Small Phonon Research Articles

Phonon Thermal Hall Effect in Strontium Titanate

It has been known for more than a decade that phonons can produce an off-diagonal thermal conductivity in the presence of a magnetic field. Recent studies of thermal Hall conductivity, κ_{xy}, in a variety of contexts, however, have assumed a negligibly small phonon contribution. We present a study of κ_{xy} in quantum paraelectric SrTiO_{3}, which is a nonmagnetic insulator and find that its peak value exceeds what has been reported in any other insulator, including those in which the signal has been qualified as "giant." Remarkably, κ_{xy}(T) and κ(T) peak at the same temperature and the former decreases faster than the latter at both sides of the peak. Interestingly, in the case of La_{2}CuO_{4} and α-RuCl_{3}, κ_{xy}(T) and κ(T) peak also at the same temperature. We also studied KTaO_{3} and found a small signal, indicating that a sizable κ_{xy}(T) is not a generic feature of quantum paraelectrics. Combined to other observations, this points to a crucial role played by antiferrodistortive domains in generating κ_{xy} of this solid.

Low-energy phonons in Bi2Sr2CaCu2O8+δ and their possible interaction with electrons measured by inelastic neutron scattering

Electron-phonon interaction in copper oxide superconductors is still enigmatic. Strong coupling for certain optic phonons is now well established experimentally, but theoretical understanding is challenging. Scattering of electrons near the Fermi surface by the longitudinal acoustic (LA) phonons is expected from basic theory because these phonons modulate electron density. We used inelastic neutron scattering on a large single crystal sample of optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ to show that low-energy LA phonons could couple to electronic density fluctuations only at small phonon wavevectors, which naturally limits any interaction to forward scattering. Such scattering should not be pairbreaking in the case of the d-wave gap. We also found that previously the reported low energy phonon spectral weight half-way to the zone boundary is consistent with conventional lattice dynamics and does not reflect an incipient charge density wave.

Superioniclike Diffusion in an Elemental Crystal: bcc Titanium.

Recent theoretical investigations [A. B. Belonoshko etal. Nat. Geosci. 10, 312 (2017)1752-089410.1038/ngeo2892] revealed the occurrence of the concerted migration of several atoms in bcc Fe at inner-core temperatures and pressures. Here, we combine first-principles and semiempirical atomistic simulations to show that a diffusion mechanism analogous to the one predicted for bcc iron at extreme conditions is also operative and of relevance for the high-temperature bcc phase of pure Ti at ambient pressure. The mechanism entails a rapid collective movement of numerous (from two to dozens) neighbors along tangled closed-loop paths in defect-free crystal regions. We argue that this phenomenon closely resembles the diffusion behavior of superionics and liquid metals. Furthermore, we suggest that concerted migration is the atomistic manifestation of vanishingly small ω-mode phonon frequencies previously detected via neutron scattering and the mechanism underlying anomalously large and markedly non-Arrhenius self-diffusivities characteristic of bcc Ti.

Thermal conductivity of amorphous silica nanoparticles

Understanding the heat transfer through confined systems at nanometer scale is of both scientific interest and technical importance. We report in this paper, a case study on the thermal conductivity of amorphous silica nanoparticles. It is found that the solid-state thermal conductivity of amorphous silica nanoparticles is size-dependent and can be quantitatively calculated by using a phenomenological theory. The observed size effect on thermal conductivity of silica nanoparticles is however not significant, mostly due to the very small phonon mean free path of amorphous silica, l0 = 0.59 nm. It is also found that the phonon-defect scattering at the nanoparticle surfaces has a predominate effect on the involved heat transfer process. The calculated thermal conductivity of 5-nm silica nanoparticles is about 0.539 W/(mK).

Low-temperature anharmonicity and the thermal conductivity of cesium iodide

Cesium halide has a simple cubic crystal structure and hosts low thermal conductivity, but its microscopic mechanism has not been fully understood. In the present work, we took cesium iodide (CsI) single crystal as an example, to investigate the lattice dynamics and thermal conductivity by performing inelastic neutron scattering (INS), heat transport measurements, and first-principles calculations. The temperature dependent phonon dispersions of CsI were obtained from INS and the low temperature anharmonicity of transverse optic $(o)$ and transverse acoustic $(a)$ phonon modes in CsI was observed. By performing the thermal conductivity measurement and first-principles calculations, it is shown that the low thermal conductivity of CsI originates from the combined effect of the small phonon group velocities and the large phonon scattering rates, which is dominated by the ($a,\phantom{\rule{0.16em}{0ex}}a,\phantom{\rule{0.16em}{0ex}}a$) and ($a,\phantom{\rule{0.16em}{0ex}}a,\phantom{\rule{0.16em}{0ex}}o$) phonon scattering processes. This work highlights the importance of phonon anharmonicity in lattice dynamics, which sheds light on the design of materials with low thermal conductivity.

Real time and real space analysis for coherent QENS and its application to liquid bismuth

Real time and real space analysis for quasielastic neutron scattering (QENS) by means of the regularization method to estimate a spectrum has been developed. In our formalism, a mediation function between a dynamic structure factor, S(Q, ω), and a time-space correlation function, P(r, t), is estimated to minimize the sum of the least square fitting term for S(Q, ω) in a Q-ω space and the regularization term for P(r, t) in a r-t space. On the application to liquid bismuth which shows complicate static local structure, it is shown that a distribution function for Gaussian superposition to represent S(Q, ω) has good characters as a mediating function. The S(Q, ω) calculated from the estimated mediation function showed a good agreement with the experimental Sexp(Q, ω) even for small acoustic phonon excitation in a low Q region and gave more or less reasonable estimation of dynamic structure beyond the measured Q-ω range. In addition of the main property of P(r, t) that the ability to focus onto the relaxation process at a specific atomic distance helped us to understand a complicated layered structure in liquid bismuth, possibility of easy separation of self-diffusion from coherent QENS is shown.

Predicted Janus monolayer ZrSSe with enhanced n-type thermoelectric properties compared with monolayer [formula omitted

In analogy to transition-metal dichalcogenide (TMD) monolayers, which have wide applications in photoelectricity, piezoelectricity and thermoelectricity, Janus MoSSe monolayer has been successfully synthesized by substituting the top S atomic layer in MoS2 by Se atoms. In this work, Janus monolayer ZrSSe is proposed by ab initio calculations. For the electron part, the generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) is used as exchange-correlation potential, while GGA for lattice part. Calculated results show that the ZrSSe monolayer is dynamically, mechanically and thermally stable, which exhibits mechanical flexibility due to small Young’s modulus. It is found that ZrSSe monolayer is an indirect-gap semiconductor with band gap of 0.60 eV. The electronic and phonon transports of ZrSSe monolayer are investigated by semiclassical Boltzmann transport theory. In n-type doping, the ZTe between ZrSSe and ZrS2 monolayers is almost the same due to similar outlines of conduction bands. The p-type ZTe of ZrSSe monolayer is lower than that of ZrS2 monolayer, which is due to larger spin-orbit splitting for ZrSSe than ZrS2 monolayer. The room-temperature sheet thermal conductance is 33.6 WK-1 for ZrSSe monolayer, which is lower than 47.8 WK-1 of ZrS2 monolayer. Compared to ZrS2 monolayer, the low sheet thermal conductance of ZrSSe monolayer is mainly due to small group velocities and short phonon lifetimes of ZA mode. Considering their ZTe and lattice thermal conductivities, the ZrSSe monolayer may have better n-type thermoelectric performance than ZrS2 monolayer. These results can stimulate further experimental works to synthesize ZrSSe monolayer.

Strong phonon anharmonicity and low thermal conductivity of monolayer tin oxides driven by lone-pair electrons

Motivated by the excellent electronic and optoelectronic properties of two-dimensional (2D) tin oxides, we systematically investigated the thermal conductivity (κ) of monolayers SnO and SnO2 by the first-principles calculations. The room-temperature κ of monolayers SnO and SnO2 reaches 9.6 W/(m K) and 98.8 W/(m K), respectively. The size effect is much weaker for monolayer SnO than for monolayer SnO2, due to the coexistence of size dependent and independent components in the κ value of monolayer SnO. The large difference between the κ values of 2D tin oxides can be attributed to the small phonon group velocity and strong anharmonicity strength of monolayer SnO. Further electronic structure analysis reveals that the existence of sterically active lone-pair electrons is the key factor for the small κ value of monolayer SnO. These results provide a guide for the manipulation of thermal transport in electronic or thermoelectric devices based on 2D tin oxides.

First-principles study of the thermoelectric properties of quaternary tetradymite BiSbSeTe2

The electronic and phonon transport properties of quaternary tetradymite BiSbSeTe2 are investigated by using a first-principles approach and Boltzmann transport theory. Unlike the binary counterpart Bi2Te3, we obtain a pair of Rashba splitting bands induced by the absence of an inversion center. Such unique characteristics could lead to a large Seebeck coefficient even at a relatively higher carrier concentration. Besides, we find an ultralow lattice thermal conductivity of BiSbSeTe2, especially along the interlayer direction, which can be traced to the extremely small phonon relaxation time mainly induced by the mixed covalent bonds. As a consequence, a considerably large ZT value of ~2.0 can be obtained at 500 K, indicating that the unique lattice structure of BiSbSeTe2 caused by isoelectronic substitution could be an advantage to achieve high thermoelectric performance.

High-pressure phase diagram of hydrogen and deuterium sulfides from first principles: Structural and vibrational properties including quantum and anharmonic effects

We study the structural and vibrational properties of the high-temperature superconducting sulfur trihydride and trideuteride in the high-pressure $Im\bar{3}m$ and $R3m$ phases by first-principles density-functional-theory calculations. On lowering pressure, the rhombohedral transition $Im\bar{3}m \rightarrow R3m$ is expected, with hydrogen bond desymmetrization and occurrence of trigonal lattice distortion. In hydrostatic conditions we find that, contrary to what suggested in some recent experiments, if the rhombohedral distortion exists it affects mainly the hydrogen-bonds, whereas the resulting cell distortion is minimal. We estimate that the occurrence of a stress anisotropy of approximately $10\%$ could explain this discrepancy. Assuming hydrostatic conditions, we calculate the critical pressure at which the rhombohedral transition occurs. Quantum and anharmonic effects, which are relevant in this system, are included at nonperturbative level with the stochastic self-consistent harmonic approximation (SSCHA). Within this approach, we determine the transition pressure by calculating the free energy Hessian. We find that quantum anharmonic effects are responsible for a strong reduction of the critical pressure with respect to the one obtained with the classical harmonic approach. Moreover, we observe a prominent isotope effect, as we estimate higher pressure transition for D${}_3$S than for H${}_3$S. Finally, within SSCHA we calculate the anharmonic phonon spectral functions in the $Im\bar{3}m$ phase. The strong anharmonicity of the system is confirmed by the occurrence of very large anharmonic broadenings leading to complex non-Lorentzian line shapes. However, for the vibrational spectra at zone center, accessible e.g. by infrared spectroscopy, the broadenings are very small (linewidth at most around 2~meV) and anharmonic phonon quasiparticles are well defined.

Breakdown of the Migdal-Eliashberg theory: A determinant quantum Monte Carlo study

The superconducting (SC) and charge-density-wave (CDW) susceptibilities of the two dimensional Holstein model are computed using determinant quantum Monte Carlo (DQMC), and compared with results computed using the Migdal-Eliashberg (ME) approach. We access temperatures as low as 25 times less than the Fermi energy, $E_F$, which are still above the SC transition. We find that the SC susceptibility at low $T$ agrees quantitatively with the ME theory up to a dimensionless electron-phonon coupling $\lambda_0 \approx 0.4$ but deviates dramatically for larger $\lambda_0$. We find that for large $\lambda_0$ and small phonon frequency $\omega_0 \ll E_F$ CDW ordering is favored and the preferred CDW ordering vector is uncorrelated with any obvious feature of the Fermi surface.

A First-Principles Study on the Vibrational and Electronic Properties of Zr-C MXenes**Supported by the National Natural Science Foundation of China under Grant Nos. 11605273, 21571185, U1404111, 11504089, 21501189, 21676291, the Shanghai Municipal Science and Technology Commission 16ZR1443100, the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA02040104)

Within the framework of density functional theory calculations, the structural, vibrational, and electronic properties of ZrnCn − 1 (n = 2, 3, and 4) and their functionalized MXenes have been investigated. We find that the most stable configurations for Zr-C MXene are the ones that the terminal groups F, O, and OH locate on the common hollow site of the superficial Zr layer and its adjacent C layer. F and OH-terminated Zr3C2 and Zr4C3 have small imaginary acoustic phonon branches around Γ point while the others have no negative phonon modes. The pristine MXenes (Zr2C, Zr3C2 and Zr4C3) are all metallic with large DOS contributed by the Zr atom at the Fermi energy. When functionalized by F, O and OH, new hybridization states appear and the DOS at the Fermi level are reduced. Moreover, we find that their metallic characteristic increases with an increase in n. For (ZrnCn − 1)O2, Zr2CO2 is a semiconductor, Zr3C2O2 is a semimetal, and Zr4C3O2 becomes a metal.

Ground state cooling in a hybrid optomechanical system with a three-level atomic ensemble

Cooling mechanical resonators are of great importance in both fundamental study and applied science. We investigate the hybrid optomechanical cooling with a three-level atomic ensemble fixed in a strong excited optical cavity. By using the quantum noise approach, we find the upper bound of the noise spectrum and further present three optimal parameter conditions, which can yield a small heating coefficient, a large cooling coefficient, and thus a small final phonon number. Moreover, through the covariance matrix approach, results of numerical simulation are obtained, which are consistent with the theoretical expectations. It is demonstrated that our scheme can achieve ground state cooling in the highly unresolved sideband regime, within the current experimental technologies. Compared with the previous cooling methods, in our scheme, there are fewer constraints on the drive strength of atomic ensemble and number of atoms in the ensemble. In addition, the tolerable ranges of parameters for ground state cooling are extended. As a result, our scheme is very suitable for experiments and can be a guideline for the research of hybrid optomechanical cooling.

Photoinduced Enhancement of Excitonic Order.

We study the dynamics of excitonic insulators coupled to phonons using the time-dependent mean-field theory. Without phonon couplings, the linear response is given by the damped amplitude oscillations of the order parameter with a frequency equal to the minimum band gap. A phonon coupling to the interband transfer integral induces two types of long-lived collective oscillations of the amplitude, one originating from the phonon dynamics and the other from the phase mode, which becomes massive. We show that, even for small phonon coupling, a photoinduced enhancement of the exciton condensation and the gap can be realized. Using the Anderson pseudospin picture, we argue that the origin of the enhancement is a cooperative effect of the massive phase mode and the Hartree shift induced by the photoexcitation. We also discuss how the enhancement of the order and the collective modes can be observed with time-resolved photoemission spectroscopy.

The current-induced heat generation in a spin-flip quantum dot sandwiched between a ferromagnetic and a superconducting electrode

Using non-equilibrium Green's functions' theory based on extended Nambu representation and small polaron transformation, we studied the current-induced heat generation in a spin-flip quantum dot sandwiched between a ferromagnetic and a superconducting electrode. We focused on moderate dot-leads coupling and relative small phonon energy, and derived the detailed expression of heat generation. The numerical results show (i) the heat generation decreases with polarization degree increasing, (ii) the intradot spin-flip can have a great effect on the heat generation at both zero temperature and finite temperature and (iii) at finite temperature an optimal workspace of keeping spin current and tuning heat generation by modulating the spin-flip intensity can be found.

Potential 2D thermoelectric material ATeI (A = Sb and Bi) monolayers from a first-principles study

Lots of two-dimensional (2D) materials have been predicted theoretically and further confirmed in experiments, and have wide applications in nanoscale electronic, optoelectronic and thermoelectric devices. In this work, the thermoelectric properties of ATeI (A = Sb and Bi) monolayers are systematically investigated according to semiclassical Boltzmann transport theory. It is found that spin–orbit coupling (SOC) has an important effect on the electronic transport coefficients of p-type doping, but a negative influence on n-type doping. The room-temperature sheet thermal conductance is 14.2 for SbTeI and 12.6 for BiTeI, which is lower than that of most well-known 2D materials, such as the transition-metal dichalcogenide, group IV–VI, group VA and group IV monolayers. The very low sheet thermal conductance of ATeI (A = Sb and Bi) monolayers is mainly due to their small group velocities and short phonon lifetimes. The strongly polarized covalent bonds between A and Te or I atoms induce strong phonon anharmonicity, which gives rise to low lattice thermal conductivity. It is found that the high-frequency optical branches contribute significantly to the total thermal conductivity, which is obviously different from the usual picture, where there is little contribution from the optical branches. According to cumulative lattice thermal conductivity with respect to the phonon mean free path (MFP), it is difficult to further reduce the lattice thermal conductivity using nanostructures. Finally, the possible thermoelectric figure of merit ZT values of the ATeI (A = Sb and Bi) monolayers are calculated. It is found that p-type doping has much better thermoelectric properties than n-type doping. At room temperature, the peak ZT can reach 1.11 for SbTeI and 0.87 for BiTeI, respectively. These results make us believe that ATeI (A = Sb and Bi) monolayers may be potential 2D thermoelectric materials, which could stimulate further experimental work towards the synthesis of these monolayers.

Investigation of upconversion luminescence in Er^3+/Yb^3+ co-doped Nb_2O_5-based glasses prepared by aerodynamic levitation method

The aerodynamic levitation method has been employed to fabricate Er3+/Yb3+ co-doped Nb2O5-La2O3-Ta2O5 glasses. The maximum phonon energy is about 735 cm−1 and the phonon density decreases with the increase of Yb3+. Small phonon density is favorable for upconversion luminescence. The absorption intensity at ~976 nm is improved by Yb3+. The glasses show high thermal stability and glass forming ability is enhanced by Yb3+. Green, red, and near infrared emissions are obtained and upconversion luminescence increases with Yb3+, which agrees well with the analysis of Raman and absorption spectra. As Yb3+ increases, the lifetime is enhanced. The results of lifetime measurement confirm the tendency of upconversion luminescence with Yb3+ ions.

Photoinduced charge-order melting dynamics in a one-dimensional interacting Holstein model

Transient quantum dynamics in an interacting fermion-phonon system are investigated. In particular, a charge order (CO) melting after a short optical-pulse irradiation and roles of the quantum phonons on the transient dynamics are focused on. A spinless-fermion model in a one-dimensional chain coupled with local phonons is analyzed numerically. The infinite time-evolving block decimation algorithm is adopted as a reliable numerical method for one-dimensional quantum many-body systems. Numerical results for the photoinduced CO melting dynamics without phonons are well interpreted by the soliton picture for the CO domains. This interpretation is confirmed by the numerical simulation for an artificial local excitation and the classical soliton model. In the case of the large phonon frequency corresponding to the antiadiabatic condition, the CO melting is induced by propagations of the polaronic solitons with the renormalized soliton velocity. On the other hand, in the case of the small phonon frequency corresponding to the adiabatic condition, the first stage of the CO melting dynamics occurs due to the energy transfer from the fermionic to phononic systems, and the second stage is brought about by the soliton motions around the bottom of the soliton band. Present analyses provide a standard reference for the photoinduced CO melting dynamics in low-dimensional many-body quantum systems.

Excess vibrational modes of a crystal in an external non-affine field

Thermal displacement fluctuations in a crystal may be classified as either “affine” or “non-affine”. While the former couples to external stress with familiar consequences, the response of a crystal when non-affine displacements are enhanced using the thermodynamically conjugate field, is relatively less studied. We examine this using a simple model of a crystal in two dimensions for which analytical calculations are possible. Enhancing non-affine fluctuations destabilises the crystal. The population of small frequency phonon modes increases, with the phonon density of states shifting, as a whole, towards zero frequency. Even though the crystal is free of disorder, we observe growing length and time scales. Our results, which may have implications for the glass transition and structural phase transitions in solids, are compared to molecular dynamics simulations. Possibility of experimental verification of these results is also discussed. We show that it is possible to enhance non-affine displacement fluctuations in a crystal using an external field. Although transient, these fluctuations can shift the phonon density of states to low frequencies and cause “Boson peak” like phenomenon in an otherwise perfect crystal without quenched disorder.

Co-precipitation of rare-earth-doped Y2O3 and MgO nanocomposites for mid-infrared solid-state lasers.

Mid-infrared, solid-state laser materials face three main challenges: (1) need to dissipate heat generated in lasing; (2) luminescence quenching by multiphonon relaxation; and (3) trade-off in high thermal conductivity and small maximum phonon energy. We are tackling these challenges by synthesizing a ceramic nanocomposite in which multiple phases will be incorporated into the same structure. The undoped majority species, MgO, will be the main carrier of high thermal conductivity, and the minority species, Er:Y2O3, will have low maximum phonon energy. There is also an inherent challenge in attempting to make a translucent part from a mixture of two different materials with two different indexes of refraction. A simple, co-precipitation technique has been developed in which both components are synthesized in situ to obtain intimate mixing. These powders compare well to commercially available ceramics, including their erbium spectroscopy, even when mixed as a composite, and can be air-fired to ∼96% of theoretical density, yielding translucent parts. As the amount of Er:Y2O3 increases, the translucency decreases as the number of scattering sites start to coalesce into large patches. If the amount of Er:Y2O3 is sufficiently small and dispersed, the yttria grains will be pinned as individuals in a sea of MgO, leading to optimal translucency.