arXiv subjects

Zhi-Qing Li

Publications and source records attributed to Zhi-Qing Li.

At least 37 records · Page 2Linked to original sources

arXiv2020

An explanation for high defect tolerance in metal halide perovskite quantum dots

We propose Auger-like process assisted by quantum defects in metal halide perovskite quantum dots, where a charge carrier in the ground state of the quantum dot is trapped by quantum defects, resulting in another charge carrier in defect is excited and returns back to the ground state of the quantum dot. We find that the whole process is on the femtosecond scale. More importantly, the process is independent of the depth and species of the defects, which is in good agreement with the recent theoretical prediction using ab initio nonadiabatic molecular dynamics simulation. This Auger-like process may provide a potential explanation of high defect tolerance in metal halide perovskite materials.

cond-mat.mes-hall↗

arXiv2020

Disorder and magnetic field induced Bose-metal state in two-dimensional Ta$_x$(SiO$_2$)$_{1-x}$ granular films

The origin of the intermediate anomalous metallic state in two-dimensional superconductor materials remains enigmatic. In the present paper, we observe such a state in a series of $\sim$9.0 nm thick Ta$_x$(SiO$_2$)$_{1-x}$ ($x$ being the volume fraction of Ta) nanogranular films. At zero field, the $x$ $\gtrsim$ 0.75 films undergo a Berezinskii-Kosterlitz-Thouless transition as transform from normal to superconducing states upon cooling. For the $x$ $\lesssim$ 0.71 films, the resistance increases with decreasing temperature from 2 K down to 40 mK. A normal state to anomalous metallic state transition is observed in the $x$ $\simeq$ 0.73 film, i.e., near the transition temperature, the resistance of the film decreases sharply upon cooling as if the system would cross over to superconducting state, but then saturates to a value far less than that in normal state. When a small magnetic field perpendicular to the film plane is applied, the anomalous metallic state occurs in the $x$ $\gtrsim$ 0.75 films. It is found that both disorder and magnetic field can induce the transition from superconductor to anomalous metal and their influences on the transition are similar. For the the magnetic field induced case, we find the sheet resistance $R_{\square}(T,H)$ ($T$ and $H$ being the temperature and the magnitude of magnetic field) data near the crossover from the anomalous metal to superconductor and in the vicinity of the anomalous metal to insulator transition, respectively, obey unique scaling laws deduced from the Bose-metal model. Our results strongly suggest that the anomalous metallic state in the Ta$_x$(SiO$_2$)$_{1-x}$ granular films is bosonic and dynamical gauge field fluctuation resulting from superconducting quantum fluctuations plays a key role in its formation.

cond-mat.supr-con↗

arXiv2019

Universal Hall coefficient correction in strongly coupled Cr-SiO$_2$ nanogranular metals

The microstructure and electrical transport of Cr$_x$(SiO$_2$)$_{1-x}$ nanogranular films with Cr volume faction $x$$\simeq$0.67 and 0.72 are systematically investigated. The transmission electron microscopy images and elemental mappings indicate that the films are quite inhomogeneous: some Cr granules directly connect to others while some Cr granules with size $\sim$1 to $\sim$3\,nm disperse in the SiO$_2$ dielectric matrix. For each film, the Hall coefficient $R_H$ varies linearly with the natural logarithm of temperature, i.e., $ΔR_H$$\propto$$\ln T$, above $\sim$60\,K, saturates at $\sim$60\,K, and retains the saturating value below $\sim$60\,K. The temperature dependence of Hall coefficient can be explained by the recent theory in granular metals and originates from \emph{virtual diffusion} of electrons through the metallic granules. For the conductivity $σ$, a robust $Δσ$$\propto$$\sqrt{T}$ law is observed from $\sim$50 down to 2\,K. The behavior of the conductivity stems from the ``Altshuler-Aronov" correction, whose influence on the Hall coefficient is not present in the films.

cond-mat.mes-hall↗

arXiv2018

Electron effective mass and electronic structure in nonstoichiometric a-IGZO films

The transport properties and optical transmittance and absorption spectra for the nostoichiometric amorphous Indium Gallium Zinc Oxide (a-IGZO) films with Gallium and Zinc deficiencies are investigated. The resistivity and carrier concentration variation with temperature both reveal that the films possess degenerate semiconductor (or metal) characteristics. The thermopower is negative and decreases linearly with decreasing temperature, indicating the electron diffusion thermopower governs the thermal transport process in each film. Using free-electron-like model, we extracted the electron effective mass, which is about three times as large as that of the stoichiometric one and increases with increasing carrier (electron) concentration. Neglecting the variation in the energy with the wavevector near the valence band maximum and using the free-electron-like model, we also obtained the electron effective mass via the optical absorption spectra measurement. The magnitude of the effective mass obtained via optical spectra measurement is comparable to that obtained via thermopower measurement for each film. Our results strongly suggest that the nostoichiometric a-IGZO films possess free-electron-like pseudo-energy-bandstructure.

cond-mat.mtrl-sci↗

arXiv2018

Hopping conductance and macroscopic quantum tunneling effect in three dimensional Pb$_x$(SiO$_2$)$_{1-x}$ nanogranular films

We have studied the low-temperature electrical transport properties of Pb$_x$(SiO$_2$)$_{1-x}$ ($x$ being the Pb volume fraction) nanogranular films with thicknesses of $\sim$1000 nm and $x$ spanning the dielectric, transitional, and metallic regions. It is found that the percolation threshold $x_c$ lies between 0.57 and 0.60. For films with $x$$\lesssim$0.50, the resistivities $ρ$ as functions of temperature $T$ obey $ρ\propto\exp(Δ/k_BT)$ relation ($Δ$ being the local superconducting gap and the $k_B$ Boltzmann constant) below the superconducting transition temperature $T_c$ ($\sim$7 K) of Pb granules. The value of the gap obtained via this expression is almost identical to that by single electron tunneling spectra measurement. The magnetoresistance is negative below $T_c$ and its absolute value is far larger than that above $T_c$ at a certain field. These observations indicate that single electron hopping (or tunneling), rather than Cooper pair hopping (or tunneling) governs the transport processes below $T_c$. The temperature dependence of resistivities shows reentrant behavior for the 0.50$<$$x$$<$0.57 films. It is found that single electron hopping (or tunneling) also dominates the low-temperature transport process for these films. The reduction of the single electron concentration leads to an enhancement of the resisivity at sufficiently low temperature. For the 0.60$\lesssim$$x$$\lesssim$0.72 films, the resistivities sharply decrease with decreasing temperature just below $T_c$, and then show dissipation effect with further decreasing temperature. Treating the conducting paths composed of Pb particles as nanowires, we have found that the $R(T)$ data below $T_c$ can be well explained by a model that includes both thermally activated phase slips and quantum phase slips.

cond-mat.mes-hall↗

arXiv2018

Effect of exciton-phonon coupling on the interlayer excitons in transition metal dichalcogenides double layers

We investigate the correction of interlayer exciton binding energy in transition metal dichalcogenides double layers arising from the exciton-optical phonon coupling using the method of Lee-Low-Pines unitary transformation. We find that the binding energy varies in several tens of meV, depending on the polarizability of materials and interlayer distance between double layers. Moreover, the correction of binding energy results in the remarkable increasing of the critical temperature for the condensation of dilute excitonic gas basing on the Berezinskii-Kosterlitz-Thouless model. These results not only enrich the knowledge for the modulation of interlayer exciton, but also provide potential insights for the Bose-Einstein condensation and superfluid transport of interlayer exciton in two-dimensional heterostructures.

cond-mat.mes-hall↗

arXiv2018

Granular-composite-like electrical transport properties of polycrystalline cubic TaN$_{x}$ thin films prepared by rf sputtering method

We have systematically investigated the electrical transport properties of polycrystalline TaN$_x$ ($0.83$$\lesssim$$x$$\lesssim$1.32) films with rocksalt structure from 300 down to 2 K. It is found that the conductivity varies linearly with $\ln T$ from $\sim$6 K to $\sim$30 K, which does not originate from the conventional two dimensional weak-localization or electron-electron interaction effects, but can be well explained by the intergrain Coulomb effect which was theoretically proposed in the granular metals. While the fluctuation-induced tunneling conduction process dominates the temperature behaviors of the conductivities (resistivities) above $\sim$60 K. Normal state to superconductive state transition is observed in the $x$$\gtrsim$1.04 films in low temperature regime. The superconductivity can still be retained at a field of 9 T. The upper critical field increases linearly with decreasing temperature in the vicinity of the superconductive transition temperature, which is the typical feature of granular superconductors or dirty type-II superconductors. The granular-composite-like electrical transport properties of the polycrystalline TaN$_x$ films are related to their microstructure, in which the TaN$_x$ grains with high conductivity are separated by the poorly conductive amorphous transition layers (grain boundaries).

cond-mat.mes-hall↗

arXiv2017

Multiphonons resonance Raman scattering in Landau-quantized graphene

We theoretically investigate multiphonons resonance Raman scattering between the Landau levels in graphene on the polar substrate using the Huang-Rhys's model. We not only present the single and multiple surface optical (SO) phonons scattering, but also propose the combined multiphonons scattering (CMS), which is composed of the SO phonon and longitudinal acoustic phonon. We find that the CMS has a blue-shift behavior with increasing the magnetic field, differing from these SO phonon resonance scattering at a special magnetic field. This behavior may be used to explain the changing shoulder of the Raman spectrum of optical phonon resonance scattering in experiments. The theoretical model could be expanded to analyze the fine structure of Raman spectrum in two-dimensional materials.

cond-mat.mes-hall↗

arXiv2017

Granular hopping conduction in (Ag,Mo)$_x$(SnO$_2$)$_{1-x}$ films in the dielectric regime

We have studied the temperature dependence of conductivity, $σ(T)$, in two series of Ag$_{x}$(SnO$_2$)$_{1-x}$ and Mo$_{x}$(SnO$_2$)$_{1-x}$ granular films lying below the percolation threshold, where $x$ is the metal volume fraction. The metal grains in the former series have approximately spherical shape, whereas in the latter series contain numerous small substructures. In both series of films, we have observed the $σ\propto \exp[-(T_0/T)^m]$ temperature dependence, with $m\,\simeq\,1/2$, over the wide $T$ range from 2 to 80 (100) K, where $T_0$ is a characteristic temperature. Our $σ$($T$) results are explained in term of the Abeles-Sheng model which considers the structural effect of metal grains. The extracted values of the optimal separation between grains ($s_{m}$) and the charging energy ($E_{c}$) slightly increase with decreasing $x$. The variation of the parameter $C(x)$, defined in the Abeles-Sheng model, is in consistency with a simplified expression which depends only on $x$ and the dielectric constant of the insulating matrix. As the temperature is increased to above $\sim$\,100 K, a crossover to the thermally fluctuation-induced tunneling conduction processes is observed.

cond-mat.dis-nn↗

arXiv2017

Electronic conduction properties of indium tin oxide: single-particle and many-body transport

Indium tin oxide (Sn-doped In$_2$O$_{3-δ}$ or ITO) is an interesting and technologically important transparent conducting oxide. This class of material has been extensively studied for decades, with research efforts focusing on the application aspects. The fundamental issues of the electronic conduction properties of ITO from 300 K down to low temperatures have rarely been addressed. Studies of the electrical-transport properties over a wide range of temperature are essential to unraveling the underlying electronic dynamics and microscopic electronic parameters. We show that one can learn rich physics in ITO material, including the semi-classical Boltzmann transport, the quantum-interference electron transport, and the electron-electron interaction effects in the presence of disorder and granularity. To reveal the opportunities that the ITO material provides for fundamental research, we demonstrate a variety of charge transport properties in different forms of ITO structures, including homogeneous polycrystalline films, homogeneous single-crystalline nanowires, and inhomogeneous ultrathin films. We not only address new physics phenomena that arise in ITO but also illustrate the versatility of the stable ITO material forms for potential applications. We emphasize that, microscopically, the rich electronic conduction properties of ITO originate from the inherited free-electron-like energy bandstructure and low-carrier concentration (as compared with that in typical metals) characteristics of this class of material. Furthermore, a low carrier concentration leads to slow electron-phonon relaxation, which causes ($i$) a small residual resistance ratio, ($ii$) a linear electron diffusion thermoelectric power in a wide temperature range 1-300 K, and ($iii$) a weak electron dephasing rate. We focus our discussion on the metallic-like ITO material.

cond-mat.mes-hall↗

arXiv2017

Variable-range-hopping conduction processes in oxygen deficient polycrystalline ZnO films

We have fabricated oxygen deficient polycrystalline ZnO films by the rf sputtering deposition method. To systematically investigate the charge transport mechanisms in these samples, the electrical resistivities have been measured over a wide range of temperature from 300 K down to liquid-helium temperatures. We found that below about 100 K, the variable-range-hopping (VRH) conduction processes govern the charge transport properties. In particular, the Mott VRH conduction process dominates at higher temperatures, while crossing over to the Efros-Shklovskii (ES) VRH conduction process at lower temperatures. The crossover occurred at temperatures as high as a few tens degrees Kelvin. Moreover, the temperature behavior of resistivity over the entire VRH conduction regime from the Mott-type to the ES-type process can be well described by a universal scaling law.

cond-mat.dis-nn↗

arXiv2017

Correction of exciton binding energy in monolayer transition metal dichalcogenides

We theoretically investigate the corrections of exciton binding energy in monolayer transition metal dichalcogenides (TMDs) due to the exciton-optical phonon coupling in the Fr$\ddot{o}$hlich interaction model by using the linear operator combined Lee-Low-Pines variational method. We not only consider the excitons couple with the intrinsic longitudinal optical (LO) phonon modes, but also the surface optical phonon modes that induced by the polar substrates underneath the TMDs. We find that exciton binding energies are corrected in a large scale due to these exciton-optical phonon couplings. We discuss the dependences of exciton binding energy on the cut-off wave vector of optical phonon modes, the polarization parameters of materials and the interlayer distance between the polar substrates and TMDs. These results provide potential explanations for the divergence of the exciton binding energy between experiment and theory in TMDs.

cond-mat.mes-hall↗

arXiv2016

The granularity effect in amorphous InGaZnO$_4$ films prepared by rf sputtering method

We systematically investigated the temperature behaviors of the electrical conductivity and Hall coefficient of two series of amorphous indium gallium zinc oxides (a-IGZO) films prepared by rf sputtering method. The two series of films are $\sim$700\,nm and $\sim$25\,nm thick, respectively. For each film, the conductivity increases with decreasing temperature from 300\,K to $T_{\rm max}$, where $T_{\rm max}$ is the temperature at which the conductivity reaches its maximum. Below $T_{\rm max}$, the conductivity decreases with decreasing temperature. Both the conductivity and Hall coefficient vary linearly with $\ln T$ at low temperature regime. The $\ln T$ behaviors of conductivity and Hall coefficient cannot be explained by the traditional electron-electron interaction theory, but can be quantitatively described by the current electron-electron theory due to the presence of granularity. Combining with the scanning electron microscopy images of the films, we propose that the boundaries between the neighboring a-IGZO particles could make the film inhomogeneous and play an important role in the electron transport processes.

cond-mat.mes-hall↗

arXiv2015

Simultaneous observation of small- and large-energy-transfer electron-electron scattering in three dimensional indium oxide thick films

In three dimensional (3D) disordered metals, the electron-phonon (\emph{e}-ph) scattering is the sole significant inelastic process. Thus the theoretical predication concerning the electron-electron (\emph{e}-\emph{e}) scattering rate $1/τ_φ$ as a function of temperature $T$ in 3D disordered metal has not been fully tested thus far, though it was proposed 40 years ago [A. Schmid, Z. Phys. \textbf{271}, 251 (1974)]. We report here the simultaneous observation of small- and large-energy-transfer \emph{e}-\emph{e} scattering in 3D indium oxide thick films. In temperature region of $T\gtrsim100$\,K, the temperature dependence of resistivities curves of the films obey Bloch-Grüneisen law, indicating the films possess degenerate semiconductor characteristics in electrical transport property. In the low temperature regime, $1/τ_φ$ as a function of $T$ for each film can not be ascribed to \emph{e}-ph scattering. To quantitatively describe the temperature behavior of $1/τ_φ$, both the 3D small- and large-energy-transfer \emph{e}-\emph{e} scattering processes should be considered (The small- and large-energy-transfer \emph{e}-\emph{e} scattering rates are proportional to $T^{3/2}$ and $T^2$, respectively). In addition, the experimental prefactors of $T^{3/2}$ and $T^{2}$ are proportional to $k_F^{-5/2}\ell^{-3/2}$ and $E_F^{-1}$ ($k_F$ is the Fermi wave number, $\ell$ is the electron elastic mean free path, and $E_F$ is the Fermi energy), respectively, which are completely consistent with the theoretical predications. Our experimental results fully demonstrate the validity of theoretical predications concerning both small- and large-energy-transfer \emph{e}-\emph{e} scattering rates.

cond-mat.mes-hall↗

arXiv2015

Crossover of electron-electron interaction effect in Sn-doped indium oxide films

We systematically study the structures and electrical transport properties of a series of Sn-doped indium oxide (ITO) films with thickness $t$ ranging from $\sim$5 to $\sim$53\,nm. Scanning electron microscopy and x-ray diffraction results indicate that the $t\lesssim 16.8$\,nm films are polycrystalline, while those $t\gtrsim 26.7$\,nm films are epitaxially grown along [100] direction. For the epitaxial films, the Altshuler and Aronov electron-electron interaction (EEI) effect governs the temperature behaviors of the sheet conductance $σ_\square$ at low temperatures, and the ratios of relative change of Hall coefficient $ΔR_H/R_H$ to relative change of sheet resistance $ΔR_\square/R_\square$ are $\approx$2, which is quantitatively consistent with Altshuler and Aronov EEI theory and seldom observed in other systems. For those polycrystalline films, both the sheet conductance and Hall coefficient vary linearly with logarithm of temperature below several tens Kelvin, which can be well described by the current EEI theories in granular metals. We extract the intergranular tunneling conductance of each film by comparing the $σ_\square(T)$ data with the predication of EEI theories in granular metals. It is found that when the tunneling conductance is less than the conductance of a single indium tin oxide (ITO) grain, the ITO film reveals granular metal characteristics in transport properties, conversely, the film shows transport properties of homogeneous disordered conductors. Our results indicate that electrical transport measurement can not only reveal the underlying charge transport properties of the film but also be a powerful tool to detect the subtle homogeneity of the film.

cond-mat.mes-hall↗

arXiv2014

Linear temperature behavior of thermopower and strong electron-electron scattering in thick F-doped SnO$_{2}$ films

Both the semi-classical and quantum transport properties of F-doped SnO$_2$ thick films ($\sim$1\,$μ$m) were investigated experimentally. It is found that the resistivity caused by the thermal phonons obeys Bloch-Grüneisen law from $\sim$90 to 300\,K, while only the diffusive thermopower, which varies linearly with temperature from 300 down to 10\,K, can be observed.The phonon-drag thermopower is completely suppressed due to the long electron-phonon relaxation time in the compound. These observations, together with the temperature independent characteristic of carrier concentration, indicate that the conduction electron in F-doped SnO$_2$ films behaves essentially like a free electron. At low temperatures, the electron-electron scattering dominates over the electron-phonon scattering and governs the inelastic scattering process. The theoretical predicated scattering rates for both large- and small-energy-transfer electron-electron scattering processes, which are negligibly weak in three-dimensional disordered conventional conductors, are quantitatively tested in this lower carrier concentration and free-electron-like highly degenerate semiconductor.

cond-mat.mtrl-sci↗

arXiv2013

Electron-electron scatttering in Sn-doped indium oxide thick films

We have measured the low-field magnetoresistances (MRs) of a series of Sn-doped indium oxide thick films in the temperature $T$ range 4--35 K. The electron dephasing rate $1/τ_φ$ as a function of $T$ for each film was extracted by comparing the MR data with the three-dimensional (3D) weak-localization theoretical predictions. We found that the extracted $1/τ_φ$ varies linearly with $T^{3/2}$. Furthermore, at a given $T$, $1/τ_φ$ varies linearly with $k_F^{-5/2}l^{-3/2}$, where $k_{F}$ is the Fermi wavenumber, and $l$ is the electron elastic mean free path. These features are well explained in terms of the small-energy-transfer electron-electron scattering time in 3D disordered conductors. This electron dephasing mechanism dominates over the electron-phonon ($e$-ph) scattering process because the carrier concentrations in our films are $\sim$ 3 orders of magnitude lower than those in typical metals, which resulted in a greatly suppressed $e$-ph relaxation rate.

cond-mat.mes-hall↗

arXiv2013

Observation of double percolation transitions in Ag-SnO$_2$ nanogranular films

Two percolation transitions are observed in Ag$_x$(SnO$_2)_{1-x}$ nanogranular films with Ag volume fraction $x$ ranging from $\sim$0.2 to $\sim$0.9. In the vicinity of each percolation threshold $x_{ci}$ ($i$$=$1, 2), the variation in $σ$ with $x$ obeys a power law for $x$$>$$x_{ci}$. The origin of the first percolation transition at $x_{c1}$ ($x_{c1}$$>$$x_{c2}$) is similar to that of the classical one, while the second transition is explained as originating from the tunneling to the second-nearest neighboring Ag particles. These observations provide strong experimental support for the validity of current theories concerning tunneling effect in conductor-insulator nanogranular composites.

cond-mat.mes-hall↗