arXiv subjects

Nianbei Li

Publications and source records attributed to Nianbei Li.

29 records · Page 2Linked to original sources

arXiv2012

Thermal conductivities of one-dimensional anharmonic/nonlinear lattices: renormalized phonons and effective phonon theory

Heat transport in low-dimensional systems has attracted enormous attention from both theoretical and experimental aspects due to its significance to the perception of fundamental energy transport theory and its potential applications in the emerging filed of phononics: manipulating heat flow with electronic anologs. We consider the heat conduction of one-dimensional nonlinear lattice models. The energy carriers responsible for the heat transport have been identified as the renormalized phonons. Within the framework of renormalized phonons, a phenomenological theory, effective phonon theory, has been developed to explain the heat transport in general one-dimensional nonlinear lattices. With the help of numerical simulations, it has been verified that this effective phonon theory is able to predict the scaling exponents of temperature-dependent thermal conductivities quantitatively and consistently.

nlin.CD↗

arXiv2012

Phononics: Manipulating heat flow with electronic analogs and beyond

The form of energy termed heat that typically derives from lattice vibrations, i.e. the phonons, is usually considered as waste energy and, moreover, deleterious to information processing. However, with this colloquium, we attempt to rebut this common view: By use of tailored models we demonstrate that phonons can be manipulated like electrons and photons can, thus enabling controlled heat transport. Moreover, we explain that phonons can be put to beneficial use to carry and process information. In a first part we present ways to control heat transport and how to process information for physical systems which are driven by a temperature bias. Particularly, we put forward the toolkit of familiar electronic analogs for exercising phononics; i.e. phononic devices which act as thermal diodes, thermal transistors, thermal logic gates and thermal memories, etc.. These concepts are then put to work to transport, control and rectify heat in physical realistic nanosystems by devising practical designs of hybrid nanostructures that permit the operation of functional phononic devices and, as well, report first experimental realizations. Next, we discuss yet richer possibilities to manipulate heat flow by use of time varying thermal bath temperatures or various other external fields. These give rise to a plenty of intriguing phononic nonequilibrium phenomena as for example the directed shuttling of heat, a geometrical phase induced heat pumping, or the phonon Hall effect, that all may find its way into operation with electronic analogs.

cond-mat.mes-hall↗

arXiv2011

Thermal conductivity of nonlinear waves in disordered chains

We present computational data on the thermal conductivity of nonlinear waves in disordered chains. Disorder induces Anderson localization for linear waves and results in a vanishing conductivity. Cubic nonlinearity restores normal conductivity, but with a strongly temperature-dependent conductivity $κ(T)$. We find indications for an asymptotic low-temperature $κ\sim T^4$ and intermediate temperature $κ\sim T^2$ laws. These findings are in accord with theoretical studies of wave packet spreading, where a regime of strong chaos is found to be intermediate, followed by an asymptotic regime of weak chaos (EPL 91 (2010) 30001).

cond-mat.stat-mech↗

arXiv2010

Energy carriers in the Fermi-Pasta-Ulam $β$ lattice: Solitons or Phonons?

We investigate anomalous energy transport processes in the Fermi-Pasta-Ulam $β$ lattice. They are determined by the maximum sound velocity of the relevant weakly damped energy carriers. That velocity can be numerically resolved by measuring the propagating fronts of the correlation functions of energy/momentum fluctuations at different times. The numerical results are compared with the predictions for solitons and effective (renormalized) phonons, respectively. Excellent agreement has been found for the prediction of effective long wavelength phonons, giving strong evidence that the energy carriers should be effective phonons rather than solitons.

cond-mat.stat-mech↗

arXiv2009

Molecular wires acting as quantum heat ratchets

We explore heat transfer in molecular junctions between two leads in the absence of a finite net thermal bias. The application of an unbiased, time-periodic temperature modulation of the leads entails a dynamical breaking of reflection symmetry, such that a directed heat current may emerge (ratchet effect). In particular, we consider two cases of adiabatically slow driving, namely (i) periodic temperature modulation of only one lead and (ii) temperature modulation of both leads with an ac driving that contains a second harmonic, thus generating harmonic mixing. Both scenarios yield sizeable directed heat currents which should be detectable with present techniques. Adding a static thermal bias, allows one to compute the heat current-thermal load characteristics which includes the ratchet effect of negative thermal bias with positive-valued heat flow against the thermal bias, up to the thermal stop-load. The ratchet heat flow in turn generates also an electric current. An applied electric stop-voltage, yielding effective zero electric current flow, then mimics a solely heat-ratchet-induced thermopower (``ratchet Seebeck effect''), although no net thermal bias is acting. Moreover, we find that the relative phase between the two harmonics in scenario (ii) enables steering the net heat current into a direction of choice.

cond-mat.mes-hall↗

arXiv2009

Shuttling heat across 1D homogenous nonlinear lattices with a Brownian heat motor

We investigate directed thermal heat flux across 1D homogenous nonlinear lattices when no net thermal bias is present on average. A nonlinear lattice of Fermi-Pasta-Ulam-type or Lennard-Jones-type system is connected at both ends to thermal baths which are held at the same temperature on temporal average. We study two different modulations of the heat bath temperatures, namely: (i) a symmetric, harmonic ac-driving of temperature of one heat bath only and (ii) a harmonic mixing drive of temperature acting on both heat baths. While for case (i) an adiabatic result for the net heat transport can be derived in terms of the temperature dependent heat conductivity of the nonlinear lattice a similar such transport approach fails for the harmonic mixing case (ii). Then, for case (ii), not even the sign of the resulting Brownian motion induced heat flux can be predicted a priori. A non-vanishing heat flux (including a non-adiabatic reversal of flux) is detected which is the result of an induced dynamical symmetry breaking mechanism in conjunction with the nonlinearity of the lattice dynamics. Computer simulations demonstrate that the heat flux is robust against an increase of lattice sizes. The observed ratchet effect for such directed heat currents is quite sizable for our studied class of homogenous nonlinear lattice structures, thereby making this setup accessible for experimental implementation and verification.

cond-mat.stat-mech↗

arXiv2009

Thermal conductivity of one-dimensional lattices with self-consistent heat baths: a heuristic derivation

We derive the thermal conductivities of one-dimensional harmonic and anharmonic lattices with self-consistent heat baths (BRV lattice) from the Single-Mode Relaxation Time (SMRT) approximation. For harmonic lattice, we obtain the same result as previous works. However, our approach is heuristic and reveals phonon picture explicitly within the heat transport process. The results for harmonic and anharmonic lattices are compared with numerical calculations from Green-Kubo formula. The consistency between derivation and simulation strongly supports that effective (renormalized) phonons are energy carriers in anharmonic lattices although there exist some other excitations such as solitons and breathers.

cond-mat.stat-mech↗

arXiv2008

Ratcheting Heat Flux against a Thermal Bias

Merely rocking the temperature in one heat bath can direct a steady heat flux from cold to hot against a non-zero thermal bias in stylized nonlinear lattice junctions that are sandwiched between two heat baths. Likewise, for an average zero-temperature difference between the two contacts a net, ratchet-like heat flux emerges. Computer simulations show that this very heat flux can be controlled and reversed by suitably tailoring the frequency ($\lesssim$ 100 MHz) of the alternating temperature field.

cond-mat.stat-mech↗

arXiv2007

Thermal rectification and negative differential thermal resistance in lattices with mass gradient

We study thermal properties of one dimensional(1D) harmonic and anharmonic lattices with mass gradient. It is found that the temperature gradient can be built up in the 1D harmonic lattice with mass gradient due to the existence of gradons. The heat flow is asymmetric in the anharmonic lattices with mass gradient. Moreover, in a certain temperature region the {\it negative differential thermal resistance} is observed. Possible applications in constructing thermal rectifier and thermal transistor by using the graded material are discussed.

cond-mat.dis-nn↗

arXiv2007

Temperature dependence of thermal conductivity in 1D nonlinear lattices

We examine the temperature dependence of thermal conductivity of one dimensional nonlinear (anharmonic) lattices with and without on-site potential. It is found from computer simulation that the heat conductivity depends on temperature via the strength of nonlinearity. Based on this correlation, we make a conjecture in the effective phonon theory that the mean-free-path of the effective phonon is inversely proportional to the strength of nonlinearity. We demonstrate analytically and numerically that the temperature behavior of the heat conductivity $κ\propto1/T$ is not universal for 1D harmonic lattices with a small nonlinear perturbation. The computer simulations of temperature dependence of heat conductivity in general 1D nonlinear lattices are in good agreements with our theoretic predictions. Possible experimental test is discussed.

cond-mat.dis-nn↗

arXiv2006

Effective phonons in anharmonic lattices: anomalous vs normal heat conduction

We study heat conduction in one dimensional (1D) anharmonic lattices analytically and numerically by using an effective phonon theory. It is found that every effective phonon mode oscillates quasi-periodically. By weighting the power spectrum of the total heat flux in the Debye formula, we obtain a unified formalism that can explain anomalous heat conduction in momentum conserved lattices without on-site potential and normal heat conduction in lattices with on-site potential. Our results agree very well with numerical ones for existing models such as the Fermi-Pasta-Ulam model, the Frenkel-Kontorova model and the $ϕ^4$ model etc.

cond-mat.stat-mech↗