98 research outputs found
Field-theoretical renormalization group for a flat two-dimensional Fermi surface
We implement an explicit two-loop calculation of the coupling functions and
the self-energy of interacting fermions with a two-dimensional flat Fermi
surface in the framework of the field theoretical renormalization group (RG)
approach. Throughout the calculation both the Fermi surface and the Fermi
velocity are assumed to be fixed and unaffected by interactions. We show that
in two dimensions, in a weak coupling regime, there is no significant change in
the RG flow compared to the well-known one-loop results available in the
literature. However, if we extrapolate the flow to a moderate coupling regime
there are interesting new features associated with an anisotropic suppression
of the quasiparticle weight Z along the Fermi surface, and the vanishing of the
renormalized coupling functions for several choices of the external momenta.Comment: 16 pages and 22 figure
Possible Magnetic Chirality in Optically Chiral Magnet [Cr(CN)][Mn()-pnH(HO)](HO) Probed by Muon Spin Rotation and Relaxation
Local magnetic fields in a molecule-based optically chiral magnet
[Cr(CN)][Mn()-pnH(HO)](HO) (GN-S) and its enantiomer (GN-R) are
studied by means of muon spin rotation and relaxation (muSR). Detailed analysis
of muon precession signals under zero field observed below T_c supports the
average magnetic structure suggested by neutron powder diffraction. Moreover,
comparison of muSR spectra between GN-S and GN-R suggests that they are a pair
of complete optical isomers in terms of both crystallographic and magnetic
structure. Possibility of magnetic chirality in such a pair is discussed.Comment: 5 pages, 5 figures, submitted to J. Phys. Soc. Jp
Origin of adiabatic and non-adiabatic spin transfer torques in current-driven magnetic domain wall motion
A consistent theory to describe the correlated dynamics of quantum mechanical
itinerant spins and semiclassical local magnetization is given. We consider the
itinerant spins as quantum mechanical operators, whereas local moments are
considered within classical Lagrangian formalism. By appropriately treating
fluctuation space spanned by basis functions, including a zero-mode wave
function, we construct coupled equations of motion for the collective
coordinate of the center-of-mass motion and the localized zero-mode coordinate
perpendicular to the domain wall plane. By solving them, we demonstrate that
the correlated dynamics is understood through a hierarchy of two time scales:
Boltzmann relaxation time when a non-adiabatic part of the spin-transfer torque
appears, and Gilbert damping time when adiabatic part comes up.Comment: 4 pages, 2 figure
Nonlinear magnetic responses at the phase boundaries around helimagnetic and skyrmion lattice phases in MnSi: Evaluation of robustness of noncollinear spin texture
The phase diagram of a cubic chiral magnet MnSi with multiple Dzyaloshinskii-Moriya (DM) vectors as a function of temperature T and dc magnetic field Hdc was investigated using intensity mapping of the odd-harmonic responses of ac magnetization (M1ω andM3ω), and the responses at phase boundaries were evaluated according to a prescription [J. Phys. Soc. Jpn. 84, 104707 (2015)]. By evaluating M3ω/M1ω appearing at phase boundaries, the robustness of noncollinear spin texture in both the helimagnetic (HM) and the skyrmion lattice (SkL) phases of MnSi was discussed. The robustness of vortices-type solitonic texture SkL in MnSi is smaller than those of both the single DM HM and chiral soliton lattice phases of a monoaxial chiral magnet Cr1/3NbS2, and furthermore the robustness of the multiple DM HM phase in MnSi is smaller than that of its SkL. Through magnetic diagnostics over the wide T -Hdc range, we found a new paramagnetic (PM) region with ac magnetic hysteresis, where spin fluctuations have been observed via electrical magnetochiral effect. The anomalies observed in the previous ultrasonic attenuation measurement correspond to the peak positions of out-of-phase M1ω. The appearance of a new PM region occurs at a characteristic magnetic field, above which indeed the SkL phase appears. It has us suppose that the new PM region could be a phase with spin fluctuation like the skyrmion gas phase
Origin for the enhanced copper spin echo decay rate in the pseudogap regime of the multilayer high-T_c cuprates
We report measurements of the anisotropy of the spin echo decay for the inner
layer Cu site of the triple layer cuprate, Hg_0.8Re_0.2Ba_2Ca_2Cu_3O_8 (T_c=126
K) in the pseudogap T regime below T_pg ~ 170 K and the corresponding analysis
for their interpretation. As the field alignment is varied, the shape of the
decay curve changes from Gaussian (H_0 \parallel c) to single exponential (H_0
\perp c). The latter characterizes the decay caused by the fluctuations of
adjacent Cu nuclear spins caused by their interactions with electron spins. The
angular dependence of the second moment (T_{2M}^{-2} \equiv )
deduced from the decay curves indicates that T_{2M}^{-2} for H_0 \parallel c,
which is identical to T_{2G}^{-2} (T_{2G} is the Gaussian component), is
substantially enhanced, as seen in the pseudogap regime of the bilayer systems.
Comparison of T_{2M}^{-2} between H_0 \parallel c and H_0 \perp c indicates
that this enhancement is caused by electron spin correlations between the inner
and the outer CuO_2 layers. These results provide the answer to the
long-standing controversy regarding the opposite T dependences of (T_1T)^{-1}
and T_{2G}^{-2} in the pseudogap regime of bi- and trilayer systems.Comment: 4 pages, 4 figure
Quantum phase transitions and collapse of the Mott gap in the dimensional half-filled Hubbard model
We study the low-energy asymptotics of the half-filled Hubbard model with a
circular Fermi surface in continuous dimensions, based on the
one-loop renormalization-group (RG) method. Peculiarity of the
dimensions is incorporated through the mathematica structure of the elementary
particle-partcile (PP) and particle-hole (PH) loops: infrared logarithmic
singularity of the PH loop is smeared for . The RG flows indicate
that a quantum phase transition (QPT) from a metallic phase to the Mott
insulator phase occurs at a finite on-site Coulomb repulsion for
. We also discuss effects of randomness.Comment: 12 pages, 10 eps figure
Signature of the staggered flux state around a superconducting vortex in underdoped cuprates
Based on the SU(2) lattice gauge theory formulation of the t-J model, we
discuss possible signature of the unit cell doubling associated with the
staggered flux (SF) state in the lightly doped spin liquid. Although the SF
state appears only dynamically in a uniform d-wave superconducting (SC) state,
a topological defect [SU(2) vortex] freezes the SF state inside the vortex
core. Consequently, the unit cell doubling shows up in the hopping
() and pairing () order parameters of physical
electrons. We find that whereas the center in the vortex core is a SF state, as
one moves away from the core center, a correlated staggered modulation of
and becomes predominant. We predict that over the
region outside the core and inside the internal gauge field penetration depth
around a vortex center, the local density-of-states (LDOS) exhibits staggered
peak-dip (SPD) structure inside the V-shaped profile when measured on the
bonds. The SPD structure has its direct origin in the unit cell doubling
associated with the SF core and the robust topological texture, which has
little to do with the symmetry of the d-wave order parameter. Therefore the
structure may survive the tunneling matrix element effects and easily be
detected by STM experiment.Comment: 27 pages, 14 figures in GIF format, typo correcte
Staggered local density-of-states around the vortex in underdoped cuprates
We have studied a single vortex with the staggered flux (SF) core based on
the SU(2) slave-boson theory of high superconductors. We find that
whereas the center in the vortex core is a SF state, as one moves away from the
core center, a correlated staggered modulation of the hopping amplitude
and pairing amplitude becomes predominant. We predict that in this
region, the local density-of-states (LDOS) exhibits staggered modulation when
measured on the bonds, which may be directly detected by STM experiments.Comment: 4 pages, 3 figure
Charge Ordering in the One-Dimensional Extended Hubbard Model: Implication to the TMTTF Family of Organic Conductors
We study the charge ordering (CO) in the one-dimensional (1D) extended
Hubbard model at quarter filling where the nearest-neighbor Coulomb repulsion
and dimerization in the hopping parameters are included. Using the cluster
mean-field approximation to take into account the effect of quantum
fluctuations, we determine the CO phase boundary of the model in the parameter
space at T=0 K. We thus find that the dimerization suppresses the stability of
the CO phase strongly, and in consequence, the realistic parameter values for
quasi-1D organic materials such as (TMTTF)PF are outside the region of
CO. We suggest that the long-range Coulomb interaction between the chains
should persist to stabilize the CO phase.Comment: 5 pages, 4 eps figures, to appear in 15 Nov. 2001 issue of PR
Effects of Spin Fluctuations in Quasi-One-Dimensional Organic Superconductors
We study the electronic states of quasi-one-dimensional organic conductors
using the single band Hubbard model at half-filling. We treat the effects of
the on-site Coulomb interaction by the fluctuation-exchange (FLEX) method, and
calculate the phase diagram and physical properties. The calculated pressure
dependence of the Neel temperature coincides well with the experimental one. We
also show that a pseudogap is formed in the density of states near the chemical
potential and that d-wave superconductivity appears next to the
antiferromagnetic state. Moreover the NMR relaxation rate increases on cooling
in the low-temperature region.Comment: 4 pages, 5 figures, to apprear in J. Phys. Soc. Jp
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