaF$_6$, a staggered magnetic field h is induced perpendicular to H owing to the staggered g tensor and the Dzyaloshinsky-Moriya (DM) interaction with an alternating D vector. Consequently, KCuGaF$_6$ in a magnetic field is represented by the quantum sine-Gordon (SG) model. We observed many resonance modes including a soliton resonance, breathers, interbreather transitions and two-breather excitation. Their resonance conditions are beautifully described by the quantum SG field theory with one adjustable parameter $c_ s=h/H$. To investigate the relationship between the Curie term due to the DM interaction and the proportional coefficient $c_s$, magnetic susceptibility measurements were also performed varying the external field direction.

cond-mat.str-el↗

arXiv2008

Pressure-Induced Reentrant Oblique Antiferromagnetic Phase in Spin Dimer System TlCuCl_3

Magnetization measurements under a hydrostatic pressure of P = 1.4 GPa were performed on the coupled spin dimer system TlCuCl_3, which exhibits a pressure-induced quantum phase transition from a gapped singlet state to an antiferromagnetic state at P_c = 0.042 GPa. Antiferromagnetic ordering with ordered moments parallel to the ac plane was observed at T_N = 16.7 K at 1.4 GPa. The spin reorientation phase transition was observed at T_R ~ 9.2 K for zero magnetic field, at which the ordered moments start to incline towards the b axis. With increasing external field parallel to th b axis, a second-order phase transition from the oblique antiferromagnetic (OAF) phase to the spin-flop (SF) phase occurs below T_R. We argue that the OAF phase arises from the competition between the anisotropic energies of the conventional second order and the fourth order, that increases with increasing pressure. We discuss the OAF-SF transition within the framework of the mean field approximation.

cond-mat.str-el↗

arXiv2008

Magnetic field-induced one-magnon Raman scattering in the magnon Bose-Einstein condensation phase of TlCuCl$_{3}$

We report the observation of the $A_{\rm g}$-symmetric one-magnon Raman peak in the magnon Bose-Einstein condensation phase of TlCuCl$_{3}$. Its Raman shift traces the one-magnon energy at the magnetic $Γ$ point, and its intensity is proportional to the squared transverse magnetization. The appearance of the one-magnon Raman scattering originates from the exchange magnon Raman process and reflects the change of the magnetic-state symmetry. Using the bond-operator representation, we theoretically clarify the Raman selection rules, being consistent with the experimental results.

cond-mat.str-el↗

arXiv2007

Magnetic-Field Induced Bose-Einstein Condensation of Magnons and Critical Behavior in Interacting Spin Dimer System TlCuCl$_3$

Magnetization measurements were performed to investigate the critical behavior of the field-induced magnetic ordering in gapped spin system TlCuCl$_3$. The critical density of the magnons was obtained as a function of temperature and the magnon-magnon interaction constant was evaluated. The experimental phase boundary for $T < 5$ K agrees almost perfectly with the magnon BEC theory based on the Hartree-Fock approximation with realistic dispersion relations. The phase boundary can be described by the power law $[H_{N}(T)-H_{c}] propto T^{phi}$. With decreasing fitting temperature range, the critical exponent ${phi}$ decreases and converges at $phi_{BEC} =3/2$ predicted by the magnon BEC theory.

cond-mat.str-el↗

arXiv2007

Kinematical Constraints on QCD Factorization in the Drell-Yan Process

We study factorization schemes for parton shower models in hadron-hadron collisions. As an example, we calculate lepton pair production mediated by a virtual photon in quark--anti-quark annihilation, and we compare factorized cross sections obtained in the conventional $\bar{\rm MS}$ scheme with those obtained in a factorization scheme in which a kinematical constraint due to parton radiation is taken into account. We discuss some properties of factorized cross sections.

hep-ph↗

arXiv2007

Thermodynamic Properties and Elementary Excitations in Quantum Sine-Gordon Spin System KCuGaF6

Thermodynamic properties and elementary excitations in $S=1/2$ one-dimensional Heisenberg antiferromagnet KCuGaF$_6$ were investigated by magnetic susceptibility, specific heat and ESR measurements. Due to the Dzyaloshinsky-Moriya interaction with alternating $D$-vectors and/or the staggered $g$-tensor, the staggered magnetic field is induced when subjected to external magnetic field. Specific heat in magnetic field clearly shows the formation of excitation gap, which is attributed to the staggered magnetic field. The specific heat data was analyzed on the basis of the quantum sine-Gordon (SG) model. We observed many ESR modes including one soliton and three breather excitations characteristic of the quantum SG model.

cond-mat.str-el↗

arXiv2007

The effect of randomness on the quantum spin system Tl$_{1-x}$K$_x$CuCl$_3$ with ${\boldmath $x$}$ = 0.44 studied by the Zero-field Muon-Spin-Relaxation (ZF-$μ$SR) method

Zero-field muon-spin-relaxation (ZF-{\boldmath $μ$}SR) measurements were carried out down to 80 mK on the randomness bond system Tl$_{1-x}$K$_x$CuCl$_3$ with $x$ = 0.44. Time spectra are well fitted by the stretched exponential function $\exp (-λt)^β$. The muon spin relaxation rate $λ$ increases rapidly with decreasing temperature, and $β$ tends to 0.5 at 80 mK. The divergent increase of $λ$ suggests the critical slowing down of the frequency of the Cu-3{\it d} spin fluctuations toward a spin frozen state below 80 mK, and the root-exponential-like behavior of the time spectrum indicates that the origin of the relaxation is possibly the spatially-fixed fluctuating dilute moments.

cond-mat.str-el↗

arXiv2006

Pressure-Induced Magnetic Quantum Phase Transition in Gapped Spin System KCuCl3

Magnetization and neutron elastic scattering measurements under a hydrostatic pressure were performed on KCuCl3, which is a three-dimensionally coupled spin dimer system with a gapped ground state. It was found that an intradimer interaction decreases with increasing pressure, while the sum of interdimer interactions increases. This leads to the shrinkage of spin gap. A quantum phase transition from a gapped state to an antiferromagnetic state occurs at Pc ? 8.2 kbar. For P > P c, magnetic Bragg reflections were observed at reciprocal lattice points equivalent to those for the lowest magnetic excitation at zero pressure. This confirms that the spin gap decreases and closes under applied pressure.

cond-mat.stat-mech↗

arXiv2005

Factorization Algorithm for Parton Showers beyond the Leading Logarithmic Order of QCD

A factorization algorithm for a patron shower model based on the evolution of momentum distributions proposed in a previous work is studied. The scaling violation of initial state parton distributions is generated using parton showers to an accuracy of the next-to-leading logarithmic (NLL) order of quantum chromodynamics (QCD) using the information from only splitting functions and initial parton distributions at some fixed low energy. In the algorithm proposed in this paper, the total momentum of the initial state partons is conserved in any factorization scheme for mass singularities. As an example, the scaling violation of the parton distributions and the transverse momentum distributions due to initial state parton radiation are calculated with the jet calculs scheme.

hep-ph↗

arXiv2005

Dust Growth and Settling in Protoplanetary Disks and Disk Spectral Energy Distributions. I. Laminar Disks

Dust growth and settling considerably affect the spectral energy distributions (SEDs) of protoplanetary disks. We investigated dust growth and settling in protoplanetary disks through numerical simulations to examine time-evolution of the disk optical thickness and SEDs. In the present paper, we considered laminar disks as a first step of a series of papers. As a result of dust growth and settling, a dust layer forms around the mid-plane of a gaseous disk. After the formation of the dust layer, small dust grains remain floating above the layer. Although the surface density of the floating small grains is much less than that of the dust layer, they govern the disk optical thickness and the emission. The floating small grains settle onto the dust layer in a long time scale compared with the formation of the dust layer. Rapid grain growth in the inner part of disks makes the radial distribution of the disk optical thickness less steep than that of the disk surface density. At t > 10^6yr, the optical thickness of the inner disk (> a few AU) almost vanishes, which may correspond to disk inner holes observed by Spitzer Space Telescope. Furthermore, we examined time-evolution of disk SEDs, using our numerical results and the two-layer model. The grain growth and settling decrease the magnitude of the SEDs especially. Our results indicate that grain growth and settling can explain the decrease in observed energy fluxes at millimeter/sub-millimeter wavelengths with time scales of 10^{6-7}yr without depletion of the disks.

astro-ph↗

arXiv2005

Structural Transitions and Magnetic Structure in NH4CuCl3 via 14N-NMR

We report results of 14N-NMR experiments on NH4CuCl3 at the magnetic field of 7 T, where the 1/4-magnetization plateau is observed at low temperatures. The quadrupole splitting parameter $ν_{z}$ splits below 70 K, indicating a structural phase transition. At 4.2 K, eight N sites with distinct values of both $ν_{z}$ and the magnetic hyperfine shift $K_{z}$ are resolved in the NMR spectrum for general field directions. We then conlude that the magnetic structure in the 1/4-plateau does not break the symmetry of the crystal. Based on the NMR and the recent neutron scattering results by Ruegg et al. [Phys. Rev. Lett. 93 (2004) 037207], we propose that triplet dimers in the 1/4-plateau is formed not between the nearest neighbor pairs but over different chains.

cond-mat.str-el↗

arXiv2004

Pressure-Induced Magnetic Quantum Phase Transitions from Gapped Ground State in TlCuCl3

Magnetization maesurements under hydrostatic pressure were performed on an S=1/2 coupled spin system TlCuCl3 with a gapped ground state under magnetic field H parallel to the [2,0,1] direction. With increasing applied pressure P, the gap decreases and closes completely at Pc=0.42 kbar. For P>Pc, TlCuCl3 undergoes antiferromagnetic ordering. A spin-flop transition was observed at Hsf=0.7T. The spin-flop field is approximately independent of pressure, although the sublattice magnetization increases with pressure. The gap and Neel temperature are presented as function is attributed to to the relative enhancement of the interdimer exchange interactions compared with the intradimer exchange interaction.

cond-mat.stat-mech↗

arXiv2004

Electronic Structure of Strained Manganite Thin Films with Room Temperature Ferromagnetism Investigated by Hard X-ray Photoemission Spectroscopy

We report the bulk sensitive Hard X-ray (hv=5.95keV) core level photoemission spectroscopy to investigate the intrinsic electronic structure of strained (La0.85Ba0.15)MnO3 thin films. In a 20nm thick well-strained film with strongly enhanced ferromagnetism, a new sharp satellite peak appeared at the low energy site of the Mn 2p3/2 main peak, whereas a broader signal was observed for the unstrained film with 300nm thickness. Cluster calculations revealed that the intensity corresponded to the density of the state at the Fermi level relating to the magnitude of the ferromagnetic order. The satellite intensity also agreed quantitatively with the square of the magnetization.

cond-mat.str-el↗

arXiv2004

Triplon Localization Effect in Tl_{1-x}$K_xCuCl_3

The effect of randomness on field-induced magnetic ordering was investigated through specific heat measurements in Tl$_{1-x}$K$_x$CuCl$_3$ with $x\leq 0.22$. The isostructural parent compounds TlCuCl$_3$ and KCuCl$_3$ are coupled spin dimer systems with a gapped ground state and their field-induced antiferromagnetic ordering is described by the Bose condensation of spin triplets (triplons). Well-defined field-induced phase transitions were observed in Tl$_{1-x}$K$_x$CuCl$_3$. The critical exponent $ϕ$ of the phase boundary defined by $T(H) \propto (H-H_{\rm c})^{1/ϕ}$ is reevaluated in TlCuCl$_3$ as $ϕ=1.67 \pm 0.07$, which is close to $ϕ_{\rm BEC} =3/2$ derived from the triplon BEC theory. For $x \neq 0$, the exponent $ϕ$ decreases systematically with $x$. The phase boundary observed at low temperatures for $x>0.1$ is almost a linear function of temperature $T$. In the low-field region for $H<H_{\rm c}$, no magnetic ordering is observed in spite of finite susceptibility. These properties are discussed in connection with the Bose glass phase argued by Fisher {\it et al.} [Phys. Rev. B {\bf 40}, 546 (1989)].

cond-mat.str-el↗

arXiv1996

Automatic Calculation of SUSY-particle Production

We introduce a new method to treat Majorana fermions and interactions with fermion-number violation on the GRACE system which has been developed for the automatic computation of the matrix elements for the processes of the standard model. Thus we have constructed a system for the automatic computation of cross-sections for the processes of the minimal SUSY standard model (MSSM).

hep-ph↗