84 research outputs found
Role of Interchain Hopping in the Magnetic Susceptibility of Quasi-One-Dimensional Electron Systems
The role of interchain hopping in quasi-one-dimensional (Q-1D) electron
systems is investigated by extending the Kadanoff-Wilson renormalization group
of one-dimensional (1D) systems to Q-1D systems. This scheme is applied to the
extended Hubbard model to calculate the temperature () dependence of the
magnetic susceptibility, . The calculation is performed by taking
into account not only the logarithmic Cooper and Peierls channels, but also the
non-logarithmic Landau and finite momentum Cooper channels, which give relevant
contributions to the uniform response at finite temperatures. It is shown that
the interchain hopping, , reduces at low temperatures,
while it enhances at high temperatures. This notable
dependence is ascribed to the fact that enhances the
antiferromagnetic spin fluctuation at low temperatures, while it suppresses the
1D fluctuation at high temperatures. The result is at variance with the
random-phase-approximation approach, which predicts an enhancement of by over the whole temperature range. The influence of both the
long-range repulsion and the nesting deviations on is further
investigated. We discuss the present results in connection with the data of
in the (TMTTF) and (TMTSF) series of Q-1D organic
conductors, and propose a theoretical prediction for the effect of pressure on
magnetic susceptibility.Comment: 17 pages, 19figure
A field-induced reentrant insulator state of a gap-closed topological insulator (Bi_{1-x}Sb_x) in quantum-limit states
In the extreme quantum limit states under high magnetic fields, enhanced
electronic correlation effects can stabilize anomalous quantum states. Using
band-tuning with a magnetic field, we realized a spin-polarized quantum limit
state in the field-induced semimetallic phase of a topological insulator
Bi_{1-x}Sb_x. Further increase in the field injects more electrons and holes to
this state and results in an unexpected reentrant insulator state in this
topological semimetallic state. A single-particle picture cannot explain this
reentrant insulator state, reminiscent of phase transitions due to many-body
effects. Estimates of the binding energy and spacing of electron-hole pairs and
the thermal de Broglie wavelength indicate that Bi_{1-x}Sb_x may host the
excitonic insulator phase in this extreme environment.Comment: 26pages, 6figure
Pressure-induced unconventional superconductivity near a quantum critical point in CaFe2As2
75As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole
resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P =
4.7 and 10.8 kbar, the temperature dependences of nuclear-spin-lattice
relaxation rate (1/T1) measured in the tetragonal phase show no coherence peak
just below Tc(P) and decrease with decreasing temperature. The
superconductivity is gapless at P = 4.7 kbar but evolves to that with multiple
gaps at P = 10.8 kbar. We find that the superconductivity appears near a
quantum critical point under pressures in the range 4.7 kbar < P < 10.8 kbar.
Both electron correlation and superconductivity disappear in the collapsed
tetragonal phase. A systematic study under pressure indicates that electron
correlations play a vital role in forming Cooper pairs in this compound.Comment: 5pages, 5figure
Renormalization Group Technique Applied to the Pairing Interaction of the Quasi-One-Dimensional Superconductivity
A mechanism of the quasi-one-dimensional (q1d) superconductivity is
investigated by applying the renormalization group techniques to the pairing
interaction. With the obtained renormalized pairing interaction, the transition
temperature Tc and corresponding gap function are calculated by solving the
linearized gap equation. For reasonable sets of parameters, Tc of p-wave
triplet pairing is higher than that of d-wave singlet pairing due to the
one-dimensionality of interaction. These results can qualitatively explain the
superconducting properties of q1d organic conductor (TMTSF)2PF6 and the ladder
compound Sr2Ca12Cu24O41.Comment: 18 pages, 9 figures, submitted to J. Phys. Soc. Jp
Quantum Monte Carlo study of the pairing symmetry competition in the Hubbard model
To shed light into the pairing mechanism of possible spin-triplet
superconductors (TMTSF)X and SrRuO, we study the competition among
various spin singlet and triplet pairing channels in the Hubbard model by
calculating the pairing interaction vertex using the ground state quantum Monte
Carlo technique. We model (TMTSF)X by a quarter-filled quasi-one
dimensional (quasi-1D) Hubbard model,and the band of SrRuO by
a two dimensional (2D) Hubbard model with a band filling of . For the
quasi-1D system, we find that triplet -wave pairing not only dominates over
triplet p-wave in agreement with the spin fluctuation theory, but also looks
unexpectedly competitive against d-wave. For the 2D system, although the
results suggest presence of attractive interaction in the triplet pairing
channels, the d-wave pairing interaction is found to be larger than those of
the triplet channels
In-plane electronic anisotropy revealed by interlayer resistivity measurements on the iron-based superconductor parent compound CaFeAsF
Both cuprates and iron-based superconductors demonstrate nematicity, defined
as the spontaneous breaking of rotational symmetry in electron systems. The
nematic state can play a role in the high-transition-temperature
superconductivity of these compounds. However, the microscopic mechanism
responsible for the transport anisotropy in iron-based compounds remains
debatable. Here, we investigate the electronic anisotropy of CaFeAsF by
measuring its interlayer resistivity under magnetic fields with varying field
directions. Counterintuitively, the interlayer resistivity was larger in the
longitudinal configuration () than in the transverse
one (). The interlayer resistivity exhibited a so-called
coherence peak under in-plane fields and was highly anisotropic with respect to
the in-plane field direction. At = 4 K and = 14 T, the
magnetoresistance was seven times larger in the than in the configuration. Our theoretical
calculations of the conductivity based on the first-principles electronic band
structure qualitatively reproduced the above observations but underestimated
the magnitudes of the observed features. The proposed methodology can be a
powerful tool for probing the nematic electronic state in various materials.Comment: 25 pages, 5 figure
Simple Real-Space Picture of Nodeless and Nodal s-wave Gap Functions in Iron Pnictide Superconductors
We propose a simple way to parameterize the gap function in iron pnictides.
The key idea is to use orbital representation, not band representation, and to
assume real-space short-range pairing. Our parameterization reproduces fairly
well the structure of gap function obtained in microscopic calculation. At the
same time the present parameterization is simple enough to obtain an intuitive
picture and to develop a phenomenological theory. We also discuss
simplification of the treatment of the superconducting state.Comment: 4 page
Single Impurity Problem in Iron-Pnictide Superconductors
Single impurity problem in iron-pnictide superconductors is investigated by
solving Bogoliubov-de Gennes (BdG) equation in the five-orbital model, which
enables us to distinguish s and s superconducting states. We
construct a five-orbital model suitable to BdG analysis. This model reproduces
the results of random phase approximation in the uniform case. Using this
model, we study the local density of states around a non-magnetic impurity and
discuss the bound-state peak structure, which can be used for distinguishing
s and s states. A bound state with nearly zero-energy is found
for the impurity potential eV, while the bound state peaks stick to
the gap edge in the unitary limit. Novel multiple peak structure originated
from the multi-orbital nature of the iron pnictides is also found.Comment: 5 page
d-Wave Spin Density Wave phase in the Attractive Hubbard Model with Spin Polarization
We investigate the possibility of unconventional spin density wave (SDW) in
the attractive Hubbard model with finite spin polarization. We show that
pairing and density fluctuations induce the transverse d-wave SDW near the
half-filling. This novel SDW is related to the d-wave superfluidity induced by
antiferromagnetic spin fluctuations, in the sense that they are connected with
each other through Shiba's attraction-repulsion transformation. Our results
predict the d-wave SDW in real systems, such as cold Fermi atom gases with
population imbalance and compounds involving valence skipper elements
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