Publications

Recent studies have demonstrated that the band structure of a carbon nanotube (CNT) depends not only on its geometry but also on various factors such as atmosphere chemical composition and dielectric environment. Systematic studies of these effects require an efficient tool for an in situ investigation of a CNT band structure. In this work, we fabricate tunneling contacts to individual semiconducting carbon nanotubes through a thin layer of alumina and perform tunneling spectroscopy measurements. We use field-effect transistor configuration with four probe contacts (two tunnel and two ohmic) and bottom gates. Bandgap values extracted from tunneling measurements match the values estimated from the diameter value within the zone-folding approximation. We also observe the splitting of Van-Hove singularities of the density of states under an axial magnetic field.

We present a measurement of the branching fractions of the Cabibbo favored ¯B0→D+π− and the Cabibbo suppressed ¯B0→D+K− decays. We find B(¯B0→D+π−)=(2.48±0.01±0.09±0.04)×10−3 and B(¯B0→D+K−)=(2.03±0.05±0.07±0.03)×10−4 decays, where the first uncertainty is statistical, the second is systematic, and the third uncertainty is due to the D+→K−π+π+ branching fraction. The ratio of branching fractions of ¯B0→D+K− and ¯B0→D+π− is measured to be RD=[8.19±0.20(stat)±0.23(syst)]×10−2. These measurements are performed using the full Belle dataset, which corresponds to 772×106B¯B pairs and use the Belle II software framework for data analysis.

In ZnO-based two-dimensional electron systems with strong Coulomb interaction, the anomalous spin-exciton branch is revealed. As probed by inelastic light scattering, in a ferromagnetic quantum-Hall state with the filling factor ν = 2, a spin exciton has a negative momentum dispersion with steepness dependent on the electron density. The negative dispersion of the spin exciton is associated with its interaction with higher-energy spin-flip modes that exist near ν = 2. Surprisingly, the anti-Stokes light scattering on spin excitons at ferromagnetic state ν = 2 is amplified by orders of magnitude, indicating the macroscopic accumulation of these excitations. The experimental findings are confirmed by the exact diagonalization method—these calculations show a magnetoroton minimum in the dispersion of spin excitons and also the attractive interaction between magnetoroton spin excitations.

The electron spin resonance (ESR) of two-dimensional electrons with large effective mass was studied experimentally near even filling factors of the integer quantum Hall effect. Surprisingly, the electron spin resonance did not vanish at the exact even fillings even in the case of large electron densities, where the ground state of the system was previously reported to be paramagnetic. Furthermore, the ESR amplitude was comparable between even and odd fillings. Such anomalous behavior suggests a substantial degree of spin polarization of the even fillings and was observed in two different material systems, namely AlAs quantum wells and ZnO/MgZnO heterojunctions. In AlAs quantum wells, spin resonance tended to split into two well-resolved lines near even fillings whereas it remained a single peak near odd ones. The evolution of ESR with varying filling factor around even fillings is studied in detail. The reported nontrivial findings suggest the modification of the ground state around even filling factors with the aid of strong e-e interaction and may be viewed as a precursor for the Stoner instability

Nematic superconductors are characterized by an apparent crystal symmetry breaking that results in the anisotropy of the in-plane upper critical magnetic field Hc2 . The symmetry breaking is usually attributed to the strain of the crystal lattice. The nature and the value of the strain are debatable. We perform systematic measurements of the Hc2 anisotropy in the high-quality SrxBi2 Se3 single crystals in the temperature range 1.8 K< T < Tc ≈ 2.7 K using temperature stabilization with an accuracy of 0.0001 K. We observe that in all tested samples the anisotropy is weakly temperaure dependent when T < 0.8 Tc and smoothly decreases at higher temperatures without any sign of singularity when T → Tc . Such a behavior is in a drastic contradiction with the prediction of the Ginzburg-Landau theory for the nematic superconductors. We discuss possible reasons for this discrepancy.  

We investigate the coherent vortex produced by two-dimensional turbulence excited in a finite box. We establish analytically the mean velocity profile of the vortex for the case where the bottom friction is negligible and express its characteristics via the parameters of pumping. Our theoretical predictions are verified and confirmed by direct numerical simulations in the framework of two-dimensional weakly compressible hydrodynamics with zero boundary conditions.

A nonmonotonic dependence of the critical Josephson supercurrent on the injection current through a normal metal/ferromagnet weak link from a single domain ferromagnetic strip has been observed experimentally in nanofabricated planar crosslike S-N/F-S Josephson structures. This behavior is explained by 0–π and π–0 transitions, which can be caused by the suppression and Zeeman splitting of the induced superconductivity due to interaction between N and F layers, and the injection of spin-polarized current into the weak link. A model considering both effects has been developed. It shows the qualitative agreement between the experimental results and the theoretical model in terms of spectral supercurrent-carrying density of states of S-N/F-S structures and the spin-dependent double-step nonequilibrium quasiparticle distribution.

A nonmonotonic dependence of the critical Josephson supercurrent on the injection current through a normal metal/ferromagnet weak link from a single domain ferromagnetic strip has been observed experimentally in nanofabricated planar crosslike S-N/F-S Josephson structures. This behavior is explained by 0–π and π–0 transitions, which can be caused by the suppression and Zeeman splitting of the induced superconductivity due to interaction between N and F layers, and the injection of spin-polarized current into the weak link. A model considering both effects has been developed. It shows the qualitative agreement between the experimental results and the theoretical model in terms of spectral supercurrent-carrying density of states of S-N/F-S structures and the spin-dependent double-step nonequilibrium quasiparticle distribution.

We explore correlations of eigenstates around the many-body localization (MBL) transition in their dependence on the energy difference (frequency) ω and disorder W. In addition to the genuine many-body problem, XXZ spin chain in random field, we consider localization on random regular graphs that serves as a toy model of the MBL transition. Both models show a very similar behavior. On the localized side of the transition, the eigenstate correlation function β(ω) shows a power-law enhancement of correlations with lowering ω; the corresponding exponent depends on W. The correlation between adjacent-in-energy eigenstates exhibits a maximum at the transition point W_c, visualizing the drift of W_c with increasing system size towards its thermodynamic-limit value. The correlation function β(ω) is related, via Fourier transformation, to the Hilbert-space return probability. We discuss measurement of such (and related) eigenstate correlation functionson state-of-the-art quantum computers and simulators.

The spin resonance of two-dimensional conduction electrons in a ZnO/MgZnO heterojunction in tilted magnetic fields is studied near the filling factor . The analysis of the spin resonance intensity at various ν values indicates that a phase transition accompanied by drastic change in the spin polarization occurs in a two-dimensional electron system at a certain angle near . For ν values larger than a certain critical value νc, an intense spin resonance is observed, clearly demonstrating that the system turns out to be in a spinpolarized state. For ν < νc, the resonance amplitude drops by more than an order of magnitude, and the spin polarization of the ground state decreases significantly. In the immediate vicinity of the transition, the spin resonance is broadened significantly and split into several independent peaks. Such behavior of the resonance can most likely be explained by the division of the system into domains with different spin polarizations.

Superconductor-ferromagnet heterostructures hosting vortices and skyrmions are a new area of the interplay between superconductivity and magnetism. We study the interaction of a Néel-type skyrmion and a Pearl vortex in thin heterostructures due to stray fields. Surprisingly, we find that it can be energetically favorable for the Pearl vortex to be situated at some nonzero distance from the center of the Néel-type skyrmion. The presence of a vortex-antivortex pair is found to result in the increase of the skyrmion radius. Our theory predicts that a spontaneous generation of a vortex-antivortex pair is possible under some conditions in the presence of a Néel-type skyrmion.

Two-dimensional electron systems in a quantizing magnetic field are regarded as of exceptional interest, considering the possible role of anyons—quasiparticles with non-boson and non-fermion statistics—in applied physics. To this day, essentially none but the fractional states of the quantum Hall effect (FQHE) have been experimentally realized as a system with anyonic statistics. In determining the thermodynamic properties of anyon matter, it is crucial to gain insight into the physics of its neutral excitations. We form a macroscopic quasiequilibrium ensemble of neutral excitations - spin one anyon complexes in the Laughlin state ν = 1/3, experimentally, where ν is the electron filling factor. The ensemble is found to have such a long lifetime that it can be considered the new state of anyon matter. The properties of this state are investigated by optical techniques to reveal its Bose properties.

We have investigated the spectrum of two-dimensional (2D) plasmon polaritons over the full range of magnetic fields. In our study, we investigate a disk-shaped two-dimensional electron system (2DES) with a metallic gate on the backside of the substrate. Importantly, we show that 2D plasmon polaritons hybridize with the TM0 photonic mode of a dielectric waveguide formed by a sample substrate.We have developed a theory for plasmon-polaritons in an infinite 2DES. We find the experimental data to be in good agreement with the developed theory.

Using 980.6  fb−1 of data collected with the Belle detector operating at the KEKB asymmetric-energy e+e− collider, we present a measurement of the branching fraction of the singly Cabibbo-suppressed decay Λс+→pω. A clear Λс+ signal is observed for Λс+→pω with a statistical significance of 9.1 standard deviations, and we measure the ratio of branching fractions B(Λс+→pω)/B(Λс+→pK−π+)=(1.32±0.12(stat)±0.10(syst))×10−2, from which we infer the branching fraction B(Λс+→pω)=(8.27±0.75(stat)±0.62(syst)±0.42(ref))×10−4. The first quoted uncertainty is statistical, the second systematic, and the third from the reference mode Λс+→pK−π+.

In this paper, we present a feasibility study of the first measurement of the longitudinal polarization of a muon from the tau-lepton decay to extract the Michel parameter xi' at the future Super Charm-Tau Factory (SCTF). The expected number of muon decays events on the full SCTF data was estimated, possible background processes were studied, and methods of their suppression were suggested. Statistical accuracy was estimated to be 0.02, which enables a new verification of the Standard Model at high precision level.

A method is suggested that allows the first measurement of the polarization of muons from tau-lepton decays at the Super charm-tau Factory. To measure the polarization, it is proposed to use muons decayed in flight in the drift chamber of the detector, when both tracks, mother and daughter, were reconstructed by the reconstruction program. The method is based on the correlation between the muon spin and daughter electron momentum. We used a Monte Carlo simulation with the Super charm-tau Factory detector parameters to estimate the number of decayed-in-flight muons. Possible background processes were studied and methods for their suppression were proposed. The expected statistical uncertainty for muon polarization measurement is about 2%.

We present the measurement of the first to fourth order moments of the four-momentum transfer squared, q(2), of inclusive B -> X(c)l(+)upsilon(l) decays using the full Belle dataset of 711 fb(-1) of integrated luminosity at the gamma(4S) resonance where l = e, mu. The determination of these moments and their systematic uncertainties open new pathways to determine the absolute value of the Cabibbo-Kobayashi-Maskawa matrix element V (cb) using a reduced set of matrix elements of the heavy quark expansion. In order to identify and reconstruct the X-c system, we reconstruct one of the two B-mesons using machine learning techniques in fully hadronic decay modes. The moments are measured with progressively increasing threshold selections on q(2) starting with a lower value of 3.0 GeV2 in steps of 0.5 GeV2 up to a value of 10.0 GeV2. The measured moments are further unfolded, correcting for reconstruction and selection effects as well as QED final state radiation. We report the moments separately for electron and muon final states and observe no lepton flavor universality violating effects.

We consider a long diffusive Josephson junction where the weak link is a thin (normal metal (N)–ferromagnet (F)) bilayer (N and F form parallel links between superconductors (Ss)). We show that superconductivity in the weak link can be described by an effective one-dimensional Usadel equation containing a “diluted” exchange field as well as a weak depairing term that is caused by the inherent inhomogeneity of the bilayer. The depairing mechanism distinguishes the S(N/F)S system from an SFS junction and affects the density of states of the S(N/F)S junction. It results in the suppression of the minigap in the spin-resolved density of states. The depairing rate and the minigap are expressed in terms of geometrical parameters, the Thouless energy and the effective exchange field. The effective one-dimensional theory can be applied to various structures with thin inhomogeneous links and shows good agreement with numerical solutions of the original two-dimensional equations. We also discuss ways to reveal the predicted effect experimentally.

Mesoscopic fluctuations of the local density of states encode multifractal correlations in disordered electron systems. We study fluctuations of the local density of states in a superconducting state of weakly disordered films. We perform numerical computations in the framework of the disordered attractive Hubbard model on two-dimensional square lattices. Our numerical results are explained by an analytical theory. The numerical data and the theory together form a coherent picture of multifractal correlations of local density of states in weakly disordered superconducting films.