We present a theoretical model of spin transitions in stacks of molecular layers. Our model captures the
already established physics of these systems (thermal hysteretic transitions and crossovers) and suggests a way
towards* in situ* control of this physics by means of an external electric field. Our results pave the way toward
both temperature and voltage controllable organic memory.

We investigate heat and charge transport through a diffusive SIF1F2N tunnel junction, where N (S) is a normal (superconducting) electrode, I is an insulator layer and F1,2 are two ferromagnets with arbitrary direction of magnetization. The flow of an electric current in such structures at subgap bias is accompanied by a heat transfer from the normal metal into the superconductor, which enables refrigeration of electrons in the normal metal. We demonstrate that the refrigeration efficiency depends on the strength of the ferromagnetic exchange field h and the angle {\alpha} between the magnetizations of the two F layers. As expected, for values of h much larger than the superconducting order parameter \Delta, the proximity effect is suppressed and the efficiency of refrigeration increases with respect to a NIS junction. However, for h \sim \Delta the cooling power (i.e. the heat flow out of the normal metal reservoir) has a non-monotonic behavior as a function of h showing a minimum at h \approx \Delta. We also determine the dependence of the cooling power on the lengths of the ferromagnetic layers, the bias voltage, the temperature, the transmission of the tunneling barrier and the magnetization misalignment angle {\alpha}.

We study the diffusive electron-electron interaction correction to conductivity by analyzing simultaneously ρxx and ρxy for disordered 2D electron systems in Si in a tilted magnetic field. Tilting the field is shown to be a straightforward tool to disentangle spin and orbital effects. In particular, by changing the tilt angle we prove experimentally that in the field range gμBB>kBT the correction depends on the modulus of the magnetic field rather than on its direction, which is expected for a system with isotropic g factor. In the high-field limit, the correction behaves as ln(B), as expected theoretically [Lee and Ramakrishnan, Phys. Rev. B 26, 4009 (1982)]. Our data prove that the diffusive electron-electron interaction correction to conductivity is not solely responsible for the huge and temperature-dependent magnetoresistance in a parallel field, typically observed in Si-MOSFETs.

We report on the inelastic-scattering rate of electrons on phonons and relaxation of electron energy studied by means of magnetoconductance, and photoresponse, respectively, in a series of strongly disordered superconducting NbN films. The studied films with thicknesses in the range from 3 to 33 nm are characterized by different Ioffe-Regel parameters but an almost constant product qT l (qT is the wave vector of thermal phonons and l is the elastic mean free path of electrons). In the temperature range 14–30 K, the electron-phonon scattering rates obey temperature dependencies close to the power law 1/τe-ph ∼ T n with the exponents n ≈ 3.2–3.8.We found that in this temperature range τe-ph and n of studied films vary weakly with the thickness and square resistance. At 10 K electron-phonon scattering times are in the range 11.9–17.5 ps. The data extracted from magnetoconductance measurements were used to describe the experimental photoresponse with the two-temperature model. For thick films, the photoresponse is reasonably well described without fitting parameters, however, for thinner films, the fit requires a smaller heat capacity of phonons. We attribute this finding to the reduced density of phonon states in thin films at low temperatures. We also show that the estimated Debye temperature in the studied NbN films is noticeably smaller than in bulk material.We report on the inelastic-scattering rate of electrons on phonons and relaxation of electron energy studied by means of magnetoconductance, and photoresponse, respectively, in a series of strongly disordered superconducting NbN films. The studied films with thicknesses in the range from 3 to 33 nm are characterized by different Ioffe-Regel parameters but an almost constant product qT l (qT is the wave vector of thermal phonons and l is the elastic mean free path of electrons). In the temperature range 14–30 K, the electron-phonon scattering rates obey temperature dependencies close to the power law 1/τe-ph ∼ T n with the exponents n ≈ 3.2–3.8.We found that in this temperature range τe-ph and n of studied films vary weakly with the thickness and square resistance. At 10 K electron-phonon scattering times are in the range 11.9–17.5 ps. The data extracted from magnetoconductance measurements were used to describe the experimental photoresponse with the two-temperature model. For thick films, the photoresponse is reasonably well described without fitting parameters, however, for thinner films, the fit requires a smaller heat capacity of phonons. We attribute this finding to the reduced density of phonon states in thin films at low temperatures. We also show that the estimated Debye temperature in the studied NbN films is noticeably smaller than in bulk material.

Electron spin resonance (ESR) was studied in an asymmetrically doped 16-nm AlAs quantum well grown in the [001] direction. Surprisingly the ESR was detectable even if the magnetic field was parallel to the surface of the two-dimensional system. This allowed us to investigate precisely the in-plane anisotropy of the electron g factor. In the case of the magnetic field aligned along the [110] or [11¯0] crystallographic axes only one ESR peak was observed, whereas it tended to split into two well-separated peaks when the in-plane component of the magnetic field was between these directions. This fact clearly indicates that two in-plane valleys at X points of the Brillouin zone were occupied with electrons and each valley was characterized by the anisotropic g factor with the [100], [010], and [001] principal axes. The principal g-factor values were extracted.

We consider the pairing of massless Dirac electrons and holes located on opposite surfaces of thin films of “strong” three-dimensional topological insulators. Such pairing is predicted to give rise to a topological exciton condensate with unusual properties. We estimate the quantitatively achievable critical temperature of the pairing while taking into account the self-consistent screening of the Coulomb interaction, disorder, and hybridization of electron and hole states caused by tunneling through the film. The increase of the gap above the hybridization value when the temperature is lowered can be an observable signature of the pairing. System parameters required to observe the electron-hole pairing are discussed.

We propose controlling an electron-hole superfluid in semiconductor coupled quantum wells and double layers of a two-dimensional (2D) material by an external periodic field. This can be created either by the gates periodically located and attached to the quantum wells or double layers of the 2D material or by the moiré pattern of two twisted layers. The dependence of the electron-hole pairing order parameter on the temperature, the charge carrier density, and the gate parameters is obtained by minimization of the mean-field free energy. The second-order phase transition between superfluid and electron-hole plasma, controlled by the external periodic gate field, is analyzed for different parameters.

We report a comprehensive study of physical properties of the binary superconductor compound SnAs. The electronic band structure of SnAs was investigated using both angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and density functional theory (DFT) within generalized gradient approximation (GGA). The DFT/GGA calculations were done including spin-orbit coupling for both bulk and (111) slab crystal structures. Comparison of the DFT/GGA band dispersions with ARPES data shows that (111) slab much better describes ARPES data than just bulk bands. Superconducting properties of SnAs were studied experimentally by specific heat, magnetic susceptibility, magnetotransport measurements and Andreev reflection spectroscopy. Temperature dependences of the superconducting gap and of the specific heat were found to be well consistent with those expected for the single band BCS superconductors with an isotropic s-wave order parameter. Despite spin-orbit coupling is present in SnAs, our data shows no signatures of a potential unconventional superconductivity, and the characteristic BCS ratio 2/Tc = 3.48 − 3.73 is very close to the BCS value in the weak coupling limit.

The electron spin resonance spectrum of a quasi-1D S=1/2 antiferromagnet K2CuSO4Br2 was found to demonstrate an energy gap and a doublet of resonance lines in a wide temperature range between the Curie-Weiss and Neèl temperatures. This type of magnetic resonance absorption corresponds well to the two-spinon continuum of excitations in S=1/2 antiferromagnetic spin chain with a uniform Dzyaloshinskii-Moriya interaction between the magnetic ions. A resonance mode of paramagnetic defects demonstrating strongly anisotropic behavior due to interaction with spinon excitations in the main matrix is also observed.

We present a combined experimental and theoretical investigation of the low-frequency ESR dynamics in the ordered phases of magnetic mineral linarite. This material consists of weakly coupled spin-1/2 chains of copper ions with frustrated ferro- and antiferromagnetic interactions. In zero magnetic field, linarite orders into a spiral structure and exhibits a peculiar magnetic phase diagram sensitive to the field orientation. The resonance frequencies and their field dependence are analyzed combining microscopic and macroscopic theoretical approaches and precise values of magnetic anisotropy constants are obtained. We conclude that possible realization of exotic multipolar quantum states in this material is greatly influenced by the biaxial anisotropy.

We have studied electron spin resonance (ESR) absorption spectra for the nonmagnetically diluted strong-leg spin ladder magnet (C7H10N)2Cu(1−x)ZnxBr4 (abbreviated as DIMPY) down to 450 mK. Formation of the clusters with nonzero net magnetization is confirmed; the cluster-cluster interaction is evidenced by the concentration dependence of ESR absorption. High-temperature spin-relaxation time was found to increase with nonmagnetic dilution. The ESR linewidth analysis proves that the Dzyaloshinskii-Moriya (DM) interaction remains the dominant spin-relaxation channel in diluted DIMPY. Experimental data indicate that the dilution results in the weakening of the effective DM interaction, which can be interpreted as total suppression of DM interaction in the close vicinity of impurity atom.

We analyse the control of Majorana zero-energy states by mapping the fermionic system onto a chain of Ising spins. Although the topological protection is lost for the Ising system, the mapping provides additional insight into the nature of the quantum states. By controlling the local magnetic field, one can separate the Ising chain into ferromagnetic and paramagnetic phases, corresponding to topological and non-topological sections of the fermionic system. In this paper we propose (topologically non-protected) protocols performing the braiding operation, and in fact also more general rotations. We first consider a T-junction geometry, but we also propose a protocol for a purely one-dimensional system. Both setups rely on an extra spin-1/2 coupler. By including the extra spin in the T-junction geometry, we overcome limitations due to the 1D character of the Jordan-Wigner transformation. In the 1D geometry the coupler, which controls one of the Ising links, should be manipulated once the ferromagnetic (topological) section of the chain is moved far away. We also propose experimental implementations of our scheme. One is based on a chain of flux qubits which allows for all needed control fields. We also describe how to translate our scheme for the 1D setup to a chain of superconducting wires hosting each a pair of Majorana edge states.

The transverse autocorrelation function at an arbitrary temperature is calculated rigorously for a system with an arbitrary number of quantum spins, each of which is coupled to all the other spins with equal exchange. It is shown that the cluster approximation, formed by a model for a real magnet, explains short- and intermediate-time (up to the time to reach the spin-diffusion regime) experimental measurements in the paramagnetic phase.

Evolution of the ESR absorption in a strong-leg spin ladder magnet (C7H10N)2CuBr4 (abbreviated as DIMPY) is studied from 300 K to 400 mK. Temperature dependence of the ESR relaxation follows a staircase of crossovers between different relaxation regimes. We argue that the main mechanism of ESR line broadening in DIMPY is uniform Dzyaloshinskii-Moriya interaction (|D| = 0.31 K) with an effective longitudinal component along an exchange bond of Cu ions within the legs resulting from the low crystal symmetry of DIMPY and nontrivial orbital ordering. The same Dzyaloshinskii-Moriya interaction along with other weaker anisotropic spin-spin interactions results in the lifting of the triplet excitation degeneracy, revealed through the weak splitting of the ESR absorption at low temperatures.

Electron transport properties of titanium nanowires were experimentally studied. Below the effective diameter 50nm all samples demonstrated a pronounced broadening of the R(T ) dependencies, which cannot be accounted for by thermal fluctuations. Extensive microscopic and elemental analyses indicate the absence of structural or/and geometrical imperfections capable of broadening the R(T ) transition to such an extent. We associate the effect with quantum fluctuations of the order parameter

Using intrinsic multiple Andreev reflections effect spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors Sm1-xThxOFeAs with various thorium doping and critical temperatures TC=21-54 K. We observe a scaling between both superconducting gaps and TC. The determined BCS ratio for the large gap 2ΔL/kBTC=5.0-5.7 and its eigen-BCS ratio (in a hypothetical case of zero interband coupling) 2ΔL/kBTCL=4.1-4.6 both exceeding the weak-coupling limit 3.52, and for the small gap 2ΔS/kBTC=1.2-1.6, remain nearly constant within all the TC range studied. The temperature dependences ΔL,S(T) agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas Coulomb repulsion constants μ∗ are finite in Sm-based oxypnictides.

We consider a Josephson contact mediated by 1D chiral modes on a surface of a 3D topological insulator with superimposed superconducting and magnetic layers. The system represents an interferometer in which 1D chiral Majorana modes on the boundaries of superconducting electrodes are linked by ballistic chiral Dirac channels. This model may be realized also in recently fabricated heterostructures based on quantum anomalous Hall insulators. We investigate the noise of the Josephson current as a function of the dc phase bias and the Aharonov-Bohm flux. Starting from the scattering formalism, a Majorana representation of the Keldysh generating action for cumulants of the transmitted charge is found. At temperatures higher than the Thouless energy ETh, we obtain the usual Johnson-Nyquist noise, 4G0kBT, characteristic for a single-channel wire with G0≡e2/(2πℏ). At lower temperatures, the behavior is much richer. In particular, the equilibrium noise is strongly enhanced to a temperature-independent value ∼G0ETh if the Aharonov-Bohm and superconducting phases are both close to 2πn, which are points of emergent degeneracy in the ground state of the junction. The equilibrium noise is related to the Josephson junction's impedance via the fluctuation-dissipation theorem. In a striking contrast to usual Josephson junctions (tunnel junctions between two s-wave superconductors), the real part of the impedance does not vanish, reflecting the gapless character of Majorana modes in the leads.

An unusual behavior of the exchange energy scale of a quantum Hall ferromagnet with ν=1 was found in strongly correlated two-dimensional electron systems based on MgZnO/ZnO heterostructures. The exchange contribution, entering the energy of a collective excitation, was probed by means of inelastic light scattering. It was established that, in a wide range of electron densities corresponding to the Wigner-Seitz parameter 7<rs<11, this contribution is on the order of the cyclotron energy, which is notably different from the typical scale of e2/ɛℓB that is typical for weakly interacting systems. The same trend was confirmed via numerical calculations.