- Theoretical and numerical study of extreme type-II superconductors subjected to high magnetic field. This research is done in collaboration with Goucher undergraduate students and the theoretical condensed matter group at Johns Hopkins University (JHU).
- Past and
present research students: Oskar Vafek '98 ('03 Ph.
D.
in Theoretical Physics, JHU), John Oleszkiewicz, '00 ('02
M.S.
in Computer Science, JHU), Richard Howard '02, Amanda Carr
'02,
T.Paul Powell '03, J. Trafton '03, Joel Tenenbaum '06 (now in Ph.
D. program at Boston University), Daniel Pines
'06 (now in Ph. D. program at University of Maryland at College
Park), Joe Porembski '07, Karl Tata '09.
1. Gapless Superconductivity
at High Magnetic Fields
In
recent years, and particularly since the discovery of high temperature
superconductors (HTS's), almost all of the superconducting systems that
capture our curiosity and imagination and simultaneously hold the
greatest
promise for practical applications are of the extreme type-II
variety.
They are characterized by high transition temperatures (Tc),
high upper critical fields (Hc2), and can be defined as
materials
in which the semiclassical Hc2(0) in units of Tesla becomes
comparable to, or even larger than, Tc in units of Kelvin.
In
such systems at low temperatures and high magnetic fields Landau level
(LL) quantization of the electronic energies is well defined and has to
be included in a description of the superconducting instability in this
regime. Such a quantum regime, where the LL structure within the
superconducting
state is well defined, represents a large portion of the H-T phase
diagram
of a clean, intrinsically extreme type-II superconductor (it can extend
to fields as low as ~0.5 Hc2(0) and temperatures as high as
0.3Tc(0)), while in a conventional superconductor (like Nb)
it is expected to be negligible. Within this high-field, low
temperature
regime the superconducting state fundamentally differs from the
familiar
low-field mixed phase of the Abrikosov-Gorkov theory, primarily by the
appearance of gapless quasiparticle excitations at the
Fermi
surface. (S.
Dukan and Z. Tešanović, Phys. Rev. B, 49, 13017
(1994) cond-mat/9402016,
S. Dukan and Z. Tešanović, Phys. Rev B, 56, 838 (1997)
cond-mat/9706290)
2. de Haas-van Alphen Effect in the Superconducting State
It has been almost twenty years since the historic observation of
magnetothermal
and de Haas-van Alphen (dHvA) oscillations in the mixed state of
the superconducting dichalcogenide 2H-NbSe2 by Graebner and
Robbins. In the last decade the dHvA effect was also detected in the
A-15
superconductors V3Si and Nb3Sn, as well as the
borocarbide
YNi2B2 and the high temperature superconductors
YBCO
and Ba(K)BiO3. These observations represent the strongest
evidence
so far for the quantization of the electronic orbits within the
superconducting
state, i.e. LL quantization. The persistence of the dHvA signal deep
within
the mixed state can be attributed to the presence of a small portion of
the Fermi surface containing gapless quasiparticle excitations,
surrounded by regions where the gap is large. (S. Dukan and Z. Tešanović,
Phys. Rev. Lett., 74, 2311 (1995) cond-mat/9501109)
3. Anomalous Behavior of Hc2(T) at Low Temperatures
Within the high-field, low temperature regime in the H-T phase diagram
the superconducting state fundamentally differs from the familiar
low-field
mixed phase of the Abrikosov-Gorkov theory. The presence of Landau
quantization
of the electronic energies in the magnetic field induces an upward
curvature
in the upper critical field Hc2(T) at temperatures ~0.1Tc0.
This behavior could be observed in extreme type-II superconductors in
which
the slope of Hc2(T) at Tc0 is
>0.2Tesla/Kelvin. (S.
Dukan and O. Vafek*, Physica C, 309, 295 (1998) cond-mat/9810304)
4. Thermal Transport in Extreme Type-II Superconductors
A
powerful probe of low-energy excitations in superconductors is
measurement
of their thermal transport. Simultaneous measurements of the field
dependent
longitudinal and transverse (Hall) thermal conductivities are now
feasible experimentally and can yield information on both quasiparticle
dynamics and the pairing mechanism. We developed a theory of the
quasiparticle contribution to the thermal transport of an extreme
type-II
superconductor in a high magnetic field and low temperatures. We found
that there was considerable thermal transport in the mixed state of the
superconductor due to the presence of gapless excitations at the Fermi
surface. We have numerically computed the longitudinal transport
coefficient
in borocarbide and A-15 superconductors. The agreement with recent
experimental
data on LuNi2B2C is very good. (S.
Dukan, T. P. Powell* and Z. Tešanović, Phys. Rev. B, 66(1),
014517
(2002) cond-mat/0204414)
5. Specific Heat
of Extreme Type-II Superconductors
Measurement
of the electronic specific heat C(T,H) represents yet another way of
probing
the quasiparticle excitations in the mixed state of the superconductor.
In a fully gapped s-wave superconductor at low
fields there is an exponentially small contribution to C(T,H) since the
quasiparticles excitations are gapped by a large BCS gap. On the other
hand the quasiparticle excitation spectrum in high magnetic field is
gapless and there should be a contribution coming from the
excitations
around the nodes at the Fermi surface leading to the algebraic
temperature
dependence of the superconducting specific heat at low temperature.
Starting
from the high-field regime, we develop a theory for the
quasiparticle
contribution to the low-temperature specific heat in the mixed state of
an extreme type-II superconductor . Using experimentally
available
parameters for the microscopic properties of the borocarbide
superconductor
YNi2B2C we numerically compute its C(T,H)/T as
temperature
T approaches zero and find a non-linear H-dependence. For certain
values of the disorder parameters the agreement with experimental data
is very good. A. L. Carr*, J. J. Trafton*,
S.
Dukan
and Z. Tešanović, Phys. Rev B, 68, 174519 (2003).
6. Tunneling Conductance of Extreme
Type-II Superconductors
The tunneling conductance between a Scanning Tunneling Microscope (STM) tip and a surface of disordered LuNi2B2C superconductor separated by a thin insulating layer in a high magnetic field and at zero temperatures is calculated. In a clean system we find that when the STM tip is placed at the position of a vortex, the differential conductance σ(V) has an algebraic dependence on the bias voltage V reflecting the presence of gapless points in the quasiparticle excitation spectrum of a superconductor in high magnetic fields. When non-magnetic impurities are introduced into the system, the differential conductance at zero bias voltage becomes finite, indicating the broadening of the gapless or near gapless regions in the quasiparticle excitation spectrum. We plot the differential conductance σ(V) as a function of disorder parameters for wide range of magnetic field strengths H in the mixed state. J. Tenenbaum*, J. Porembski*, K. Tata* and S. Dukan, in preparation (2007).