Superconductivity

Phase transitions in Lu2Ir3Si5

by Anand Awasthi

Co-authors: Yogesh Singh, Dilip Pal, S. Ramakrishnan, and S. K. Malik

We report the results of our investigations on a polycrystalline sample of Lu2Ir3Si5, which crystallizes in the U2Co3... more We report the results of our investigations on a polycrystalline sample of Lu2Ir3Si5, which crystallizes in the U2Co3 Si5-type structures (Ibam). These investigations comprise powder x-ray diffraction, magnetic susceptibil-ity, electrical resistivity, and high-temperatures (120–300 K) heat-capacity studies. Our results reveal that the sample undergoes a superconducting transition below 3.5 K. It also undergoes a first-order phase transition between 150–250 K as revealed by an upturn in the resistivity, a diasmagnetic drop in the magnetic susceptibility, and a large anomaly (20–30 J/mol K) in the specific heat data. We observe a huge thermal hysteresis of almost 45 K between the cooling and warming data across this high-temperature transition in all our measurements. Low-temperature x-ray diffraction measurements at 87 K reveals that the compound undergoes a structural change at the high-temperature transition. Resistivity data taken in repeated cooling and warming cycles indicate that at the high-temperature transition, the system goes into a highly metastable state; successive heating and/or cooling curves are found to lie above the previous one, and the resistance keeps increasing with every thermal cycle. The room-temperature resistance of a thermally cycled piece of the sample decays exponentially with time with a decay-time constant estimated to be about 10^4 s. The anomaly (upturn) in the resistivity and the large drops (~40%) in the susceptibility across the high-temperature transition suggest that the observed structural change is accompanied or induced by an electronic transition.

Skyrmionic state in chiral p-wave superconductors

by Julien Garaud

Semi-Meissner state and non-pairwise intervortex interactions in type-1.5 superconductors

by Julien Garaud

Topological solitons in three-band superconductors with broken time reversal symmetry

by Julien Garaud

Length scales, collective modes, and type-1.5 regimes in three-band superconductors

by Julien Garaud

Molecular-dynamics-simulation study of the two-dimensional lattice structure of the Cu-1 plane in YBCO

by Taner Edis

Authors: J. D. Fan, G. L. Zhao, T. Edis, and Y. M. Malozovsky.
Physical Review B, 56:17 10747 (1997)

A molecular-dynamics simulation was used to study the two-dimensional structure of a YBCO cuprate plane, which forms a... more A molecular-dynamics simulation was used to study the two-dimensional structure of a YBCO cuprate plane, which forms a square lattice in the insulating state. The simulation was per- formed on an arbitrary Cu-1 plane in a cell, taking into account interactions with atoms located on nearest- neighbor planes. A layered 2D screened-Coulomb potential developed by Fan and Malozovsky is found to be sufficient to produce the square lattice structure. This pair potential is derived microscopically, accounting for the exchange correlation effects in a layered lattice with an isotropic 2D electron spectrum; the MD results support the validity of the potential.

Kosterlitz-Thouless transition and charge redistribution in the superconductivity of YBCO/PBCO superlattices

by Taner Edis

Authors: M. Rasolt, T. Edis and Z. Tesanovic.
Physical Review Letters, 66:22 2927 (1991)

Recent experimental measurements of the resistive transition in YBCO/PrBCO superlattices show a dramatic suppression... more Recent experimental measurements of the resistive transition in YBCO/PrBCO superlattices show a dramatic suppression in the resistive transition temperature Tc as the thickness of the Pr layer increases while the thickness of the Y layer decreases. We show that the qualitative features of these experiments (and in particular most of the suppression in Tc) can be accounted for within the Kosterlitz-Thouless theory. A small further suppression in Tc is shown to occur due to charge transfer from Y to the Pr layers.

Weak-link magnetically modulated resistance response in granular superconducting systems

by Taner Edis

Co-authored with Kishin Moorjani

Physical Review B, 51:2 1124 (1995)

Experimentally, magnetically modulated resistance (MMR) techniques are effective in ascertaining the properties of... more Experimentally, magnetically modulated resistance (MMR) techniques are effective in ascertaining the properties of granular superconductors, which can be modeled as a network of Josephson junctions. Each junction is parametrized by a critical current, with a capacitance and resistance in parallel to ac- count for the nonsupercurrent components. The overall current-voltage behavior of the network, in the presence of an external magnetic field, is given by a set of coupled nonlinear ordinary differential equa- tions, which are studied numerically. Features ofinterest in the simulated experiments arise from effects of the applied field on couplings, and the transitions between multiple solution classes of the differential equations. Through these simulations of the phase dynamics, details of the weak link MMR signal can be understood, the typical signal being reproduced with large enough networks.

A Two-Dimensional Structure Factor Calculation for the Cu-1 Plane in YBa2Cu3O6

by Taner Edis

Authors: T. Edis, J.D. Fan, D. Bagayoko and J.T. Wang.
International Journal of Modern Physics B, Vol. 12, Nos. 29, 30 & 31 (1998) 3091–3094

The anisotropic radial distribution function and two-dimensional (2D) structure factor was calculated for the Cu-1... more The anisotropic radial distribution function and two-dimensional (2D) structure factor was calculated for the Cu-1 plane in YBa2Cu3O6+x when x ≈ 0, based on data generated by a molecular dynamics simulation. The results indicate a stable square lattice and support the validity of layered two-dimensional screened Coulomb potential used in the simulation.

Critical temperature oscillations and reentrant superconductivity due to the FFLO like state in F/S/F trilayers

by Vladimir Zdravkov

Ann. Phys. (Berlin) 524, No. 1, 37–47 (2012)
DOI: 10.1002/andp.201100133

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a
Fulde-Ferrell... more Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a
Fulde-Ferrell Larkin-Ovchinnikov (FFLO) like state leads to interference effects of the
superconducting pairing wave function, form the core of the superconducting spin valve. The
realization of strong critical temperature oscillations in such trilayers, as a function of the
ferromagnetic layer thicknesses or, even more efficient, reentrant superconductivity, are the
key condition to obtain a large spin valve effect, i.e. a large shift in the critical temperature.
Both phenomena have been realized experimentally in the Cu 41 Ni 59 /Nb/Cu 41 Ni 59 trilayers
investigated in the present work.

SuperGreen Computing: Superconducting Computers as Green Technology

by Frank Ortmann

Original article at http://www.niocad.co.za/blog/2012/01/25/superconducting-computers-as-green-technology/

The growth in supercomputer power and the number of data centres means that their electricity use has increased to... more The growth in supercomputer power and the number of data centres means that their electricity use has increased to 1.4% of the world's electricity production. Individual supercomputers are requiring more energy than is easily provided and at a huge cost. Recent advances in superconducting circuit design suggest that superconducting computers may solve the energy efficiency problem faced by semiconductor technology.

Pairing in disordered s-wave superconductors and the effect of their coupling

by Budhaditya Chatterjee

Low-temperature anomaly of microwave absorption and ac susceptibility of single-wall carbon nanotubes: Bulk superconductivity and weak ferromagnetism

by Björn Corzilius

B. Corzilius, K.-P. Dinse, J. van Slageren, K. Hata

The electromagnetic properties of a bulk sample of high quality single-wall carbon nanotubes with a mean diameter of... more The electromagnetic properties of a bulk sample of high quality single-wall carbon nanotubes with a mean diameter of 2.5 nm were investigated by noninvasive methods. The observed sudden increase in the microwave absorptivity below 12 K is taken as indicative of the formation of a superconducting phase of a small part of the sample. The increase in the ac susceptibility peaking at 20 K points to the existence of a weak ferromagnetism probably of a different set of tubes in the heterogeneous sample. Above 20 K, the microwave loss function apparently originates predominantly from a narrow-band-gap (7 meV) one-dimensional conductor.

Raman scattering versus infrared conductivity:  Evidence for one-dimensional conduction in La2-xSrxCuO4

by Andrea Lucarelli

F. Venturini, Q.-M. Zhang, R. Hackl, A. Lucarelli, S. Lupi, M. Ortolani, P. Calvani, N. Kikugawa, and T. Fujita
Phys. Rev. B 66, 060502 (2002) – Published August 6, 2002

Raman and infrared (IR) spectra of an underdoped La1.90Sr0.10CuO4 single crystal have been measured as a function of... more Raman and infrared (IR) spectra of an underdoped La1.90Sr0.10CuO4 single crystal have been measured as a function of temperature. Both techniques provide unconventional low-energy spectra. The IR conductivity exhibits features peaked at finite frequencies which do not have a counterpart in the Raman response. Below approximately 100 K a transfer of both Raman and IR spectral weight towards lower energies is found and a new component in the Raman response builds up being characterized by a very long lifetime of electrons propagating along the Cu-O bonds.

Dynamic investigation of the transport current in YBa2Cu3O7−δ thin films

by Andrea Lucarelli

A Lucarelli , A Frey , R Yang , G Lüpke , T J Haugan , G A Levin and  P N Barnes
2008 Supercond. Sci. Technol. 21 115003 doi: 10.1088/0953-2048/21/11/115003

The current density evolution in YBa 2Cu 3O 7−δ thin films is studied by time-resolved magneto-optical imaging as a... more The current density evolution in YBa 2Cu 3O 7−δ thin films is studied by time-resolved magneto-optical imaging as a function of the phase of an ac current applied simultaneously with a perpendicular dc magnetic field. We present a new empirical method to separate the total current distribution into a circulating current, which screens the applied field, and the applied transport current. The latter shows an asymmetric profile with pronounced peaks at the edges of the sample and its phase-dependent self-field is contained in the flux region bound by the circulating current. Threading dislocations provide the necessary pinning sites for the observed high local values of the transport current.

Possible routes for synthesis of new boron-rich Fe–B and Fe1−xCrxB4 compounds

by Sheena Shah

We use ab initio calculations to examine thermodynamic factors that could promote the formation of recently proposed... more We use ab initio calculations to examine thermodynamic factors that could promote the formation of recently proposed unique oP10-FeB4 and oP12-FeB2 compounds. We demonstrate that these compact boron-rich phases are stabilized further under pressure. We also show that chromium tetraboride is more stable in the new oP10 rather than the reported oI10 structure which opens up the possibility of realizing an oP10-(FexCr1−x)B4 pseudobinary material. In addition to exhibiting remarkable electronic features, oP10-FeB4 and oP12-FeB2 are expected to be harder than the known Fe–B compounds commonly used for hard coating applications.

New Superconducting and Semiconducting Fe-B Compounds Predicted with an Ab Initio Evolutionary Search

by Sheena Shah

New candidate ground states at 1∶4, 1∶2, and 1∶1 compositions are identified in the well-known Fe-B system via a... more New candidate ground states at 1∶4, 1∶2, and 1∶1 compositions are identified in the well-known Fe-B system via a combination of ab initio high-throughput and evolutionary searches. We show that the proposed oP12-FeB2 stabilizes by a break up of 2D boron layers into 1D chains while oP10-FeB4 stabilizes by a distortion of a 3D boron network. The uniqueness of these configurations gives rise to a set of remarkable properties: oP12-FeB2 is expected to be the first semiconducting metal diboride and oP10-FeB4 is shown to have the potential for phonon-mediated superconductivity with a Tc of 15–20 K.

SQUID systems for quantum computing

by fabio chiarello

Measurements of the decoherence time from energy level quantization in Josephson junctions and rf SQUIDs

by fabio chiarello

W e have measured the effective resistance of a
Josephson junction and of a rf SQUID cooled at a more W e have measured the effective resistance of a
Josephson junction and of a rf SQUID cooled at a
temperature between 18mK and 35mK. From a fit of the data
with a simplified solution of the master equation describing
the system dynamics we found for both devices an effective
resistance R=4-5 MQ. This value, in the same system cooled at
5 mK, would lead to a decoherence time of approximately 1.1
11s allowing to perform measurements of the Rabi oscillations
having a tunneling frequency of the order of few M H z This
value is very promising also for the realization of qu-bits
realized by means of Josephson devices.

A new flux/phase qubit with integrated readout

by fabio chiarello