Research

Details; Study of coherent light-matter interaction is of considerable interest both for better understanding of the fundamentals of resonance optics and due to numerous applications. Several important models of the resonant light-matter interaction described by Maxwell-Bloch system are integrable in terms of inverse scattering transform or Riemann problem. Property of the integrability offers unique opportunity to get insight of fine details of the resonant light-matter interaction. Among them are classical system of two level atoms and so called lambda-configuration model describing system of two level atoms with degenerated ground levels. Transitions from upper state to degenerated sublevels correspond to left and right polarized light. Direct transitions between sublevels are prohibited. This project considers electrodynamics of both systems in presence of inhomogeneous broadening, when individual atomic transitions are randomly detuned from the field’s carrier frequency, and effects of switching in lambda-systems.

Objectives for 2016; 1) to find and analyze localized solutions (pulses) propagating on the continuous wave background; and 2) to obtain and analyze solutions describing polarization switching regime together with asymptotic polarization states in lambda system.

Results/current status; Using spectral deformation of the original spectral problem for the Lax operator new class of solutions for two-level classical Maxwell-Bloch system with inhomogeneous broadening was discovered. Optical pulses describing polarization switching in presence of sublevel virtual polarization of lambda system were predicted.

Team@Collaborations: Biondini Jino (State University of New York at Buffalo), Kovacic Gregor (Rensselaer Polytechnic Institute), Maimistov Andrei (Moscow Engineering Physics Institute), Newhall Katherine (University of North Carolina)

Potential Impact: Related to EIT applications and to coherent electrodynamics.

Details: Currently phase modulation format is of considerable importance for optical fiber communications. After propagation through fiber system optical pulses encoded in a phase modulation format are acquiring amplitude gitter which provides negative impact to decoding process. In-line LIXELs proposed to reduce this amplitude gitter. Due to their nonlinear properties, VIXELs, driven by external signals with amplitudes varying in a certain interval, can potentially reproduce output signals of standard amplitudes and with the same phases as an input signals. Therefore appropriately selected and tuned VIXEL can potentially reduce amplitude gitter and eventually improve bit-error-rate of transmission system.

Objectives for 2016: Design and assemble experimental setup to conduct fiber loop experiment width in-line VIXEl as an amplitude gitter equalizer with signal’s phase preserving functionality. Using standard telecom equipment analyze feasibility of this approach.

Results/current status: Two master students of Skoltech Lukaschuk Anton and Neskorenyuk Vladislav visited Institute of Microwave Technology and Photonics at the Technical University Darmstadt, and completed the first phase of the project. Experimental setup is tested and ready for experimental work. Currently both students are processing paperwork for continuation of their visit and research project.

Team@Collaborations: Lukaschuk Anton and Neskornyuk Vladislav (Skoltech), Kueppers Franko and Shipulin Arkadi (Technical University of Darmstadt)

Potential Impact: Improvement of optical fiber system performance

We have developed the scattering matrix based method for the calculation of the intensity and polarization of the point dipoles located in the periodical chiral nanostructures. The polarization state and the angular diagram can be controlled by the appropriate design of the periodical nanostructure unit sell. The calculations show good agreement with the published experimental data [K. Konishi et al., Phys. Rev. Lett. 106, 057402 (2011)].

Within this project we collaborate with

(1) group of  Prof. S.G. Tikhodeev (General Physics Institute, RAS)

(2) group of Prof. V.D. Kulakovskii (Institute of Solid State Physics, RAS)

RFBR 13-02-12144       Polarization photonics based on semiconductor nanostructures

RFBR 16-29-03283       Anisotropic semiconductor nanostructures for polarization photonics

Prof. S.G. Tikhodeev, General Physics Institute, RAS

RFBR 13-02-12139       Microcavity spinor exciton-polariton systems for integrated optoelectronics

Prof. V.D. Kulakovskii (Institute of Solid State Physics, RAS)

Prof. Leonid Butov, UCSD, USA

Physics of indirect excitons. Recently we present theoretical studies of condensation of indirect excitons in a trap. Our model quantifies the effect of screening of the trap potential by indirect excitons on the exciton condensation. The theoretical studies are applied to a system of indirect excitons in a GaAs/AlGaAs coupled quantum well structure in a diamond-shaped electrostatic trap where exciton condensation was studied in earlier experiments. The estimated condensation temperature of the indirect excitons in the trap reaches hundreds of milliKelvin.

Prof. Albert Nasibulin, CPQM, Skoltech

We explore the possibility to control the absorption properties of carbon nanotube film by it perforation. We show that drilling a regular array of the holes in the film can result in the increase of absorption in a particular spectral range. This counterintuitive result stems from the excitation of the photonic crystal resonance in CNT and it spectral position can be tuned by scaling the geometrical parameters of the array of holes.

Prof. Alexander Pevtsov, Ioffe Institute, RAS

An important class of photonic structures are opal films formed by a regular array of dielectric spheres. Its optical properties can be modified by covering the opal film surface with a thin layer of high refractive index dielectric. The theoretical modeling shows that in this system modes with strong local increase of electromagnetic field can appear. This type of the resonances can be used in nonlinear optical applications.

Prof. Yuri Vainer, Institute of Spectroscopy, Troitsk

Plasmon resonance based single molecule spectroscopy.

Details: The detailed understanding of the intricate dynamics of quantum fluids, in particular in the rapidly growing subfield of quantum turbulenceб which elucidates the evolution of a vortex tangle in a superfluid, requires an in-depth understanding of the role of finite temperature in such systems. The Landau two-fluid model is the most successful hydrodynamical theory of superfluid helium, but by the nature of the scale separations it cannot give an adequate description of the processes involving vortex dynamics and interactions. In our research we introduced a framework based on a nonlinear classical-field equation that is mathematically identical to the Landau model and provides a mechanism for severing and coalescence of vortex lines, so that the questions related to the behavior of quantized vortices can be addressed self-consistently. The correct equation of state as well as nonlocality of interactions that leads to the existence of the roton minimum can also be introduced in such description. We applied the ideas developed for finite-temperature description of weakly interacting Bose gases as possible extensions and numerical refinements of the proposed method. We apply this method to elucidate the behavior of the vortices during expansion and contraction following the change in applied pressure. We show that at low temperatures, during the contraction of the vortex core as the negative pressure grows back to positive values, the vortex line density grows through a mechanism of vortex multiplication. This mechanism is suppressed at high temperatures.

Team@Collaborations (if any): Prof Nick Proukakis, University of Newcastle, Prof Marc Brachet, University of Paris.

Publications by team members related to the project and with the Skoltech affiliations

N.G.Berloff, Marc Brachet, and N. Proukakis “Modeling quantum fluid dynamics at nonzero temperatures”, 111, 4675–4682, Proceedings of National Academy of Sciences, USA (2014)

Potential Impact: Introduced framework allows proper modeling of superfluid and normal fluid behavior and interactions including processes associated with quantized vortices in liquid helium experiments.

Details: Semiconductor microcavities are used to support freely flowing polariton quantum liquids allowing the direct observation and optical manipulation of macroscopic quantum states. Incoherent optical excitation at a point produces radially expanding condensate clouds within the planar geometry. By using arbitrary configurations of multiple pump spots, we discover a geometrically controlled phase transition, switching from the coherent phase-locking of multiple condensates to the formation of a single trapped condensate.

Team@Collaborations (if any): Nanophotonics group of Cavendish (University of Cambridge) led by Prof Jeremy Baumberg

Publications by team members related to the project and with the Skoltech affiliations:

1. Gabriel Christmann, Guilherme Tosi, Natalia G Berloff, Panagiotis Tsotsis, Peter S Eldridge, Zacharias Hatzopoulos, Pavlos G Savvidis, Jeremy J Baumberg “Oscillatory solitons and time-resolved phase locking of two polariton condensates,” New Journal of Physics, 16 (10), 103039 (2014)

2. A Dreismann, P Cristofolini, R Balili, G Christmann, F Pinsker, N G Berloff, Z Hatzopoulos, P G Savvidis, J. J. Baumberg, “Coupled counter-rotating polariton condensates in optically defined annular potentials”, 111(24):8770-5 Proceedings of National Academy of Siences, USA (2014)

3. Cristofolini, A. Dreismann, G. Christmann, G. Franchetti, N.G. Berloff, P. Tsotsis, Z. Hatzopoulos, P.G. Savvidis, J.J. Baumberg “Optical superfluid phase transitions and trapping of polariton condensates, ” Physical Review Letters, 110 , 186403 (2013).

Potential Impact: By creating new types of bosons that are coupled mixtures of optical and electronic states, condensates can be created on a semiconductor chip and potentially up to room temperature. One of the most useful implementations of macroscopic condensates involves forming rings, which exhibit new phenomena because the quantum wave-functions must join up in phase; these are used for some of the most sensitive magnetometer and accelerometer devices known.

Details: investigate the potential of organic semiconductor as the host for polariton condensates

Objectives for 2016: demonstrate of polariton condensates in the orange part of the spectrum; model the mean-field behavior of such a condensate

Results/current status In February 2016, and in the kitchen of TPOC3, we set up and demonstrated the first orange polariton condensate. Limitation in the ambient stability of the experiment meant that we had to await for the delivery of the Hybrid Photonics Labs in August 2016 where the experiments continue. We are expecting to publish the results in early 2017

Team@Collaborations (if any): In collaboration with the group of Prof David Lidzey from the University of Sheffield, who provided the samples.

External funding (if any): Industrial in-kind contributions: IBM provides nano-fabrication of ad-hoc organic cavities

Publications by team members related to the project and with the Skoltech affiliations in preparation

Potential Impact: there is lack of coherent optical sources in the orange part of the spectrum and our system offers a potential solution

Details Investigation of polariton graphs as a platform for quantum simulations. Several platforms are currently being explored for simulating physical systems whose complexity increases faster than polynomially with the number of particles or degrees of freedom in the system. Many of these computationally intractable problems can be mapped into classical spin models such as the Ising and the XY models and simulated by a suitable physical system. Here we investigate the potential of polariton graphs as an efficient simulator for finding the global minimum of the classical XY Hamiltonian. By imprinting polariton condensate lattices of bespoke geometries we show that we can simulate a large variety of systems undergoing the U(1) symmetry breaking transitions. We realise various magnetic phases, such as ferromagnetic, anti-ferromagnetic, and frustrated spin configurations on unit cells of various lattices: linear chain, square, triangular and a disordered graph. Our results provide a route to study unconventional superfluids, spin-liquids, Berezinskii-Kosterlitz-Thouless phase transition, classical magnetism among the many systems that are described by the XY Hamiltonian.

Results/current status: Initial work has led to a high impact paper in PRX (August 2016). An experimental/theoretical paper with polariton graphs as the platform for quantum simulators is now submitted to Nature Materials

Team@Collaborations (if any): Lagoudakis/Berloff

External funding (if any): SK-MIT

Publications by team members related to the project and with the Skoltech affiliations

1. Ohadi, R. L. Gregory, T. Freegarde, Y. G. Rubo, A. V. Kavokin, N. G. Berloff, and P. G. Lagoudakis. Nontrivial Phase Coupling in Polariton Multiplets.- Phys. Rev. X 6, 031032 (26 August 2016).
2. N.G. Berloff, K.Kalinin, M. Silva, W. Langbein, and P. G. Lagoudakis “Realizing the XY Hamiltonian in polariton simulators,” in review, ArXiv:1607.06065 (2016)

Potential Impact: Polariton driven quantum simulator

Details

The efficiency of 1.5% obtained by us for this kind of a structure is state of the art till date [(1.) Solid State Commun. 150 561–3, (2) Appl. Phys. Lett. 98 183113].

Objectives for 2016

1. Optimization of interface between CNT/a-Si layers, and device architecture for higher conversion efficiency.
2. Fabrication of SWCNT/a-Si heterostructures.
3. Extending SWCNT as front contact replacing the traditional TCO/metal owing to its very promising opto-electrical properties.
4. Preparation and paper publication of the research results in an international scientific journal indexed in the Scopus data base, or in “WEB of science”.

Results/current status

The current status is as follows:

1. Dry and chemical etching of thick native oxide layer on a-Si. Surface treatment of a-Si.
2. Interface engineering between a-Si and SWCNT for optimizing the energy levels.
3. Doping of SWCNT for higher conductivity retaining the transparency.
4. Modifying SWCNT for exploring the possibility to replace traditional metal grids as front contacts.

Team and Collaborations

1. Dr. Sergei Bereznev, Tallinn University of Technology.Department of Materials Science
   2. Dr. Oleg Sergeev, R&D Manager New Technologies, Division Photovoltaics, NEXT ENERGY

External funding (if any): H2020 program – ERA-NET

Publications by team members related to the project and with the Skoltech affiliations: Adinath M. Funde, Albert G. Nasibulin, Hashmi Gufran Syed, Anton S. Anisimov, Alexey Tsapenko, Peter Lund, J. D. Santos, I. Torres, J. J. Gandía, J. Cárabe, and A. D. Rozenberg, Igor A. Levitsky (2016) Carbon nanotube – amorphous silicon hybrid solar cell with improved conversion efficiency.  Nanotechnology. 27(18) 185401 (6pp)

Potential Impact: The efficiency of 1.5% obtained by us for this kind of a structure is state of the art till date [(1.) Solid State Commun. 150 561–3, (2) Appl. Phys. Lett. 98 183113]. The fabrication of the proposed solar cell partially eliminates the complex, expensive vacuum techniques and replace with very simple dry transfer technique [ACS Nano 5 3214–21] of high quality pristine SWCNT. Moreover, these SWCNT owing to its opto-electrical properties are very promising to replace the conventional TCO/ITO as front and back contacts. This leads the way to extending CNT for exploring the possibility of an ‘All Carbon solar cell’. Thus, making the structure very economical, environmentally stable and less hazardous.

Details: Development of high-precision compact gas sensor based on carbon nanotubes using the features of artificial neural networks for processing signals from various gas mixtures with different concentrations. The main idea based on the fact that pristine and chemically processed carbon nanotubes (CNT:) are very sensitive to different gas concentrations. The electrical properties of these structures strongly depend on the surrounding atmosphere. The artificial neural networks are proposed to be used for processing combination of electrical signals from CNT sensors treated in a different way.

Objectives for 2016

1. Start experiments on electrical engineering and chemical treatment of carbon nanotubes.
2. Launch of equipment and start measurements.

Results/current status: In progress (starts in the end of 2016), funded by Skoltech. Under going search and hire of a postdoc and PhD stidents. Almost all of the necessary equipment has already been purchased: gas distribution system for creation of different gas mixtures, temperature controlled stage Linkam for electrical measurements in different temperatures and surrounding gases, commutator Keysight. Purchasing of plasma treater for CNT processing is planned in the nearest future.

Team and Collaborations (if any): Albert Nasibulin (PI); Postdoc (electrical engineering, general management) – to be hired; Ph.D. student (chemical processing); MS Student (experiments for neural network learning)

External funding (if any): Additional funding provided to Res3arch package

Publications by team members related to the project and with the Skoltech affiliations: Not yet

Potential Impact: Artificial nose is very promising technology which can be applied in different R&D and technical areas such as quality control (detection of contamination and spoilage; monitoring of storage conditions), process and production (comparison with a reference product, measurement and comparison of the effects of manufacturing process on product, scale-up monitoring), health and security (quality control of food products, detection of dangerous and harmful contamination, detect odourless chemicals) etc.

Details: Developed a prototype of transparent supercapacitor with a specific capacitance of 17.5 F g^-1, which can be stretched up to 120% with practically no variation in the electrochemical performance after 1000 stretching cycles and 1000 charging-discharging cycles.

Objectives for 2016

1. Develop working prototype of stretchable device based on piezo-generator film (PVDF) and supercapacitors.
2. Study of optical, electrical and mechanical properties of stretchable electrodes made of CNT films.
3. Apply to STRIP Skoltech contest 2016 with the project on piezo-supercap device.
4. Patent submission on piezo-supercap device.

Results/current status

1. Almost in progress (in 2015).
2. All the necessary equipment is purchased and delivered (tensile testing mechanical device, tensile stage for SEM measurements and investigation), delivery of Test Cell (Japan) by the end of 2016 is expected.
3. STRIP proposal was submitted in 2015 and passed to a final stage of the contest, proposal was one of the finalist of “Green Chip” contest by ROSNANO.

Team@Collaborations (if any): Albert Nasibulin (Principal Inviestigator, Team: Evgenia Gilshteyn, Daler Amanbaev, Vladislav Tkachuk; Collaborator 1. Tania Kallio, Research Group of Electrochemical Energy Conversion and Storage, Department of Chemistry, School of Chemical Technology, Aalto University; Collaborator 2. Ekaterina Fedorovskaya, Nikolaev Institute of Inorganic Chemistry Siberian Branch of Russian Academy of Sciences; Collaborator 3. Maxim Silibin, MIET (National Research University of Electronic Technology),

External funding (if any): N/A

Publications by team members related to the project and with the Skoltech affiliations

Evgenia P. Gilshteyn, Tanja Kallio, Petri Kanninen, Ekaterina Fedorovskaya, Anton S. Anisimov, Albert G. Nasibulin (2016) Stretchable and transparent supercapacitors based on aerosol synthesized single-walled carbon nanotube films. RSC Advances 2016, Accepted Manuscript DOI: 10.1039/C6RA20319A

Potential Impact

Transparent energy conversion and storage devices have recently attracted increasing attention due to their great potential as integrated power sources for displays and windows in buildings, automobiles and aerospace vehicles. On the other hand, mechanical stretchability coupled with optical transparency of the energy storage devices is required for many other applications, ranging from self-powered rolled-up displays to self-powered wearable optoelectronics. The design of highly stretchable devices is an essential element in the development of many unprecedented applications such as electronic skin and smart energy storage clothes. Portable and wearable supercapacitors, coupled with either self-healability or stretchability, have particularly become a mainstream in the personalized electronics. In addition, aerosol synthesized and collected on a filter SWCNTs can be effectively transferred to any substrate, including stretchable substrate materials such as PDMS (polydimethylsiloxane). They fulfil all the requirements for low-cost, stretchable and transparent electronics.

Details: The primary goal of the project is to do a research with Single-walled carbon nanotubes (SWCNT) as a flexible, transparent, powerful, flat amplitude-frequency response transducer of ultrasound.

Objectives for 2016: Complete research of SWCNT ultrasound transducers in the gas mediums (frequency range 10Hz – 100 kHz), find the potential customers for the ultrasound based SWCNT transducers and make prototypes for them. Write an article covering current research results.

Results/current status: The first series of experiments of SWCNT transducers in the range of 10 Hz – 100 kHz in the air was successfully arranged with qualitative and quantitative results. Currently, we are waiting for equipment to make new series of measurements in different gases and liquids mediums.

Team@Collaborations (if any): Stepan Romanov, Daria Kopylova

External funding (if any):

Publications by team members related to the project and with the Skoltech affiliations: Stepan Romanov, Single-walled carbon nanotubes as a source of ultrasound, Skoltech Master thesis diploma 06.2016.

Potential Impact: In general, the technology is applicable in any field which uses ultrasound (Medicine, Industry, Science.). The main advantages of SWCNT-based transducers are: flexibility (allow to easily make focused ultrasound), organic material, production easily scalable, the transducer have a flat amplitude-frequency response dependence and large bandwidth.

Details: Installation of the reactors

Objectives for 2016

1. Optimization of hydrocarbon based process for high yield synthesis of SWCNT using spark discharge generator like catalyst source.
2. Using two chambers reactor for annealing of metallic CNTs in aerosol state.
3. Influence study of catalytic NPs on CNTs diameter distribution.

Results/current status

Development of reactor design;

Assembling of experimental setup/In progress;

Synthesis process optimization /In progress.

Team and Collaborations (if any): N/A

External funding (if any): N/A

Publications by team members related to the project and with the Skoltech affiliations

Potential Impact: One step synthesis of selected chirality/ narrow diameter distribution of CNTs; Possibility to get long unicherality CNTs.

Details: Demonstrated technology for fabrication of state of the art transparent (90%) conductive films with 50 Ohm/sq.

Objectives for 2016:

1. «Acids» treatment of CNT films for increasing transparency by removing iron catalytic nanoparticles;
2. Selection of dopants and solvents. Optimization of doping solution concentration.

Results/current status

1. Three best reagents for removing iron nanoparticles have been found in set of 10 different solutions. Like result we got increasing of transparency on 1-1.5% for 90% T samples;
2. Set of different P- type dopants like Re-salts, Pt-salts, Au-salts in different solvents has been used. Work on methods of covering dopants onto sample surface /In progress
3. Development of list potential N-type dopants/ In progress.

Team and Collaborations (if any): N/A

External funding (if any): N/A

Publications by team members related to the project and with the Skoltech affiliations: Article submission about doping effect of p-type dopants in different solvents / in progress.

Potential Impact: Potentially open opportunity of development p-n junction devises on SWCNT films; Increasing quality factor of transparent electrodes for various applications.

Details

Excitons with strong binding energy determine optical properties of novel 2D materials. In photodetectors based on novel transition metal dichalcogenites excitons must be ionized to lead to the photocurrent. Together with visiting researcher Benedict Scharf we solved the problem for the Stark effect and authorization in MoS2.

Team@Collaborations (if any): Visiting researcher Benedict Scharf

External funding (if any): funded from start up package

Publications by team members related to the project and with the Skoltech affiliations

Manuscript submitted to Phys. Rev. B

Results/current status: Contact resistance is the major bottleneck in using carbon nanotubes for field effect transistors. Together with a visiting student Roohollah Hafizi we found that nanotube deformation caused by the metal deposition dramatically modify electronic structure of the nanotubes with strong implications on contact resistance.

Team@Collaborations (if any): Visiting PhD student Roohollah Hafizi

External funding (if any): funded from start up package.

Publications by team members related to the project and with the Skoltech affiliations: Manuscript in preparation

Results/current status: Black phosphorous is an emerging new 2D material for electronic and opto-electronic applications. Together with my PhD student Yurii Trushkov we investigated from first principles transport properties of black phosphorous. We predicted phonon limited mobility as a function of temperature, doping level, and electric field.

Team@Collaborations (if any): PhD student Yurii Trushkov

External funding (if any): funded from start up package.

Publications by team members related to the project and with the Skoltech affiliations” Manuscript in preparation

Details: Theoretical investigations of interplay between superconductivity and Coulomb-frustrated electronic phase separation mostly in cuprates. It is a collaborative project with the experimental group of Prof. Nuh Gedik, MIT.

Current specific projects: (1) Fermi-surface in the presence of spin-vortex checkerboard. (2) Superconductivity model for spin-vortex checkerboard.

Results/current status: On-going

Team@Collaborations (if any):  Vivek Bhartiya  &Andrey Tarkhov (PhD students), Pavel Dolgirev (MSc student).

External funding (if any):

Funded by the 2015 Next Generation: Skoltech – MIT Partnership Program.

Publications by team members related to the project and with the Skoltech affiliations: Boris V. Fine,  Science 351, 235-a (2016).

Potential Impact: Understanding the mechanism of high-temperature superconductivity. Possibly, predicting a route to room-temperature superconductivity.

The project is aimed at establishing the dynamical foundations of quantum thermalization process, establishing new numerical methods of solving non-perturbative many-particle dynamics, using this expertise to develop quantum technologies. Current projects: (1) Effects of quantum measurements on quantum statistics. (2) Protection of macroscopic quantum superpositions. (3)  Time-reversal echoes for lattices of Bose-Einstein condensates. (4) Search for new algorithm for simulating the high-temperature dynamics of spin 1/2 lattices with application to solid-state nuclear magnetic resonance. (5) Dynamic localization in driven spin-1/2 lattices.

Team@Collaborations (if any): Postdoc Walter Hahn, Ph.D. students Grigory Starkov and Andrey Tarkhov

Funded by Boris Fine’s startup package

Publications by team members related to the project and with the Skoltech affiliations: C.P. Kropf, J. Kolrautz, J. Haase, B.V. Fine, Fortschr. Phys., 1–5 (2016) / DOI 10.1002/prop.201600023

Potential Impact: Making fundamental progress in quantum statistical physics. Finding new applicability limits of the conventional statistical description. Discovering new phenomena beyond these limits. Finding new way to engineer quantum simulators. Finding new more efficient methods of numerical simulations.

Details: Quantum materials for superconducting photon detectors have celebrated sensitivity in the THz frequency range. These materials consist of thin superconducting films operating close to the metal-insulator or “localization” transition. While the photon absorption and signal readout processes in these systems are well understood, the microscopic physics of the material, particularly in the presence of bias current, is currently poorly defined and understood. The goal of the project is to resolve this lack of fundamental understanding. We are going to develop a fundamental background for understanding and improving characteristics of disordered superconducting films currently used as materials for microwave detectors. We will use this new fundamental understanding to predict new classes of materials for superconducting nano-photonics.

Objectives for 2016

1) Theory of the critical temperature suppression in superconducting films

2) Calculation of superfluid density in inhomogeneous superconductors

Results/current status: in progress

Team@Collaborations (if any): Daniel Antonenko, Igor Poboiko (Skoltech PhD students): Collaborators: Mikhail Feigelman, Igor Burmistrov, Konstantin Tikhonov (Landau ITP)

External funding (if any): Supported by MIT NGP

Publications by team members related to the project and with the Skoltech affiliations” Not yet, project is just launched this September

Potential Impact: The research results are thus expected to have impact both on the technology of superconducting photon detectors through optimization of their characteristics, and on fundamental understanding of the properties of strongly inhomogeneous low‐dimensional superconducting materials.

Details

The goal of the project is to obtain the general expression for fluctuating conductivity in s-wave superconductors at arbitrary temperatures above the transition point and arbitrary disorder level. Due to technical complexity of the problem,  there exist several competing results even in the simplest case of dirty superconductors, whereas most of previously available predictions in the clean (ballistic) limit are doubtful. By deriving the general expression and tracing its crossover between the dirty and clean limit, we are going to identify inconsistencies in the previous approaches. The resulting theory will be a firm ground for studying fluctuation superconductivity in materials with a wide range of disorder mean free path.

Objectives for 2016: Analytic calculation of the DOS diagram crossed by an additional cooperon (other relevant diagrams are calculated previously). Analysis of the general expression for fluctuating conductivity in various asymptotic regions. Preparation of the paper for publication.

Results/current status: in progress

Team@Collaborations (if any): Nikolay Stepanov (Skoltech PhD student)

External funding (if any): No

Publications by team members related to the project and with the Skoltech affiliations

Details

In dirty superconductors, sharp energy gap is smeared by disorder fluctuations. This effect leads to the formation of the subgap localized states. In the first part of the project, we study the density of states (DOS) in diffusive superconductors with pointlike magnetic impurities of arbitrary strength described by the Poissonian statistics. Considering instantons in the replica sigma-model technique, we calculate the average DOS beyond the mean-field level and determine the smearing of the spectral edges due to fluctuations of the pair-breaking strength and via induced fluctuations of the order parameter. In the second part of the project, we are going to study the formation of fluctuation states in two-dimensional disordered superconductors, where the presence of multiple instanton solutions with large size reveals the tendency to delocalization of disorder-induced subgap states.

Objectives for 2016: Evaluation of the subgap density of states in two-dimensional superconductors due to multiple instanton solutions.

Results/current status: In progress

Team@Collaborations (if any): Yakov Fominov, Konstantin Tikhonov (Landau ITP)

Publications by team members related to the project and with the Skoltech affiliations: Ya. V. Fominov, M. A. Skvortsov, Subgap states in disordered superconductors with strong magnetic impurities, Phys. Rev. B 93, 144511 (2016).

Details

The complexity of the many-body localization (MBL) problem lies in the highly nontrivial topology of the Fock space. We have undertaken two different approaches to study MBL in the model case of an interacting chaotic quantum dot. First, we analyzed the initial stage of the temporal decay of an excited state, when it is disintegrating into 3-particle excitations By studying mesoscopic fluctuations of the energy relaxation rate, we determined the region of applicability of the classical Fermi-golden-rule picture at an arbitrary excitation energy and temperature of the quantum dot. To trace the final stage of the quasiparticle disintegration, we performed extensive numerical simulation of the Anderson localization on random regular graphs, which bear the main characteristics of the Fock-space space: (i) local tree-like structure, and (ii) the presence of loops (absent in the simplified Cayley tree approximation). We established a very slow and non-monotonous dependence of the eigenvalue and eigen-function statistics on the system size and demonstrated that the extended state is always ergodic, for all disorder strengths below the localization transition.

Results/current status: Completed

Team@Collaborations (if any): Vladyslav Kozii (MIT PhD student), Konstantin Tikhonov (Landau ITP), Alexander Mirlin (Karlsruhe)

Publications by team members related to the project and with the Skoltech affiliations: 1 paper published, 1 submitted

1. A. Kozii, M. A. Skvortsov, Energy relaxation rate and its mesoscopic fluctuations in quantum dots, Ann. Phys. 371, 20 (2016).
2. S. Tikhonov, A. D. Mirlin, M. A. Skvortsov, Anderson localization on random regular graphs, arXiv:1604.05353, submitted to Phys. Rev. Lett.

Details

We study electron transport at the edge of a generic disordered two-dimensional topological insulator, where some channels are topologically protected from backscattering. Assuming the total number of channels is large, we consider the edge as a quasi-one-dimensional quantum wire and describe it in terms of a nonlinear sigma model with a topological term. Neglecting localization effects, we calculate the average distribution function of transmission probabilities as a function of the sample length. We also study quantum interference effects in a two-dimensional chiral metal (bipartite lattice) with vacancies. We demonstrate that randomly distributed vacancies constitute a peculiar type of chiral disorder leading to strong modifications of critical properties at zero energy as compared to those of conventional chiral metals. When the average density of vacancies is different in the two sub-lattices, a finite concentration of zero modes emerges and a gap in the quasi-classical density of states opens around zero energy.

Results/current status: Completed

Team@Collaborations (if any): Pavel Ostrovsky (MPI Stuttgart)

Publications by team members related to the project and with the Skoltech affiliations: 2 papers published:

1. M. Ostrovsky, I. V. Protopopov, E. J. König, I. V. Gornyi, A. D. Mirlin, M. A. Skvortsov, Density of states in a two-dimensional chiral metal with vacancies, Phys. Rev. Lett. 113, 186803 (2014).
2. Khalaf, M. A. Skvortsov, P. M. Ostrovsky, Semiclassical electron transport at the edge of a 2D topological insulator: Interplay of protected and unprotected modes, Phys. Rev. B 93, 125405 (2016).