Low-temperature Phonon Research Articles

Electrically-driven ultrafast out-of-equilibrium light emission from hot electrons in suspended graphene/hBN heterostructures

Nanoscale light sources with high speed of electrical modulation and low energy consumption are key components for nanophotonics and optoelectronics. The record-high carrier mobility and ultrafast carrier dynamics of graphene make it promising as an atomically thin light emitter, which can be further integrated into arbitrary platforms by van der Waals forces. However, due to the zero bandgap, graphene is difficult to emit light through the interband recombination of carriers like conventional semiconductors. Here, we demonstrate ultrafast thermal light emitters based on suspended graphene/hexagonal boron nitride (Gr/hBN) heterostructures. Electrons in biased graphene are significantly heated up to 2800 K at modest electric fields, emitting bright photons from the near-infrared to the visible spectral range. By eliminating the heat dissipation channel of the substrate, the radiation efficiency of the suspended Gr/hBN device is about two orders of magnitude greater than that of graphene devices supported on SiO2 or hBN. We further demonstrate that hot electrons and low-energy acoustic phonons in graphene are weakly coupled to each other and are not in full thermal equilibrium. Direct cooling of high-temperature hot electrons to low-temperature acoustic phonons is enabled by the significant near-field heat transfer at the highly localized Gr/hBN interface, resulting in ultrafast thermal emission with up to 1 GHz bandwidth under electrical excitation. It is found that suspending the Gr/hBN heterostructures on the SiO2 trenches significantly modifies the light emission due to the formation of the optical cavity and showed a ∼440% enhancement in intensity at the peak wavelength of 940 nm compared to the black-body thermal radiation. The demonstration of electrically driven ultrafast light emission from suspended Gr/hBN heterostructures sheds the light on applications of graphene heterostructures in photonic integrated circuits, such as broadband light sources and ultrafast thermo-optic phase modulators.

Topological black holes in Einstein-Maxwell and 4D conformal gravities revisited

The thermodynamics of charged topological black holes (TBHs) with different horizon geometries in d-dimensional Einstein-Maxwell and 4-dimensional conformal gravities is revisited using the restricted phase space formalism. The concept of subsystems for black holes is introduced, which enables a precise description for the thermodynamic behaviors of (non-compact) black holes with infinitely large horizon area. The concrete behaviors can be different for TBHs in the same underlying theory but with different horizon geometries, or for those with the same horizon geometry but from different underlying theories. The high and low temperature limits of the TBHs are also considered, and the high temperature limits behave universally as low temperature phonon gases. Finally, using the concept of subsystems, some conceptual issues in the description for thermal fluctuations in black hole systems are clarified, and the relative thermal fluctuations for finite subsystems are also analyzed in some detail.

Low temperature phonon studies and evidence of structure–spin correlations in MnV2O4

We report here, temperature dependent Raman spectroscopic measurements on polycrystalline MnV2O4 over the temperature range of 10–300 K. The main aim of this study is to investigate the spin–phonon correlations across the magnetic and structural transition temperatures in this system. Temperature dependent magnetization measurements combined with the low temperature Raman study show the existence of structural changes in MnV2O4 across the two successive phase transitions. Anomalous changes in line shape parameters in some of the Raman modes are observed during temperature dependent Raman measurements, indicating the presence of spin–phonon coupling. A significantly high spin–phonon coupling strength of 5.9 cm−1 suggests alteration of spin dynamics across magnetic transition temperature, TM (57 K).

Low-Temperature Heat Capacity and Phonon Kinetics in Some Rare-Earth Pentaphosphate Single Crystals and Glasses

The influence of the Schottky low-energy excitations related to the presence of rare-earth metals on the thermodynamic and kinetic characteristics of a number of rare-earth pentaphosphate single crystals and the related glasses is analyzed. The temperature dependences of the heat capacity are found to be related to the nanosized structure of the amorphous state of the material.

Origin of Intrinsically Low Thermal Conductivity in Talnakhite Cu17.6Fe17.6S32 Thermoelectric Material: Correlations between Lattice Dynamics and Thermal Transport.

Understanding the nature of phonon transport in solids and the underlying mechanism linking lattice dynamics and thermal conductivity is important in many fields, including the development of efficient thermoelectric materials where a low lattice thermal conductivity is required. Herein, we choose the pair of synthetic chalcopyrite CuFeS2 and talnakhite Cu17.6Fe17.6S32 compounds, which possess the same elements and very similar crystal structures but very different phonon transport, as contrasting examples to study the influence of lattice dynamics and chemical bonding on the thermal transport properties. Chemically, talnakhite derives from chalcopyrite by inserting extra Cu and Fe atoms in the chalcopyrite lattice. The CuFeS2 compound has a lattice thermal conductivity of 2.37 W m-1 K-1 at 625 K, while Cu17.6Fe17.6S32 features Cu/Fe disorder and possesses an extremely low lattice thermal conductivity of merely 0.6 W m-1 K-1 at 625 K, approaching the amorphous limit κmin. Low-temperature heat capacity measurements and phonon calculations point to a large anharmonicity and low Debye temperature in Cu17.6Fe17.6S32, originating from weaker chemical bonds. Moreover, Mössbauer spectroscopy suggests that the state of Fe atoms in Cu17.6Fe17.6S32 is partially disordered, which induces the enhanced alloy scattering. All of the above peculiar features, absent in CuFeS2, contribute to the extremely low lattice thermal conductivity of the Cu17.6Fe17.6S32 compound.

Influence of sulfur doping on the electrical and thermal transport properties of FeTe1-xSx superconductors

In this study, we determined the structural, electrical, and thermal transport properties of the FeTe1−xSx system, where x = 0.0, 0.1, 0.2, and 0.3. The samples were synthesized using a standard solid-state reaction route via vacuum encapsulation and characterized by X-ray diffraction, which indicated a tetragonal phase with the P4/nmm space group. The parent compound FeTe exhibited an anomaly according to the resistivity measurements at around 67 K due to structural change. The S-substituted samples exhibited negative dependence on the temperature at an electrical resistivity above 10 K, and quantitative analysis indicated that the variable range hopping mechanism governs electrical conduction in the FeTe1−xSx system. In the low-temperature regime, the Seebeck coefficient, S(T), had an anomalous dip feature with crossover of the sign, which can be understood based on the phonon drag contributions and the multiband picture, respectively. The lattice phonons made major contributions to the measured thermal conductivity, κ(T). In addition, we observed a reduction in the height of the low-temperature phonon peak with κ due to the substitution of S for Te, thereby indicating strong enhancement of the disorder-induced phonon scattering. Moreover, a positive dκ/dT was observed in the normal state, which might be related to local anharmonic lattice distortions. Overall, based on our analyses, we conclude that phonons play important roles in the transport properties of FeTe1-xSx superconductors.

Geometrically induced broadening for phonon blocking at low temperatures

Because of the presence of robust massless modes with no excitation gap, the heat current carried by phonons tends to survive at low temperatures even when scattering mechanisms are incorporated into the system. This becomes a fundamental obstacle to thermoelectric applications at low temperatures. In this study, we investigate the effect of energy broadening on phonon transport in mesoscopic systems coupled to leads or probes in various geometries using a nonequilibrium Green's function formalism. An analytic theory derived from a minimal model consisting of a harmonic chain shows that geometrically induced broadening sizably suppresses low-temperature phonon transport. It is also demonstrated from numerical calculations that this scheme for phonon blocking is viable for realistic systems in higher dimensions.

Influence of sintering temperature on the electrical and thermal properties of bulk samples of Pr2/3Sr1/3MnO3-δ

We present temperature-dependent studies on electrical and thermal properties of bulk samples of Pr2/3Sr1/3MnO3-δ synthesized at different sintering temperatures (TS). It is established that these properties intensely influenced by the sintering temperatures. Analysis of electrical resistivity data ρ(T) by means of standard models, demonstrated that electron-electron scattering processes play a key role in the conduction process in the low-temperature region. At high temperatures, the ρ(T) follows the small polaron hopping (SPH) model. Analyses of the Seebeck coefficient data S(T) in the low temperature regime illustrates that electron-magnon scattering governs the thermoelectric transport, on contrary, the high-temperature S(T) follows the SPH model. The low-temperature phonon peak in thermal conductivity κ(T) increases with increasing TS, demonstrating that a better crystallinity of Pr2/3Sr1/3MnO3 samples was obtained at higher sintering temperatures. Magnetic transitions are observed to increase with a higher TS, and the entropy change associated with the transition also increases with TS.

Simple theory of low-temperature thermal conductivity in single- and double-walled carbon nanotubes

Low-temperature phonon thermal conductance (PTC) of any 1D system increases proportionally to the temperature. However, here we show that in single- and double-walled carbon nanotubes (CNTs), starting from 3--6 K, the PTC increases faster than the linear function, since the low-frequency modes of dispersion curves, which do not tend to zero together with the wave vector, are excited. To develop the PTC theory, we combine the Landauer's ballistic approach with the simple continuous model proposed for the calculation of the low-frequency phonon spectra of both free nanotubes and those interacting with an environment. The approach obtained is valid not only for commensurate double-walled CNTs, but also for incommensurate ones. The temperature-dependent relation between the PTC of double-walled CNT and those of its constituent SWNTs is obtained and discussed. The low-temperature heat transfer in bulk materials originated from CNTs is also considered and the upper limit of thermal conductivity of such materials is determined. We argue that the ideal material consisting of CNTs can challenge diamond only when the mean length of its defect-free nanotubes reaches at least one hundred of micrometers.

The influence of silica nano-inclusions on the structure of methane crystal observed in thermal conductivity experiment

The preliminary results of experimental investigations of the thermal conductivity of nanocomposites built of amorphous silica nanoparticles embedded in the structure of crystalline methane are presented. The investigations were carried out on the samples containing the particles of linear dimensions of 5, 10–20 and 42nm. The measurements were performed with steady-state heat flow method in the temperature interval 2–35K.A very simple thermal conductivity model shows that the dependence of the thermal conductivity coefficient of the investigated nanocomposites on temperature can be successfully described by taking into account only two mechanisms of the heat carrier dissipation: scattering of phonons by methane crystal grain boundaries and phonon-phonon interaction in U-processes. It was found that the low-temperature phonon mean free path is inversely proportional to the linear dimension of the nanopowder particles embedded in the methane crystal structure.

Effect of Co substitution on thermoelectric properties of FeSi

We have studied Fe1-xCoxSi (0 ≤ x ≤ 1) crystals using the electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements, to examine the influence of Co substitution on the electrical and thermal transport properties of FeSi. Notably, a metal-insulator transition occurs at x = 0.3 for the Fe1-xCoxSi series, as the closure of band-gap of FeSi occurs upon Co substitution. Besides, the substituent (Co) induced a significant variation in the Seebeck coefficient, including two sign crossovers. These observations are ascribed to the asymmetric electronic band structure of the Fe1-xCoxSi compounds. The analysis of thermal conductivity demonstrated that Co substitution has a strong effect on the low-temperature phonon thermal transport of FeSi via boundary scattering and point-defect scattering. Most intriguingly, the substituted end compound CoSi has the largest room temperature thermoelectric figure of merit of about 0.06 which is four orders of magnitude larger than that of undoped FeSi.

Low-temperature anharmonic phonon properties of supported graphene

Temperature-dependent Raman studies of anharmonic phonon-phonon processes in supported graphene are presented. Different G peak position temperature dependencies for graphene on glass and copper substrates were observed. Having calculated graphene-substrate total interaction energy taking into account substrate-induced strain, we estimated E2g mode energy changes due to anharmonic phonon-phonon interaction as the temperature decreases from room temperature to a value close to absolute zero for both substrates. For graphene on glass, the obtained value was about 1.8 times greater than the theoretical one for suspended graphene. For graphene on copper, the value was about 2.9 times greater. This result demonstrates a strong substrate influence on anharmonic phonon-phonon processes in graphene.

Method of the phonon modulation of cryogenic hydrogen fuel structure in ITF targets

The possibility of spatial homogenization of the cryogenic hydrogen fuel structure in laser thermonuclear targets is considered. The results of a large cycle of experimental studies on low-temperature (T = 4.2 K) phonon modulation of the structure of solid hydrogen layers with various ortho-para compositions and its effect on the homogenization process rate are presented.

Influence of induced defects on transport properties of the Bridgman-grown Bi2Se3-based single crystals

The impact of induced defects such as Bi intercalants, Bi antisites, and Se vacancies on the transport properties of the Bridgman-grown Bi2Se3-based single crystals in the ab plane were investigated by means of temperature-dependent electrical resistivity, Seebeck coefficient, and thermal conductivity measurements. We found that the Bi2Se3-based crystals can be grown either along the c-axis or perpendicular to it, depending on the different Bi-Se ratios in the starting material. All grown crystals showed a weak metallic behavior with a predominant electron-phonon scattering governing their electrical transport. The absolute value of the Seebeck coefficient of the n-type crystals grown along the c-axis is higher than those grown perpendicular to the c-axis (along the ab plane) at room temperature. The Fermi energy estimated from the Seebeck coefficient data is in the range of 0.20–0.43 eV. The thermal conductivity measurement showed that the lattice phonons dominate thermal transport in these Bi2Se3-based crystals. Analyses of lattice thermal conductivity data of the crystals by the Debye-Callaway approximation revealed that both boundary and point-defect scattering of phonons have a significant effect on the size and shape of the low-temperature phonon peak. These present findings indicate that, in addition to the change in composition, the physical properties of the Bi2Se3-based crystals are also affected significantly by the induced defects. Finally, the near-stoichiometric Bi2.04Se2.99 compound has a room-temperature thermoelectric figure of merit (ZT) of ∼0.12.

Phonon Mode Transformation Across the Orthohombic-Tetragonal Phase Transition in a Lead Iodide Perovskite CH3NH3PbI3: A Terahertz Time-Domain Spectroscopy Approach.

We study the temperature-dependent phonon modes of the organometallic lead iodide perovskite CH3NH3PbI3 thin film across the terahertz (0.5-3 THz) and temperature (20-300 K) ranges. These modes are related to the vibration of the Pb-I bonds. We found that two phonon modes in the tetragonal phase at room temperature split into four modes in the low-temperature orthorhombic phase. By use of the Lorentz model fitting, we analyze the critical behavior of this phase transition. The carrier mobility values calculated from the low-temperature phonon mode frequencies, via two theoretical approaches, are found to agree reasonably with the experimental value (∼2000 cm(2) V(-1) s(-1)) from a previous time-resolved THz spectroscopy work. Thus, we have established a possible link between terahertz phonon modes and the transport properties of perovskite-based solar cells.

Low temperature carrier transport study of monolayer MoS2 field effect transistors prepared by chemical vapor deposition under an atmospheric pressure

Large size monolayer Molybdenum disulphide (MoS2) was successfully grown by chemical vapor deposition method under an atmospheric pressure. The electrical transport properties of the fabricated back-gate monolayer MoS2 field effect transistors (FETs) were investigated under low temperatures; a peak field effect mobility of 59 cm2V−1s−1 was achieved. With the assist of Raman measurement under low temperature, this work identified the mobility limiting factor for the monolayer MoS2 FETs: homopolar phonon scattering under low temperature and electron-polar optical phonon scattering at room temperature.

Phonon thermal transport and phonon–magnon coupling in polycrystalline BiFeO3 systems

Temperature-dependent thermal conductivity of polycrystalline BiFeO3, Bi0.9Ba0.05Ca0.05FeO2.95 and Bi0.9Ca0.1FeO2.95 materials was measured using a direct heat pulse technique. Thermal conductivity of the BiFeO3-based materials is analyzed using a phonon model to probe the thermal transport mechanisms in these ferrites. It is found that the calculated thermal conductivity of the BiFeO3-based compounds is in good agreement with experimental data. The suppression of the low-temperature phonon peak in the thermal conductivity of the doped BiFeO3 materials is mainly attributed to the phonon-point-defect scattering. In addition, the contribution of optical phonon–magnon resonance scattering to optical phonon thermal transport reveals the presence of phonon–magnon coupling in these BiFeO3 materials. Finally, magneto-thermal conductivity measurements show the magnon thermal transport in pure and doped BiFeO3 systems.

Spin-boson theory for charge photogeneration in organic molecules: Role of quantum coherence

The charge photogeneration process in organic molecules is investigated by a quantum heat engine model, in which two molecules are modeled by a two-spin system sandwiched between two bosonic baths. The two baths represent the high-temperature photon emission source and the low-temperature phonon environment, respectively. We utilize the time-dependent density matrix renormalization group algorithm to investigate the quantum dynamics of the model. It is found that the transient energy current flowing through the two molecules exhibits two stages. In the first stage the energy current is of a coherent feature and represents the ultrafast delocalization of the charge-transfer state, and in the second stage a steady incoherent current is established. The power conversion efficiency is significantly high and may reach the maximum value of $93%$ with optimized model parameters. The long-lived quantum entanglement between the two spins is found to be primarily responsible for the hyperefficiency.

Looking for footprint of bulk metallic glass in electronic and phonon heat capacities of Cu55Hf45−xTix alloys

We report on the heat capacity investigation of Cu55Hf45−xTix metallic glasses. The most appropriate procedure to estimate low temperature electronic and phonon contributions has been determined. Both contributions exhibit monotonous Ti concentration dependence, demonstrating that there is no relation of either the electron density of states at the Fermi level or the Debye temperature to the increased glass forming ability in the Ti concentration range x = 15–30. The thermodynamic parameters (e.g., reduced glass temperature) remain better indicators in assessing the best composition for bulk metallic glass formation.

Geant4 Simulations of the SuperCDMS iZIP Detector Charge Carrier Propagation and FET Readout

The SuperCDMS experiment aims to directly detect dark matter particles called WIMPs (weakly interacting massive particles). The detectors measure phonon and ionization energy due to nuclear and electron recoils from incident particles. The SuperCDMS Detector Monte Carlo group uses Geant4 to simulate electron-hole pairs (\(e^-/h^+\)) and low temperature phonons. We use these simulations in order to study energy deposition in the detectors. Phonons and electron-hole pairs are tracked in a crystal detector. Because of the band structure of the crystals, the electrons undergo oblique propagation. The charge electrodes on each side of the detector are biased at different voltages while the phonon sensors are grounded. This creates a nearly uniform electric field through the bulk of the detector, with a complex shape near the surfaces. The electric field is calculated from interpolating on a tetrahedral mesh. The resulting TES phonon readout, as well as the FET charge readout are simulated. To calculate the FET readout, the Shockley-Ramo theorem is applied to simulate the current in the FET. The goal of this paper is to describe the theory and implementation of calculating the electric field, performing the charge carrier propagation, and simulating the FET readout of the SuperCDMS detectors.