Moscow Institute of Physics and Technology

Theoretical nanophysics laboratory (laboratory is closed since 31.12.2016)

Upcoming quantum-nanophysics seminars
Theory seminar, Thursday, November 22, 2018, Kapitza Institute, 11:30 am

Vladimir Krasnov (Stockholm University, Sweden)

Josephson and non-Josephson emission from Bi2Sr2CaCu2O8+δ mesa structures

Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. When voltage is applied to such mesas, they can generate various types of waves (photons, phonons, polaritons, magnons, plasmons) in a variety of different ways. In this talk I will overview both Josephson and non-Josephson mechanisms of emission, including the standard ac-Josephson effect for emission of electromagnetic waves [1], segnetoelectric effect for emission of phonons and polaritons [2] and non-equilibrium quasiparticle relaxation and recombination for emission of any type of bosons, having strong electron-boson interaction and participating in pairing [3]. Those effects are important both for applied research, e.g., creation of tunable, compact, continuous wave and monochromatic THz source with a frequency span in the whole THz gap region and beyond 0.1-15 THz [1,2], and for fundamental understanding of the mechanism of pairing in high-temperature superconductors [3].
References:
[1] E. A. Borodianskyi and V.M. Krasnov, Josephson emission with frequency span 1–11 THz from small Bi2Sr2CaCu2O8+δ mesa structures, Nature Commun. 8, 1742 (2017).
[2] S. O. Katterwe, H. Motzkau, A. Rydh, and V. M. Krasnov, Coherent generation of phonon-polaritons in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions, Phys. Rev. B 83, 100510(R) (2011).
[3] V.M. Krasnov, S.O. Katterwe, & A. Rydh, Signatures of the electronic nature of pairing in high-Tc superconductors obtained by non-equilibrium boson spectroscopy, Nature Commun. 4, 2970 (2013).

Scientific Council of the Landau Institute, Friday, November 23, 2018, Landau Institute, 11:30 am

P. A. Ioselevich, P. M. Ostrovsky, Ya. V. Fominov

Mesoscopic supercurrent fluctuations in diffusive magnetic Josephson junctions

We study the supercurrent in quasi-one-dimensional Josephson junctions with a weak link involving magnetism, either via magnetic impurities or via ferromagnetism. In the case of weak links longer than the magnetic pair-breaking length, the Josephson effect is dominated by mesoscopic fluctuations. We establish the supercurrent-phase relation (CPR) along with statistics of its sample-dependent properties in junctions with transparent contacts between leads and link. High transparency gives rise to the inverse proximity effect, while the direct proximity effect is suppressed by magnetism in the link. We find that all harmonics are present in the CPR. Each harmonic has its own sample-dependent amplitude and phase shift with no correlation between different harmonics. Depending on the type of magnetic weak link, the system can realize a \varphi_0 or \varphi junction in the fluctuational regime. Full supercurrent statistics is obtained at arbitrary relation between temperature, superconducting gap, and the Thouless energy of the weak link.

Scientific Council of the Landau Institute, Friday, November 23, 2018, Landau Institute, 11:30 am

Konstantin Tikhonov

Statistics of eigenstates near the localization transition on random regular graphs

Dynamical and spatial correlations of eigenfunctions as well as energy level correlations in the Anderson model on random regular graphs (RRG) are studied. We consider the critical point of the Anderson transition and the delocalized phase. In the delocalized phase near the transition point, the observables show a broad critical regime for system sizes below the correlation volume and then cross over to the ergodic behavior. Eigenstate correlations allow us to visualize the correlation length that controls the finite-size scaling near the transition. The critical-to-ergodic crossover is very peculiar, since the critical point is similar to the localized phase, whereas the ergodic regime is characterized by very fast diffusion which is similar to the ballistic transport. In particular, the return probability crosses over from a logarithmically slow variation with time in the critical regime to an exponentially fast decay in the ergodic regime. We find a perfect agreement between results of exact diagonalization and those resulting from the solution of the self-consistency equation obtained within the saddle-point analysis of the effective supersymmetric action. We show that the RRG model can be viewed as an intricate limit of the Anderson model in spatial dimensions.

Visiting address:

Laboratory building, room 122
Moscow Institute of Physics and Technology
Dolgoprudny, Russia

contact e-mail address: nanotheory@phystech.edu (head M.V.Feigel'man, deputy head I.V.Zagorodnev)

Research topics
  • Mesoscopic electronic systems
  • Superconducting hybrid structures
  • Quantum phase transitions
  • Spintronics
  • 2DEG and quantum Hall effect
  • Quantum magnetism and topological order
  • Physics of quantum computation
Recent quantum-nanophysics seminars
Scientific Council of the Landau Institute, Friday, November 16, 2018, Landau Institute, 11:30 am

Ya.V. Fominov

Poverkhnostnyi impedans na diffuznoi granitse kiral'nogo p-volnovogo sverkhprovodnika

Vychislena lokal'naya kompleksnaya provodimost' i obuslovlennyi eyu poverkhnostnyi impedans na diffuznoi granitse kiral'nogo p-volnovogo sverkhprovodnika. Imenno kiral'noe p-volnovoe sostoyanie schitaetsya naibolee veroyatnym v sverkhprovodyashchem rutenate strontsiya Sr2RuO4. Ono anizotropnoe i pri etom polnost'yu shchelevoe (modul' parametra poryadka fiksirovan, a faza zavisit ot napravleniya). My rassmatrivaem otklik na vneshnee elektromagnitnoe pole kak na podshchelevykh, tak i na nadshchelevykh chastotakh. Izucheny anomal'nye osobennosti poverkhnostnogo impedansa, svyazannye s generatsiei vblizi granitsy nechetnykh po chastote sverkhprovodyashchikh korrelyatsii (sostoyanie tipa sverkhprovodimosti Berezinskogo). Teoreticheskie rezul'taty sopostavleny s izmereniyami poverkhnostnogo impedansa Sr2RuO4, provedennymi v IFTT. Nablyudaetsya kachestvennoe soglasie teorii i eksperimenta.
Doklad osnovan na rabote S. V. Bakurskiy, Ya. V. Fominov, A. F. Shevchun, Y. Asano, Y. Tanaka, M. Yu. Kupriyanov, A. A. Golubov, M. R. Trunin, H. Kashiwaya, S. Kashiwaya, and Y. Maeno, "Local impedance on a rough surface of a chiral p-wave superconductor", Phys. Rev. B 98, 134508 (2018); https://arxiv.org/pdf/1807.11735.pdf

Scientific Council of the Landau Institute, Friday, November 16, 2018, Landau Institute, 11:30 am

Ya.V. Fominov

Sverkhprovodyashchii spinovyi klapan v sistemakh so splavom Geislera

Provedeno teoreticheskoe soprovozhdenie eksperimentov po izmereniyu effekta sverkhprovodyashchego spinovogo klapana v sisteme tipa FFS (gde F — ferromagnetik, S — sverkhprovodnik). Effekt sostoit v tom, chto kriticheskaya temperatura Tc sistemy zavisit ot vzaimnoi orientatsii namagnichennostei dvukh ferromagnetikov. V eksperimente v kachestve srednego F sloya byl vzyat splav Geislera Co2Cr1-xFexAl, i eto pozvolilo usilit' effekt (raznost' Tc pri parallel'noi i antiparallel'noi orientatsii) po sravneniyu s ranee issledovannymi sistemami s zhelezom. Znak effekta zavisit ot tolshchiny sloya. Teoriya ob'yasnyaet poluchennye rezul'taty. Usilenie effekta okazyvaetsya svyazano s umen'shennoi velichinoi obmennogo polya v splave Geislera.
Doklad osnovan na sleduyushchikh rabotakh:
[1] A. Kamashev, A. Validov, N. Garif’yanov, Ya. Fominov, P. Leksin, J. Schumann, J. Thomas, V. Kataev, B. Büchner, I. Garifullin, "Isolation of proximity-induced triplet pairing channel in a superconductor/ferromagnet spin valve", EPJ Web of Conferences 185, 08001 (2018).
[2] A.A. Kamashev, A.A. Validov, J. Schumann, V. Kataev, B. Büchner, Ya.V. Fominov, I.A. Garifullin, "Increasing the performance of a superconducting spin valve using a Heusler alloy", Beilstein J. Nanotechnol. 9, 1764 (2018).

Shmidtovskii seminar po sverkhprovodimosti, Thursday, November 15, 2018, MISiS, auditoriya B-607, 5:00 pm

Tim Duty (University of New South Wales, Australia), Karin Cedergren, Sergey Kafanov, Roger Ackroyd and Jared H. Cole

Magnetic-field-induced parity effect in insulating Josephson junction chains

We report the experimental manifestation of even-odd parity effects in the transport characteristics of insulating Josephson junction chains, which occur as the superconducting gap is suppressed by applied magnetic fields at millikelvin temperatures. The primary signature is a non-monotonic dependence of the critical voltage, Vc, for the onset of charge transport through the chain, with the parity crossover indicated by a maximum of Vc at the parity field B*, We also observe a distinctive change in the transport characteristics across the parity transition, indicative of Cooper-pair dominated transport below B*, giving way to single-electron dominated transport above B*, For fields applied in the plane of the superconducting aluminum films, the parity effect is found to occur at the field, B*||, such that the superconducting gap equals the single-electron charging energy, Δ(B*||)=EC. On the contrary, the parity effect for perpendicularly applied fields can occur at relatively lower fields, B*⊥≃ 2Φ0/AI, depending only on island area, AI. In this case, the parity effect occurs in sync with formation of the single-vortex state of the islands in the chain. Our results suggest a novel explanation for the insulating peak observed in disordered superconducting films and one-dimensional strips patterned from such films, which occurs at a finite magnetic field.