arXiv subjects

Alexander L. Burin

Publications and source records attributed to Alexander L. Burin.

At least 19 recordsLinked to original sources

arXiv2026

How alignment controls heat transport in polymer chains with kinks?

Thermal transport in polymer chains is commonly attributed to ballistic propagation of long-wavelength acoustic phonons, which act as Goldstone modes protected by translational symmetry, whereas transport by higher-frequency phonons is suppressed by Anderson localization. Consistent with this picture, highly aligned polymers exhibit exceptionally high thermal conductivity, while poorly aligned polymers are orders of magnitude less conductive and serve as efficient thermal insulators. Here we show that this striking sensitivity to molecular alignment originates from acoustic-phonon scattering by molecular kinks. In the long-wavelength limit, longitudinal acoustic (LA) phonons are completely reflected by a single kink, whereas transverse acoustic (TA) phonons exhibit a universal transmission coefficient of one half. We show that the strong reflection results from the breaking of translational symmetry caused by the change in molecular-axis direction at the kink, while the universal TA transmission originates from virtual scattering through an evanescent transverse Bloch mode. The resulting strong suppression of long-wavelength phonon transport dramatically reduces the thermal conductivity of poorly aligned chains. These findings identify kink engineering as a promising strategy for controlling thermal transport in polymeric materials.

cond-mat.soft↗

arXiv2026

Thermal conductivity of aligned polymers with kinks

Thermal conductivity of aligned polymer molecules can be exceptionally high along the alignment direction due to energy transport through strong covalent bonds. At the same time, it is highly sensitive to molecular conformation, varying by orders of magnitude as a result of gauche kinks. Here, we theoretically investigate phonon transport in kinked polymers by numerically evaluating thermal conductivity and interpreting the results in terms of phonon scattering from randomly distributed kinks. For strongly aligned polymers with restricted deviations from a linear backbone, we find that heat transport becomes superdiffusive at long lengths, with thermal conductivity scaling as $κ\propto L^{1/3}$. At shorter lengths, thermal conductivity exhibits non-monotonic behavior: it increases at very short scales due to ballistic transport of almost all phonons, then decreases at intermediate lengths due to the Anderson localization of most phonon modes. These results are consistent with experiments and molecular dynamics simulations, and they elucidate the microscopic mechanisms governing heat transport in polymers.

physics.chem-ph↗

arXiv2025

Many-body localization in a quantum Ising model with the long-range interaction: Accurate determination of the transition point

Many-body localization (MBL) transition emerges at strong disorder in interacting systems, separating chaotic and reversible dynamics. Although the existence of MBL transition within the macroscopic limit in spin chains with a short-range interaction was proved rigorously, the transition point is not found yet because of the dramatic sensitivity of the transition point to the chain length at computationally accessible lengths, possible due to local fluctuations destroying localization. Here we investigate MBL transition in the quantum Ising model (Ising model in a transverse field) with the long-range interaction suppressing the fluctuations similarly to that for the second-order phase transitions. We estimate the MBL threshold within the logarithmic accuracy using exact results for a somewhat similar localization problem on a Bethe lattice problem and show that our expectations are fully consistent with the estimate of the transition point using exact diagonalization. In spite of unlimited growing of the critical disorder within the thermodynamic limit, this result offers the opportunity to probe the critical behavior of the system near the transition point. Moreover, the model is relevant for the wide variety of physical systems with the long-range dipole-dipole, elastic or indirect exchange interactions.

cond-mat.dis-nn↗

arXiv2025

The origin of anomalous non-linear microwave absorption in Josephson junction qubits: mysterious nature of two level systems or their dynamic interaction?

Quantum two-level systems (TLSs) commonly found at low temperature in amorphous and disordered materials are responsible for decoherence in superconducting Josephson junction qubits particularly because they absorb energy of coherent qubit oscillations in the microwave frequency range. In planar Josephson resonators with oxide interfaces this absorption is characterized by an anomalously weak loss tangent dependence on the field in the non-linear regime that conflicts with the theoretical expectations and the observations in amorphous dielectrics. It was recently suggested that this anomalous absorption is due to TLS dynamic interactions. Here we show that such interactions cannot lead to the observed loss-tangent field dependence and suggest the alternative explanation assuming that TLS dipole moments $p$ are distributed according to the specific power law $P(p) \propto 1/p^{3-η}$ ($0\leq η<1$). This assumption, indeed, results in the observed loss tangent behavior. The hypothesis of a power law distribution is supported both by the recent measurements of individual TLS dipole moments and the theoretical model of TLS formation due to the long-range dipole-dipole interaction, thus connecting the anomalous absorption with the possible solution of the long-standing problem of the nature of TLSs.

cond-mat.dis-nn↗

arXiv2025

Super diffusive length dependent thermal conductivity in one-dimensional materials with structural defects: longitudinal to transverse phonon scattering leads to $κ\propto L^{1/3}$ law

Structural defects in one-dimensional heat conductors couple longitudinal (stretching) and transverse (bending) vibrations. This coupling results in the scattering of longitudinal phonons to transverse phonons and backwards. We show that the decay rate of longitudinal phonons due to this scattering scales with their frequencies as $ω^{3/2}$ within the long wavelength limit ($ω\rightarrow 0$), which is more efficient scattering compared to the traditionally considered Rayleigh scattering within the longitudinal band ($ω^2$). This scattering results in temperature independent thermal conductivity depending on the size as $κ\propto L^{1/3}$ for sufficiently long materials. This predicted length dependence is observed in nanowires, though the temperature dependence is seen there possibly because of deviations from pure one-dimensional behavior. The significant effect of interaction of longitudinal phonons with transverse phonons is consistent with the earlier observations of a substantial suppression of thermal energy transport by kinks, obviously leading to such interaction, though anharmonic interaction can also be significant.

physics.chem-ph↗

arXiv2024

Two stage decoherence of optical phonons in long oligomers

Intramolecular energy transport is generally responsible for chemical energy balance in molecular systems. The transport is fast and efficient if energy is transferred by optical phonons in periodic oligomers, but its efficiently is limited by decoherence emerging due to anharmonic interactions with acoustic phonons. We show that in the most common case of the optical phonon band being narrower than the acoustic bands decoherence takes place in two stages. The faster stage involves optical phonon multiple forward scattering due to absorption and emission of transverse acoustic phonons, i. e. collective bending modes with a quadratic spectrum; the transport remains ballistic and the speed can be altered. The subsequent slower stage involves phonon backscattering in multiphonon processes involving two or more acostic phonons resulting is a switch to diffusive transport. If the initially excited optical phonon possesses a relatively small group velocity, then its equilibration in the first stage is accompanied by its acceleration due to its transitions to states propagating faster. This theoretical expectation is consistent with the recent measurements of optical phonon transport in alkane chains, accelerating with increasing the chain length.

physics.chem-ph↗

arXiv2024

Ballistic Energy Transport via Long Alkyl Chains: A New Initiation Mechanism

In an effort to increase the speed and efficiency of ballistic energy transport via oligomeric chains, we performed measurements of the transport in compounds featuring long alkyl chains of up to 37 methylene units. Compounds of the N3-(CH2)n-COOMe type (denoted as aznME) were synthesized with n = 5, 10, 15, 19, 28, 37 and studied using relaxation-assisted two-dimensional infrared spectroscopy. The speed of the ballistic transport, initiated by the N3 tag excitation, increased ca. 3-fold for the longer chains (n = 19-37) compared to the shorter chains, from 14.7 Å/ps to 48 Å/ps, in line with an earlier prediction (Nawagamuwage et al. 2021, J. Phys. Chem. B, 125, 7546). Modeling, based on solving numerically the Liouville equation, was capable of reproducing the experimental data only if three wavepackets are included, involving CH2 twisting (Tw), wagging (W), and rocking (Ro) chain bands. The approaches for designing molecular systems featuring higher speed and efficiency of energy transport are discussed.

physics.chem-ph↗

arXiv2024

Superdiffusion in random two dimensional system with time-reversal symmetry and long-range hopping

Although it is recognized that Anderson localization takes place for all states at a dimension $d$ less or equal $2$, while delocalization is expected for hopping $V(r)$ decreasing with the distance slower or as $r^{-d}$, the localization problem in the crossover regime for the dimension $d=2$ and hopping $V(r) \propto r^{-2}$ is not resolved yet. Following earlier suggestions we show that for the hopping determined by two-dimensional anisotropic dipole-dipole interactions in the presence of time-reversal symmetry there exist two distinguishable phases at weak and strong disorder. The first phase is characterized by ergodic dynamics and superdiffusive transport, while the second phase is characterized by diffusive transport and delocalized eigenstates with fractal dimension less than $2$. The transition between phases is resolved analytically using the extension of scaling theory of localization and verified numerically using an exact numerical diagonalization.

cond-mat.dis-nn↗

arXiv2024

Red shift of the superconductivity cavity resonance in Josephson junction qubits as a direct signature of TLS population inversion

Quantum two-level systems (TLSs) limit the performance of superconducting qubits and superconducting and optomechanical resonators breaking down the coherence and absorbing the energy of oscillations. TLS absorption can be suppressed or even switched to the gain regime by inverting TLS populations. Here we theoretically explore the regime where the full inversion of TLS populations is attained at energies below a pump field quantization energy by simultaneously applying the pump field and the time varying bias. This regime is attained changing the bias sufficiently slowly to fully invert TLS populations when their energies cross resonance with the pump field and sufficiently fast to avoid TLS relaxation between two resonance crossing events. This population inversion is accompanied by a significant red shift of cavity resonance due to quantum level repulsion. The red-shift in frequency serves as a signature of the population inversion, as its re-entrant behavior as function of bias sweep rate and of the magnitude of the pump field allows the determination of the TLSs dipole moment and relaxation time. The predicted behavior is qualitatively consistent with the recent experimental observations in Al superconducting resonators.

cond-mat.dis-nn↗

arXiv2023

Maximum propagation speed and Cherenkov effect in optical phonon transport through periodic molecular chains

Optical phonons serve as the fast and efficient carriers of energy across periodic polymers due to their delocalization, large group velocity because of covalent bonding and large energy quantum compared to that for acoustic phonons, as it was observed in a number of recent measurements in different oligomers. However, this transport is dramatically sensitive to anharmonic interactions, including the unavoidable interaction with acoustic phonons responsible for the transport decoherence, suppressing ballistic transport at long distances. Here we show that this decoherence is substantially suppressed if the group velocity of optical phonons is less than the sound velocity of acoustic phonons; otherwise ballistic transport is substantially suppressed by a Cherenkov's like emission of acoustic phonons. This conclusion is justified considering energy and momentum conservation during phonon absorption or emission and supported by the numerical evaluation of lifetimes of the optical phonons. It is also consistent with the recent experimental investigations of ballistic optical phonon transport in oligomers with minor exception of relatively short oligophenylenes.

physics.chem-ph↗

arXiv2022

Anisotropy-mediated reentrant localization

We consider a 2d dipolar system, $d=2$, with the generalized dipole-dipole interaction $\sim r^{-a}$, and the power $a$ controlled experimentally in trapped-ion or Rydberg-atom systems via their interaction with cavity modes. We focus on the dilute dipolar excitation case when the problem can be effectively considered as single-particle with the interaction providing long-range dipolar-like hopping. We show that the spatially homogeneous tilt $β$ of the dipoles giving rise to the anisotropic dipole exchange leads to the non-trivial reentrant localization beyond the locator expansion, $a a_{AT}$. This localization emerges due to the presence of the ergodic extended states at either spectral edge, which constitute a zero fraction of states in the thermodynamic limit, decaying though extremely slowly with the system size.

cond-mat.dis-nn↗

arXiv2022

Experimentally revealing anomalously large dipoles in a quantum-circuit dielectric

Quantum two-level systems (TLSs) intrinsic to glasses induce decoherence in many modern quantum devices, such as superconducting qubits. Although the low-temperature physics of these TLSs is usually well-explained by a phenomenological standard tunneling model of independent TLSs, the nature of these TLSs, as well as their behavior out of equilibrium and at high energies above 1 K, remain inconclusive. Here we measure the non-equilibrium dielectric loss of TLSs in amorphous silicon using a superconducting resonator, where energies of TLSs are varied in time using a swept electric field. Our results show the existence of two distinct ensembles of TLSs, interacting weakly and strongly with phonons, where the latter also possesses anomalously large electric dipole moment. These results may shed new light on the low temperature characteristics of amorphous solids, and hold implications to experiments and applications in quantum devices using time-varying electric fields.

quant-ph↗

arXiv2022

Exact solution of the minimalist Stark many body localization problem in terms of spin pair hopping

Stark many body localization problem on a periodic spin chain with local four spin hopping conserving dipole moment becomes equivalent to a spin pair hopping model after overturn of spins in odd or even positions. Eigenstates of the latter problem are separated into four groups including two groups of delocalized states with translationally invariant unrestricted (group I) or restricted (group II) Krylov subspaces and other two with confined spin transport having either all mobile (group III) or some immobile spins (group IV). These groups can be examined experimentally in systems like those recently investigated in Refs. [1, 2].

cond-mat.stat-mech↗

arXiv2021

Crucial effect of transverse vibrations on the transport through polymer chains

The low temperature transport of electron, or vibrational or electronic exciton towards polymer chains turns out to be dramatically sensitive to its interaction with transverse acoustic vibrations. We show that this interaction leads to substantial polaron effect and decoherence, which are generally stronger than those associated with longitudinal vibrations. For site-dependent interactions transverse phonons form subohmic bath leading to the quantum phase transition accompanied by full suppression of the transport at zero temperature and fast decoherence characterized by temperature dependent rate of temperature to the power of two thirds at low temperature while this rate rate ois proportional to the squared temperature for site-independent interactions. The latter dependence was used to interpret recent measurements of temperature dependent vibrational energy transport in polyethylene glycol oligomers.

cond-mat.mes-hall↗

arXiv2021

Anomalous low-energy properties in amorphous solids and the interplay of electric and elastic interactions of tunneling two-level systems

Tunneling two-level systems (TLSs), generic to amorphous solids, dictate the low-temperature properties of amorphous solids and dominate noise and decoherence in quantum nano-devices. The properties of the TLSs are generally described by the phenomenological standard tunneling model. Yet, significant deviations from the predictions of this model found experimentally suggest the need for a more precise model in describing TLSs. Here we show that the temperature dependence of the sound velocity, dielectric constant, specific heat, and thermal conductivity, can be explained using an energy-dependent TLS density of states reduced at low energies due to TLS-TLS interactions. This reduction is determined by the ratio between the strengths of the TLS-TLS interactions and the random potential, which is enhanced in systems with dominant electric dipolar interactions.

cond-mat.dis-nn↗

arXiv2019

Many-body localization in spin chains with the long-range transverse interactions: scaling of critical disorder with the system size

We investigate many-body localization in the chain of interacting spins with a transverse power-law interaction, $J_{0}/r^α$, and random on-site potentials, $ϕ_i \in \left(-W/2,W/2\right)$, in the long-range limit, $α< 3/2$, which has been recently examined experimentally on trapped ions. The many-body localization threshold is characterized by the critical disordering, $W_c$, which separates localized ($W > W_c$) and chaotic ($W < W_c$) phases. Using the analysis of the instability of localized states with respect to resonant interactions complemented by numerical finite size scaling, we show that the critical disordering scales with the number of spins, $N$, as $W_c \approx [1.37 J_{0}/(4/3 - α)]N^{4/3 - α} \ln N$ for $0 < α\leq 1$, and as $W_c \approx [J_{0}/(1-2α/3)]N^{1-2α/3} \ln^{2/3} N$ for $1 < α< 3/2$ while the transition width scales as $σ_{W} \propto W_{c}/N$. We use this result to predict the spin long-term evolution for a very large number of spins ($N = 50$), inaccessible for exact diagonalization, and to suggest the rescaling of hopping interaction with the system size to attain the localization transition at finite disordering in the thermodynamic limit of infinite number of spins.

cond-mat.dis-nn↗

arXiv2018

Chaotic dynamics in a quantum Fermi-Pasta-Ulam problem

We investigate the emergence of chaotic dynamics in a quantum Fermi - Pasta - Ulam problem for anharmonic vibrations in atomic chains applying semi-quantitative analysis of resonant interactions complemented by exact diagonalization numerical studies. The crossover energy separating chaotic high energy phase and localized (integrable) low energy phase is estimated. It decreases inversely proportionally to the number of atoms until approaching the quantum regime where this dependence saturates. The chaotic behavior appears at lower energies in systems with free or fixed ends boundary conditions compared to periodic systems. The applications of the theory to realistic molecules are discussed.

cond-mat.dis-nn↗

arXiv2018

Theory of nonlinear microwave absorption by interacting two-level systems

The microwave absorption and noise caused by quantum two-level systems (TLS) dramatically suppress the coherence in Josephson junction qubits that are promising candidates for a quantum information applications. Microwave absorption by TLSs is not clearly understood yet because of the complexity of their interactions leading to the spectral diffusion. Here, the theory of the non-linear absorption in the presence of spectral diffusion is developed using the generalized master equation formalism. The theory predicts that the linear absorption regime holds while a TLS Rabi frequency is smaller than their phase decoherence rate. At higher external fields, a novel non-linear absorption regime is found with the loss tangent inversely proportional to the intensity of the field. The theory can be generalized to acoustic absorption and lower dimensions realized in superconducting qubits.

cond-mat.mes-hall↗