arXiv subjects
Publications and source records attributed to A. L. Burin.
We analyze the role of spectral diffusion in the problem of many-body delocalization in quantum dots and in extended systems. The spectral diffusion parametrically enhances delocalization, modifying the scaling of the delocalization threshold with the interaction coupling constant.
We study the heat conductivity in Anderson insulators in the presence of power-law interaction. Particle-hole excitations built on localized electron states are viewed as two-level systems randomly distributed in space and energy and coupled due to electron-electron interaction. A small fraction of these states form resonant pairs that in turn build a complex network allowing for energy propagation. We identify the character of energy transport through this network and evaluate the thermal conductivity. For physically relevant cases of 2D and 3D spin systems with $1/r^3$ dipole-dipole interaction (originating from the conventional $1/r$ Coulomb interaction between electrons), the found thermal conductivity $κ$ scales with temperature as $κ\propto T^3 $ and $κ\propto T^{4/3}$, respectively. Our results may be of relevance also to other realizations of random spin Hamiltonians with long-range interactions.
Tunneling two level systems (TLS), present in dielectrics at low temperatures, have been recently studied for fundamental understanding and superconducting device development. According to a recent theory by Burin \textit{et al.}, the TLS bath of any amorphous dielectric experiences a distribution of Landau-Zener transitions if exposed to simultaneous fields. In this experiment we measure amorphous insulating films at millikelvin temperatures with a microwave field and a swept electric field bias using a superconducting resonator. We find that the maximum dielectric loss per microwave photon with the simultaneous fields is approximately the same as that in the equilibrium state, in agreement with the generic material theory. In addition, we find that the loss depends on the fields in a way which allows for the separate extraction of the TLS bath dipole moment and density of states. This method allows for the study of the TLS dipole moment in a diverse set of disordered films, and provides a technique for continuously inverting their population.
We show that $1/f$-noise in the variable range hopping regime is related to transitions of many-electrons clusters (fluctuators) between two almost degenerate states. Giant fluctuation times necessary for $1/f$-noise are provided by slow rate of simultaneous tunneling of many localized electrons and by large activation barriers for their consecutive rearrangements. The Hooge constant steeply grows with decreasing temperature because it is easier to find a slow fluctuator at lower temperatures. Our conclusions qualitatively agree with the low temperature observations of $1/f$-noise in p-type silicon and GaAs.
The dependence of DNA assemblies conductance on relative humidity is investigated theoretically. Following earlier suggestions, we consider the ionic conductivity through the layers of water adsorbed by DNA molecules. The increase in humidity results in a growing water layer. The binding energy of ions depends on the thickness of the water layer due to change in water polarization. This dependence is very strong at smaller thicknesses of water layers due to the low-dimensional confinement of an electric field in water. We show that the associated change in ion concentration can explain the six orders of magnitude increase in conductivity, with relative humidity growing from 0.05 to 0.95
We study the low temperature dynamics in films made of molecular magnets, i. e. crystals composed of molecules having large electronic spin S in their ground state. The electronic spin dynamics is mediated by coupling to a nuclear spin bath; this coupling allows transitions for a small fraction of electronic spins between their two energy minima, Sz=+- S, under resonant conditions when the change of the Zeeman energy in magnetic dipolar field of other electronic spins is compensated by interaction with nuclear spins. Transitions of resonant spins can result in opening or closing resonances in their neighbors leading to the collective dynamics at sufficiently large density P0 of resonant spins. We formulate and solve the equivalent dynamic percolation problem for the Bethe lattice (BL) of spins interacting with z neighbors and find that depending on the density of resonant spins P0 and the number of neighbors z the system has either one (2 5) kinetic transitions at P_{0}=Pc1 ~ exp(-1/3)/(3z) and P0=Pc2 ~ exp(-1)/z. The former transition is continuous and associated with the formation of an infinite cluster of coupled resonant spins similarly to the static percolation transition occurring at P0 ~ 1/z. The latter transition, z>5, is discontinuous and associated with the instantaneous increase in the density of resonant spins from the small value ~ 1/z to near unity. Experimental implications of our results are discussed.
The interference of polariton guiding modes propagating through "traffic circle" waveguides composed of dielectric spherical particles is investigated. The dependence of intensity of the wave on the position of the particle was studied using the multisphere the Mie scattering formalism. We show that if the frequency of light belongs to the passband of the waveguide, electromagnetic waves may be considered as two optical beams running along a circle in opposite directions and interfering with each other. Indeed, the obtained intensity behavior can be represented as a simple superposition of two waves propagating around a circle in opposite directions. The applications of this interference are discussed.
We discuss memory effects in the conductance of hopping insulators due to slow rearrangements of many-electron clusters leading to formation of polarons close to the electron hopping sites. An abrupt change in the gate voltage and corresponding shift of the chemical potential change populations of the hopping sites, which then slowly relax due to rearrangements of the clusters. As a result, the density of hopping states becomes time dependent on a scale relevant to rearrangement of the structural defects leading to the excess time dependent conductivity.
We investigate the effect of exciton coupling on the optical absorption spectrum of polymer molecules under conditions of strong inhomogeneous broadening. We demonstrate that the dependence of the maximum in the rescaled absorption spectrum on the number of monomers is determined by the average monomer excitation energies and their resonant coupling and insensitive to the inhomogeneous broadening. Thus the absorption spectrum can be used to determine optical interactions between monomers. The results are applied to the absorption spectra of poly-A poly-T DNA hairpins and used to interpret the dependence of the absorption spectrum on the number of monomers. We also discuss exciton localization in these hairpins.
We discuss memory effects in the conductance of hopping insulators due to slow rearrangements of structural defects leading to formation of polarons close to the electron hopping states. An abrupt change in the gate voltage and corresponding shift of the chemical potential change populations of the hopping sites, which then slowly relax due to rearrangements of structural defects. As a result, the density of hopping states becomes time dependent on a scale relevant to rearrangement of the structural defects leading to the excess time dependent conductivity.
Microscopic mechanisms of positive charge transfer in DNA remain unclear. A quantum state of electron hole in DNA is determined by the competition of a pi-stacking interaction $b$ smearing a charge between different base pairs and interaction $λ$ with the local environment which attempts to trap charge. To determine which interaction dominates we investigated charge quantum states in various $(GC)_{n}$ sequences choosing DNA parameters satisfying experimental data for the balance of charge transfer rates $G^{+} \leftrightarrow G_{n}^{+}$, $n=2,3$ \cite{FredMain}. We show that experimental data can be consistent with theory only under an assumption $b\ll λ$ meaning that charge is typically localized within a single $G$ site. Consequently any DNA sequence including the one consisting of identical base pairs behaves more like an insulating material than a molecular conductor.
We investigate the stability of the recently discovered room temperature Bose-Einstein condensate (BEC) of magnons in Ytrrium Iron Garnet (YIG) films. We show that magnon-magnon interactions depend strongly on the external field orientation, and that the BEC in current experiments is actually metastable - it only survives because of finite size effects, and because the BEC density is very low. On the other hand a strong field applied perpendicular to the sample plane leads to a repulsive magnon-magnon interaction; we predict that a high-density magnon BEC can then be formed in this perpendicular field geometry.
We comment on recent work of Bodea et al and discuss the numerical prefactor in the interaction stimulated relaxation in amorphous solids.
We accurately treat the effect of the double and triple interactions of two-level systems (TLS) in glasses onto the energy delocalization due to the long-range interaction of TLS. Although this work qualitatively reproduces the estimates of our previous work we believe that it is important because in this paper the estimates are done with quantitative accuracy. This work can serve as the important step towards the development of quantitative theory of many-body delocalization due to the long-range interaction. Our study is compared with the recent work by Bodea et al claiming that the interaction of TLS triples leads to the energy delocalization. We cannot agree with the mentioned work because as we show the interaction of TLS triples was overestimated there and therefore the result for the TLS relaxation rate obtained there is invalid.
The anomalous magnetic field dependence of dielectric properties of insulating glasses in the temperature interval $10mK<T<50mK$ is considered. In this temperature range, the dielectric permittivity is defined by the resonant contribution of tunneling systems. The external magnetic field regulates nuclear spins of tunneling atoms. This regulation suppresses a nuclear quadrupole interaction of these spins with lattice and, thus, affects the dielectric response of tunneling systems. It is demonstrated that in the absence of an external magnetic field the nuclear quadrupole interaction $b$ results in the correction to the permittivity $δχ\sim| b| /T$ in the temperature range of interest. An application of a magnetic field results in a sharp increase of this correction approximately by a factor of two when the Zeeman splitting $m$ approaches the order of $| b| $. Further increase of the magnetic field results in a relatively smooth decrease in the correction until the Zeeman splitting approaches the temperature. This smooth dependence results from tunneling accompanied by a change of the nuclear spin projection. As the magnetic field surpasses the temperature, the correction vanishes. The results obtained in this paper are compared with experiment. A new mechanism of the low temperature nuclear spin-lattice relaxation in glasses is considered.
We consider the effect of the internal nuclear quadrupole interaction on quantum tunneling in complex multi-atomic two-level systems. Two distinct regimes of strong and weak interactions are found. The regimes depend on the relationship between a characteristic energy of the nuclear quadrupole interaction $λ_{\ast}$ and a bare tunneling coupling strength $Δ_{0}$. When $Δ_{0}>λ_{\ast}$, the internal interaction is negligible and tunneling remains coherent determined by $Δ_{0}$. When $Δ_{0}<λ_{\ast}$, coherent tunneling breaks down and an effective tunneling amplitude decreases by an exponentially small overlap factor $η^{\ast}\ll1$ between internal ground states of left and right wells of a tunneling system. This affects thermal and kinetic properties of tunneling systems at low temperatures $T<λ_{*}$. The theory is applied for interpreting the anomalous behavior of the resonant dielectric susceptibility in amorphous solids at low temperatures $T\leq 5$mK where the nuclear quadrupole interaction breaks down coherent tunneling. We suggest the experiments with external magnetic fields to test our predictions and to clarify the internal structure of tunneling systems in amorphous solids.
The standard tunneling model describes quite satisfactorily the thermal properties of amorphous solids at temperatures $T<1K$ in terms of an ensemble of two-level systems possessing logarithmically uniform distribution over their tunneling amplitudes and uniform distribution over their asymmetry energies. In particular, this distribution explains the observable logarithmic temperature dependence of the dielectric constant. Yet, experiments have shown that at ultralow temperatures $T<5mK$ such a temperature behavior breaks down and the dielectric constant becomes temperature independent (plateau effect). In this letter we suggest an explanation of this behavior exploiting the effect of the nuclear quadrupole interaction on tunneling. We show that below a temperature corresponding to the characteristic energy of the nuclear quadrupole interaction the effective tunneling amplitude is reduced by a small overlap factor of the nuclear quadrupole ground states in the left and right potential wells of the tunneling system. It is just this reduction that explains the plateau effect . We predict that the application of a sufficiently large magnetic field $B>10T$ should restore the logarithmic dependence because of the suppression of the nuclear quadrupole interaction.
We present a numerical study of lasing modes in diffusive random media with local pumping. The reabsorption of emitted light suppresses the feedback from the unpumped part of the sample and effectively reduces the system size. The lasing modes are dramatically different from the quasimodes of the passive system (without gain or absorption). Even if all the quasimodes of a passive diffusive system are extended across the entire sample, the lasing modes are still confined in the pumped volume with an exponential tail outside it. The reduction of effective system volume by absorption broadens the distribution of decay rates of quasimodes and facilitates the occurrence of discrete lasing peaks.