126 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
Mechanism of Ambipolar Field-Effect Carrier Injections in One-Dimensional Mott Insulators
To clarify the mechanism of recently reported, ambipolar carrier injections
into quasi-one-dimensional Mott insulators on which field-effect transistors
are fabricated, we employ the one-dimensional Hubbard model attached to a
tight-binding model for source and drain electrodes. To take account of the
formation of Schottky barriers, we add scalar and vector potentials, which
satisfy the Poisson equation with boundary values depending on the drain
voltage, the gate bias, and the work-function difference. The current-voltage
characteristics are obtained by solving the time-dependent Schr\"odinger
equation in the unrestricted Hartree-Fock approximation. Its validity is
discussed with the help of the Lanczos method applied to small systems. We find
generally ambipolar carrier injections in Mott insulators even if the work
function of the crystal is quite different from that of the electrodes. They
result from balancing the correlation effect with the barrier effect. For the
gate-bias polarity with higher Schottky barriers, the correlation effect is
weakened accordingly, owing to collective transport in the one-dimensional
correlated electron systems.Comment: 21 pages, 10 figures, to appear in J. Phys. Soc. Jp
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
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
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 (M1Ï andM3Ï), and the responses at phase boundaries were evaluated according to a prescription [J. Phys. Soc. Jpn. 84, 104707 (2015)]. By evaluating M3Ï/M1Ï 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 M1Ï. 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
Antiferromagnetic S=1/2 Heisenberg Chain and the Two-flavor Massless Schwinger Model
An antiferromagnetic S=1/2 Heisenberg chain is mapped to the two-flavor
massless Schwinger model at \theta=\pi. The electromagnetic coupling constant
and velocity of light in the Schwinger model are determined in terms of the
Heisenberg coupling and lattice spacing in the spin chain system.Comment: 3 pages. LaTex2
Competing phases in the high field phase diagram of (TMTSF)ClO
A model is presented for the high field phase diagram of (TMTSF)ClO,
taking into account the anion ordering, which splits the Fermi surface in two
bands. For strong enough field, the largest metal-SDW critical temperature
corresponds to the N=0 phase, which originates from two intraband nesting
processes. At lower temperature, the competition between these processes puts
at disadvantage the N=0 phase vs. the N=1 phase, which is due to interband
nesting. A first order transition takes then place from the N=0 to N=1 phase.
We ascribe to this effect the experimentally observed phase diagrams.Comment: 5 pages, 3 figures (to appear in Phys. Rev. Lett.
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
Staggered-spin contribution to nuclear spin-lattice relaxation in two-leg antiferromagnetic spin-1/2 ladders
We study the nuclear spin-lattice relaxation rate in the two-leg
antiferromagnetic spin-1/2 Heisenberg ladder. More specifically, we consider
the contribution to from the processes with momentum transfer
. In the limit of weak coupling between the two chains, this
contribution is of activation type with gap at low temperatures
( is the spin gap), but crosses over to a slowly-decaying temperature
dependence at the crossover temperature . This crossover
possibly explains the recent high-temperature NMR results on ladder-containing
cuprates by T. Imai et al.Comment: 6 pages, 2 figures, REVTeX, uses eps
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