37 research outputs found
D-wave-like nodal superconductivity in the organic conductor (TMTSF)2ClO4
We suggest theoretical explanation of the high upper critical magnetic field,
perpendicular to conducting chains, Hc2, experimentally observed in the
superconductor (TMTSF)2ClO4, in terms of singlet superconducting pairing. In
particular, we compare the results of d-wave-like nodal, d-wave-like node-less,
and s-wave scenarios of superconductivity. We show that, in d-wave-like nodal
scenario, superconductivity can naturally exceed both the orbital upper
critical magnetic field and Clogston-Shandrasekhar paramagnetic limit as well
as reach experimental value, Hc2 = 6T, in contrast to d-wave-like node-less and
s-wave scenarios. In our opinion, the obtained results are strongly in favor of
d-wave-like nodal superconductivity in (TMTSF)2ClO4, whereas, in a sister
compound, (TMTSF)2PF6, we expect either the existence of triplet order
parameter or the coexistence of triplet and singlet order parameters.Comment: Talk at the ECRYS-2011 international conferenc
Universal Field-Induced Charge-Density-Wave Phase Diagram: Theory versus Experiment
We suggest a theory of the Field-Induced Charge-Density-Wave (FICDW) phases,
generated by high magnetic fields in quasi-low-dimensional conductors. We
demonstrate that, in layered quasi-one-dimensional conductors, the
corresponding critical magnetic fields ratios are universal and do not depend
on any fitting parameter. In particular, we find that $H_1/H_0 = 0.73, \
H_2/H_0 = 0.59, H_3/H_0 = 0.49, H_4/H_0 = 0.42H_nnn+1\alpha_2_4$
material.Comment: 9 pages, 2 figure
Orbital quantization in the high magnetic field state of a charge-density-wave system
A superposition of the Pauli and orbital coupling of a high magnetic field to
charge carriers in a charge-density-wave (CDW) system is proposed to give rise
to transitions between subphases with quantized values of the CDW wavevector.
By contrast to the purely orbital field-induced density-wave effects which
require a strongly imperfect nesting of the Fermi surface, the new transitions
can occur even if the Fermi surface is well nested at zero field. We suggest
that such transitions are observed in the organic metal
-(BEDT-TTF)KHg(SCN) under a strongly tilted magnetic field.Comment: 14 pages including 4 figure
Larkin-Ovchinnikov-Fulde-Ferrell state in quasi-one-dimensional superconductors
The properties of a quasi-one-dimensional (quasi-1D) superconductor with {\it
an open Fermi surface} are expected to be unusual in a magnetic field. On the
one hand, the quasi-1D structure of the Fermi surface strongly favors the
formation of a non-uniform state (Larkin-Ovchinnikov-Fulde-Ferrell (LOFF)
state) in the presence of a magnetic field acting on the electron spins. On the
other hand, a magnetic field acting on an open Fermi surface induces a
dimensional crossover by confining the electronic wave-functions wave-functions
along the chains of highest conductivity, which results in a divergence of the
orbital critical field and in a stabilization at low temperature of a cascade
of superconducting phases separated by first order transistions. In this paper,
we study the phase diagram as a function of the anisotropy. We discuss in
details the experimental situation in the quasi-1D organic conductors of the
Bechgaard salts family and argue that they appear as good candidates for the
observation of the LOFF state, provided that their anisotropy is large enough.
Recent experiments on the organic quasi-1D superconductor (TMTSF)ClO
are in agreement with the results obtained in this paper and could be
interpreted as a signature of a high-field superconducting phase. We also point
out the possibility to observe a LOFF state in some quasi-2D organic
superconductors.Comment: 24 pages+17 figures (upon request), RevTex, ORSAY-LPS-24109
Coexistence or Separation of the Superconducting, Antiferromagnetic, and Paramagnetic Phases in Quasi One-Dimensional (TMTSF)2PF6 ?
We report on experimental studies of the character of phase transitions in
the quasi-1D organic compound (TMTSF)2PF6 in the close vicinity of the borders
between the paramagnetic metal PM, antiferromagnetic insulator AF, and
superconducting SC states. In order to drive the system through the phase
border P_0(T_0), the sample was maintained at fixed temperature T and pressure
P, whereas the critical pressure P_0 was tuned by applying the magnetic field
B. In this approach, the magnetic field was used (i) for tuning (P-P_0), and
(ii) for identifying the phase composition (due to qualitatively different
magnetoresistance behavior in different phases). Experimentally, we measured
R(B) and its temperature dependence R(B,T) in the pressure range (0 - 1)GPa.
Our studies focus on the features of the magnetoresistance at the phase
transition between the PM and AF phases, in the close vicinity to the
superconducting transition at T~1K. We found pronounced history effects arising
when the AF/PM phase border is crossed by sweeping the magnetic field: the
resistance depends on a trajectory which the system arrives at a given point of
the P-B-T phase space. In the transition from the PM to AF phase, the features
of the PM phase extends well into the AF phase. At the opposite transition from
the AF to PM phase, the features of the AF phase are observed in the PM phase.
These results evidence for a macroscopically inhomogeneous state, which
contains macroscopic inclusions of the minority phase. When the system is
driven away from the transition, the homogeneous state is restored; upon a
return motion to the phase boundary, no signatures of the minority phase are
observed up to the very phase boundary.Comment: 10 figures, 23 page
Novel Phases in the Field Induced Spin Density Wave State in (TMTSF)_2PF_6
Magnetoresistance measurements on the quasi one-dimensional organic conductor
(TMTSF)_2PF_6 performed in magnetic fields B up to 16T, temperatures T down to
0.12K and under pressures P up to 14kbar have revealed new phases on its P-B-T
phase diagram. We found a new boundary which subdivides the field induced spin
density wave (FISDW) phase diagram into two regions. We showed that a
low-temperature region of the FISDW diagram is characterized by a hysteresis
behavior typical for the first order transitions, as observed in a number of
studies. In contrast to the common believe, in high temperature region of the
FISDW phase diagram, the hysteresis and, hence, the first order transitions
were found to disappear. Nevertheless, sharp changes in the resistivity slope
are observed both in the low and high temperature domains indicating that the
cascade of transitions between different subphases exists over all range of the
FISDW state. We also found that the temperature dependence of the resistance
(at a constant B) changes sign at about the same boundary. We compare these
results with recent theoretical models.Comment: LaTex, 4 pages, 4 figure
Tomography of pairing symmetry from magnetotunneling spectroscopy -- a case study for quasi-1D organic superconductors
We propose that anisotropic -, -, or -wave pairing symmetries can be
distinguished from a tunneling spectroscopy in the presence of magnetic fields,
which is exemplified here for a model organic superconductor .
The shape of the Fermi surface (quasi-one-dimensional in this example) affects
sensitively the pairing symmetry, which in turn affects the shape (U or V) of
the gap along with the presence/absence of the zero-bias peak in the tunneling
in a subtle manner. Yet, an application of a magnetic field enables us to
identify the symmetry, which is interpreted as an effect of the Doppler shift
in Andreev bound states.Comment: 4 papegs, 4 figure
Detailed study of the ac susceptibility of Sr2RuO4 in oriented magnetic fields
We have investigated the ac susceptibility of the spin triplet superconductor
SrRuO as a function of magnetic field in various directions at
temperatures down to 60 mK. We have focused on the in-plane field configuration
(polar angle ), which is a prerequisite for inducing
multiple superconducting phases in SrRuO. We have found that the
previous attribution of a pronounced feature in the ac susceptibility to the
second superconducting transition itself is not in accord with recent
measurements of the thermal conductivity or of the specific heat. We propose
that the pronounced feature is a consequence of additional involvement of
vortex pinning originating from the second superconducting transition.Comment: Accepted for publication in Phys. Rev.
Magnetic field - temperature phase diagram of quasi-two-dimensional organic superconductor lambda-(BETS)_2 GaCl_4 studied via thermal conductivity
The thermal conductivity kappa of the quasi-two-dimensional (Q2D) organic
superconductor lambda-(BETS)_2 GaCl_4 was studied in the magnetic field H
applied parallel to the Q2D plane. The phase diagram determined from this bulk
measurement shows notable dependence on the sample quality. In dirty samples
the upper critical field H_{c2} is consistent with the Pauli paramagnetic
limiting, and a sharp change is observed in kappa(H) at H_{c2 parallel}. In
contrast in clean samples H_{c2}(T) shows no saturation towards low
temperatures and the feature in kappa(H) is replaced by two slope changes
reminiscent of second-order transitions. The peculiarity was observed below ~
0.33T_c and disappeared on field inclination to the plane when the orbital
suppression of superconductivity became dominant. This behavior is consistent
with the formation of a superconducting state with spatially modulated order
parameter in clean samples.Comment: 10 pages, 8 figures, new figure (Fig.5) and references added, title
change
Quantum Hall Effect in Three-dimensional Field-Induced Spin Density Wave Phases with a Tilted Magnetic Field
The quantum Hall effect in the three-dimensional anisotropic tight-binding
electrons is investigated in the field-induced spin density wave phases with a
magnetic field tilted to any direction. The Hall conductivity,
and , are shown to be quantized as a function of the wave vector
of FISDW, while stays zero, where is the most conducting
direction and and are perpendicular to .Comment: 18 pages, REVTeX 3.0, 1 figure is available upon request, to be
published in Physical Review