RESEARCH INTERESTS
 

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 '08, Karl Tata '10.


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

<>                                                                                                                stm

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.  S. Dukan, J. Tenenbaum*, J. Porembski*, K. Tata* and  S. Dukan, Phys. Rev. B, 82(9),  (2010).

 


 

NOTE: *  indicates an undergraduate student collaborator. home