Localized Phonon Modes Research Articles

Andreev Reflection in Transport Through a Quantum Dot Interacting With a Polaron

Spin-dependent tunneling through a quantum dot coupled to one ferromagnetic and one superconducting electrodes is studied theoretically in the Andreev re ection (AR) regime. Spectral functions for the system are calculated in terms of the nonequilibrium Green function technique. E ects due to interplay between the Coulomb correlations on the dot and the local phonon mode in the context of the AR transmission are analyzed.

The influence of local phonon modes in a wide-band matrix on the tunnel current-voltage characteristics of quasi-zero-dimensional structures

Experimental results on the visualization of the density of states in InAs/GaSa(001) quantum dots that were obtained by tunnel atomic-force microscopy in an ultrahigh vacuum are presented. A one-dimensional (1D) model of dissipative quantum tunneling is proposed for describing experimental current-voltage characteristics of a tunnel contact between an atomic force microscope probe and the surface of InAs/GaAs (001) quantum dots. It was found that the influence of two local modes of the wide-band matrix on the probability of 1D dissipative tunneling leads to the appearance of several randomly spaced peaks in the field dependence. It was shown that the theoretical dependence agrees qualitatively with experimental the current-voltage characteristic of the atomic force microscope tip and the surface of InAs/GaAs(001) quantum dots.

Graphene nanoribbon molecular sensor based on inelastic transport

Results of phonon-assisted inelastic quantum transport calculations are presented for graphene nanoribbons. We consider a single molecule attached to a carbon atom and describe the electronic structure by a tight-binding model, taking into account a local phonon mode associated with the attached molecule characteristic vibration. The calculated transmission spectra reveal a striking sensitivity for molecules attached to the edges of asymmetric zigzag graphene nanoribbons. Our results show that the differential conductance may be used to identify the presence as well as the characteristic vibration frequency of a target molecule at finite temperatures.

The “Surface Optical” Phonon in CdSe Nanocrystals

The origin of the ubiquitous low-frequency shoulder on the longitudinal optical (LO) phonon fundamental in the Raman spectra of CdSe quantum dots is examined. This feature is usually assigned as a "surface optical" (SO) phonon, but it is only slightly affected by modifying the surface through exchanging ligands or adding a semiconductor shell. Here we present excitation profile data showing that the low-frequency shoulder loses intensity as the excitation is tuned to longer wavelengths, closer to resonance with the lowest-energy 1Se-1S3/2 excitonic transition. Calculations of the resonance Raman spectra are carried out using a fully atomistic model with an empirical force field to calculate the phonon modes and the standard effective mass approximation envelope function model to calculate the electron and hole wave functions. When a force field of the Tersoff type is used, the calculated spectra closely resemble the experimental ones in showing mainly the higher-frequency LO phonon with 1Se-1S3/2 resonance but showing intensity in lower-frequency features with 1Pe-1P3/2 resonance. These calculations indicate that the main LO phonon peak involves largely motion of the interior atoms, while the low-frequency shoulder is more equally distributed throughout the crystal but not surface-localized. Interestingly, very different results are obtained with the widely used Coulomb plus Lennard-Jones force field developed by Rabani, which predicts far more disordered structures and more localized phonon modes for the nanocrystals compared with the Tersoff-type potential.

Scalable Implementation of Boson Sampling with Trapped Ions

Boson sampling solves a classically intractable problem by sampling from a probability distribution given by matrix permanents. We propose a scalable implementation of boson sampling using local transverse phonon modes of trapped ions to encode the bosons. The proposed scheme allows deterministic preparation and high-efficiency readout of the bosons in the Fock states and universal mode mixing. With the state-of-the-art trapped ion technology, it is feasible to realize boson sampling with tens of bosons by this scheme, which would outperform the most powerful classical computers and constitute an effective disproof of the famous extended Church-Turing thesis.

A model for vibrational properties of the atomic interface in A/B fcc metals

The motivation of this theoretical work is to introduce a model calculation for the elastic waves scattering and coherent phonon transport at an atomic nanojunction between face-centered cubic (fcc) leads. The model system A/B consists of two perfect semi-infinite fcc leads A and B, oriented in the same direction and joined by an atomic interface. It is applied to the system Cu/Ni and its inverse Ni/Cu. A theoretical approach based on the matching method is used to study the dynamics of the system A/B.The interface observables are numerically calculated, in the harmonic approximation, to investigate how the interface properties can respond to changes in the elastic force constants of the sub-lattice from where the elastic waves are incident. We have calculated the phonon scattering and the total coherent transmission via the fcc interface as elements of a Landauer–Büttiker-type scattering matrix. The calculated properties are presented for all accessible frequencies in the propagating interval and for the elastic force constants of the leads A and B. The system dynamics, the scattering, and transmission spectra via the atomic interface are analyzed as a function of the incident frequency per propagating mode of the perfect fcc waveguides. Our results show that the atomic interface is an effective phonon splitter and suggest that its characteristics may be controlled by varying its nanometric parameters. In addition, the localized phonon modes associated with the interface domain interact with propagating modes of the perfect fcc systems, located on both sides, leading to interference effects.

Functional renormalization group study of the Anderson–Holstein model

We present a comprehensive study of the spectral and transport properties in the Anderson–Holstein model both in and out of equilibrium using the functional renormalization group (fRG). We show how the previously established machinery of Matsubara and Keldysh fRG can be extended to include the local phonon mode. Based on the analysis of spectral properties in equilibrium we identify different regimes depending on the strength of the electron–phonon interaction and the frequency of the phonon mode. We supplement these considerations with analytical results from the Kondo model. We also calculate the nonlinear differential conductance through the Anderson–Holstein quantum dot and find clear signatures of the presence of the phonon mode.

Multiple-timescale relaxation dynamics in CsGd(MoO4)2—a dipolar magnet with a highly anisotropic layered crystal structure

The dynamic properties of the dipolar magnet CsGd(MoO4)2 have been studied. The frequency and temperature dependence of the AC susceptibility investigated in the paramagnetic region above 2 K revealed the co-existence of magnetic field induced slow and fast relaxation channels with a timescale differing by three orders of magnitude. The slow relaxation is determined by the properties of the first coordination sphere of the Gd3+ ion and has the character of a two-phonon Orbach process. The fast relaxation is potentially attributed to a two-phonon Raman process realized via a localized phonon mode associated with the layered crystal structure. The temperature dependence of the phonon mean free path in zero magnetic field indicates significant phonon scattering below 1 K resulting from the combined effect of magnetic correlations and the scattering of dominant phonons with energies corresponding to the crystal-field levels.

Structural, electronic, phonon and thermodynamic properties of hypothetical type-VIII clathrates Ba8Si46 and Ba8Al16Si30 investigated by first principles

We present the results of first principles calculations of the structural, electronic, elastic, vibrational, and thermodynamic properties of the hypothetical silicon-based, guest containing type-VIII clathrates Ba8Si46 and Ba8Al16Si30. We obtained the lattice constant, formation energy, band structure, density of states, elastic constants, sound velocity, and Debye temperature using the density functional theory with generalized gradient approximation (GGA). We calculated phonon dispersion and vibrational density of states spectra using the density functional perturbation energy within GGA. We computed the temperature dependent specific heat, vibrational entropy, and vibrational Helmholtz free energy by utilizing quasi-harmonic approximation. We found that replacing some silicon atoms in the framework with aluminum atoms leads to the decrease of the fundamental band gap from 1.0 in Ba8Si46 to 0.18 eV in Ba8Al16Si30. Moreover, the guest Ba atoms produced localized phonon modes lying below 1.2, 1.8 THz for Ba8Si46, and Ba8Al16Si30, respectively, which lead to the reduction of the acoustic bandwidth of the host material. The effect of replacing Si atoms with Al on the properties of the interest is also discussed.

Infrared absorption band and vibronic structure of the nitrogen-vacancy center in diamond

Negatively-charged nitrogen-vacancy (NV$^-$) color centers in diamond have generated much interest for use in quantum technology. Despite the progress made in developing their applications, many questions about the basic properties of NV$^-$ centers remain unresolved. Understanding these properties can validate theoretical models of NV$^-$, improve their use in applications, and support their development into competitive quantum devices. In particular, knowledge of the phonon modes of the $^1A_1$ electronic state is key for understanding the optical pumping process. Using pump-probe spectroscopy, we measured the phonon sideband of the ${^1}E\rightarrow{^1}A_1$ electronic transition in the NV$^-$ center. From this we calculated the ${^1}E\rightarrow{^1}A_1$ one-phonon absorption spectrum and found it to differ from that of the ${^3}E\rightarrow{^3}A_2$ transition, a result which is not anticipated by previous group-theoretical models of the NV$^-$ electronic states. We identified a high-energy 169 meV localized phonon mode of the $^1A_1$ level.

Nanomechanical coupling between microwave and optical photons

A variety of nanomechanical systems can now operate at the quantum limit1,2,3,4, making quantum phenomena more accessible for applications and providing new opportunities for exploring the fundamentals of quantum physics. Such mechanical quantum devices offer compelling opportunities for quantum-enhanced sensing and quantum information5,6,7. Furthermore, mechanical modes provide a versatile quantum bus for coupling hybrid quantum systems, supporting a quantum-coherent connection between different physical degrees of freedom8,9,10,11,12,13. Here, we demonstrate a nanomechanical interface between optical photons and microwave electrical signals, using a piezoelectric optomechanical crystal. We achieve coherent signal transfer between itinerant microwave and optical fields by parametric electro-optical coupling using a localized phonon mode. We perform optical tomography of electrically injected mechanical states and observe coherent interactions between microwave, mechanical and optical modes, manifested as electromechanically induced optical transparency. Our on-chip approach merges integrated photonics with microwave nanomechanics and is fully compatible with superconducting quantum circuits, potentially enabling microwave-to-optical quantum state transfer, and photonic networks of superconducting quantum bits14,15,16.

Pulse-electron paramagnetic resonance of Cr3+ centers in SrTiO3

Electron paramagnetic resonance on chromium doped SrTiO3 samples grown using the Verneuil technique shows the presence of charge-compensated Cr3+-VO as one of the dominant chromium centers. The spin-lattice relaxation processes have been investigated in samples with both isotropic Cr3+ and Cr3+-VO centers in heavily doped SrTiO3. The relaxation of longitudinal magnetization was dominated by the sum of two exponentials with two time constants (i.e., a slow and a fast constant) at liquid-helium temperatures. The results of fitting the temperature variation of T1 suggest that the dominant exponential contribution is related to the spin-phonon relaxation time arising from the local phonon mode.

Effects of electron-phonon coupling in the Kondo regime of a two-orbital molecule

We study the interplay between strong electron-electron and electron-phonon interactions within a two-orbital molecule coupled to metallic leads, taking into account Holstein-like coupling of a local phonon mode to the molecular charge as well as phonon-mediated interorbital tunneling. By combining canonical transformations with numerical renormalization-group calculations to address the interactions nonperturbatively and on equal footing, we obtain a comprehensive description of the system's many-body physics in the anti-adiabatic regime where the phonons adjust rapidly to changes in the orbital occupancies, and are thereby able to strongly affect the Kondo physics. The electron-phonon interactions strongly modify the bare orbital energies and the Coulomb repulsion between electrons in the molecule, and tend to inhibit tunneling of electrons between the molecule and the leads. The consequences of these effects are considerably more pronounced when both molecular orbitals lie near the Fermi energy of the leads than when only one orbital is active. In situations where a local moment forms on the molecule, there is a crossover with increasing electron-phonon coupling from a regime of collective Kondo screening of the moment to a limit of local phonon quenching. At low temperatures, this crossover is associated with a rapid increase in the electronic occupancy of the molecule as well as a marked drop in the linear electrical conductance through the single-molecule junction.

Seebeck effect near quantum MIT in Eu1 − xCaxB6

AbstractWe report the results of a Seebeck effect study carried out on the single crystals of Eu1 − xCaxB6 (0 ≤ x ≤ 1) at temperatures 1.8–300 K. The Seebeck coefficient is shown to be determined by the sum of the linear term S = AT and a phonon‐drag contribution, which is induced by the local phonon mode of europium (calcium) with the characteristic frequency 7.3 meV (9.1 meV). The crossover from negative (A ≈ −0.23 µV K−2 for x = 0) to positive (A ≈ +1.2 µV K−2 for x = 0.244) values of the diffusion thermopower indicates the smooth change of the band structure when approaching the hole‐like conductivity found in Eu1 − xCaxB6 at x > xC ≈ 0.2.

Inelastic Electron Tunneling Spectroscopy for Topological Insulators

Inelastic electron tunneling spectroscopy is a powerful spectroscopy that allows one to investigate the nature of local excitations and energy transfer in the system of interest. We study inelastic electron tunneling spectroscopy for topological insulators and investigate the role of inelastic scattering on the Dirac node states on the surface of topological insulators. Local inelastic scattering is shown to significantly modify the Dirac node spectrum. In the weak coupling limit, peaks and steps are induced in second derivative d2I/dV2. In the strong coupling limit, the local negative-U centers are formed at impurity sites, and the Dirac cone structure is fully destroyed locally. At intermediate coupling, resonance peaks emerge. We map out the evolution of the resonance peaks from weak to strong coupling, which interpolate nicely between the two limits. There is a sudden qualitative change of behavior at intermediate coupling, indicating the possible existence of a local quantum phase transition. We also find that, even for a simple local phonon mode, the inherent coupling of spin and orbital degrees in topological insulators leads to the spin-polarized texture in inelastic Friedel oscillations induced by the local mode.

Phonon-affected steady-state transport through molecular quantum dots

We consider the transport through a vibrating molecular quantum dot contacted to macroscopic leads acting as charge reservoirs. In the equilibrium and nonequilibrium regimes, we study the formation of a polaron-like transient state at the quantum dot for all the ratios of the dot–lead coupling to the energy of the local phonon mode. We show that the polaronic renormalization of the dot–lead coupling is a possible mechanism for negative differential conductance. Moreover, the effective dot level follows one of the lead chemical potentials to enhance resonant transport, causing novel features in the inelastic tunneling signal. In the linear response regime, we investigate the impact of the electron–phonon interaction on the thermoelectrical properties of the quantum dot device.

Surface lattice vibration and electron-phonon interaction in topological insulator Bi2Te3 (111) films from first principles

Surface phonon and electron-phonon (EP) interaction in Bi2Te3 (111) films are calculated by including spin-orbit coupling from the density-functional perturbation theory. We present a detailed analysis of the zone-center phonons by considering the interlayer relaxation of the films. The phonon density of states and the localized surface phonon modes along the and directions in the surface Brillouin zone are given. Our results clearly show that softening and stiffening of the surface phonons coexist in Bi2Te3 ultrathin films. Furthermore, we report on a first-principles study of the EP coupling constant for Bi2Te3 films. Our calculated EP interaction in Bi2Te3 films is very weak, which is in favor of application for room temperature electronic devices.

Vibrational Andreev bound states in magnetic molecules

We predict the existence of vibrational Andreev bound states in deformable magnetic molecules on superconducting surfaces. We discuss the Anderson impurity model with electron-phonon coupling to a realistic anharmonic vibrational mode that modulates the tunneling barrier and show that the vibronic features are spectroscopically most visibile near the transition point between the Kondo-screened singlet and the unscreened doublet ground state. We find competing tendencies between phonon hardening due to anharmonicity and softening due to coupling to electrons, contrary to the Anderson-Holstein model and other models with harmonic local phonon mode where the vibrational mode is always softened. In addition, we find that the singlet and doublet many-body states may experience very different effective phonon potentials.

Ferroelectric-thermoelectricity and Mott transition of ferroelectric oxides with high electronic conductivity

This paper reviews ferroelectric oxides in the unusual condition where the concentration of electronic carriers is close to a metal–insulator transition; in certain structures and compositions these materials have properties of interest for oxide based thermoelectric applications. In relaxor ferroelectrics, nanopolar regions associated with intrinsic localized phonon modes provide glass-like phonon characteristics due to the large levels of phonon scattering. The (Sr1−xBax)Nb2O6−δ relaxor ferroelectric single crystals have a high thermoelectric power factor, S2σ ∼ 40 μW/cm K2 at 277 °C along the c-axis, which is competitive with the best thermoelectrics. In the heavily reduced, nonstoichiometric n-type perovskite BaTiO3−δ and tungsten bronze (Sr1−xBax)Nb2O6−δ, it is shown that metallic-like conductivity occurs in the paraelectric phase and the onset of ferroelectricity stabilizes semiconducting character. Both the phase transition temperature dependence on the carrier concentration and evidence for polarization coupling to the conductivity mechanism will be discussed.

CsBr decreasing the phonon energy in Dy3+ doped GeGaSe glass

The electron–phonon coupling is firstly investigated in the Dy3+-doped Ge30Ga5Se65 and 0.9Ge30Ga5Se65+0.1CsBr glasses. Using the Judd–Ofelt theory, the intensity parameters, spontaneous emission probabilities and branching ratios of Dy3+ ions are calculated. Measured lifetimes (τm) of 6H11/2 manifold are measured in temperature region of 20–300K. Results show that local phonon modes and electron–phonon (e–p) coupling strengths are changed in the addition of CsBr. The frequency of the phonon vibration was decreased to 268cm−1 with CsBr addition to selenide glasses. The value of e–p coupling strength decreases from 0.456 in Ge30Ga5Se65 to 0.048 in 0.9Ge30Ga5Se65+0.1CsBr. It is clear that the e–p coupling strengths decrease considerably with the addition of CsBr to Ge30Ga5Se65 glass.