Carrier Momentum Research Articles

Ultrafast decay of carrier momentum anisotropy in graphite

Time- and angle-resolved photoelectron spectroscopy is employed to study the thermalization of a nonequilibrium carrier distribution in the Dirac cone of graphite generated upon absorption of linearly polarized near-infrared laser pulses. The data reveal a characteristic time constant of $(20\ifmmode\pm\else\textpm\fi{}3)\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$ for the decay of a photoinduced momentum anisotropy in the nascent carrier population. Additionally, the nascent carrier spectral peak shows a downshift in energy by $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{eV}$ within the first $30\phantom{\rule{0.16em}{0ex}}\mathrm{f}\mathrm{s}$ after excitation, which is associated with the emission of strongly coupled ${A}_{1}^{\ensuremath{'}}$ optical phonons at $K$. In agreement with previous findings for graphene, a pronounced momentum anisotropy is also observed for an intermediate quasithermalized state of the carrier population. A fully thermalized distribution (with respect to energy and momentum) is finally established on a characteristic timescale of $(40\ifmmode\pm\else\textpm\fi{}10)\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$. The results underscore the complexity of carrier thermalization on ultrafast timescales resulting from the interplay of carrier-carrier and carrier-phonon interaction.

Electric-dipole spin resonance and spin orbit coupling effects in odd-integer quantum Hall edge channels

Structural-inversion-asymmetry effects on charge carriers can be described by a spin-orbit (Rashba) effective magnetic field. The direction of this field is fixed by the carrier momentum and axis of asymmetry - properties that form the basis of the spin transistor. The authors have used a sensitive terahertz magnetophotoresponse method to probe electrons in an asymmetric quantum well of InAs in the integer quantum-Hall-effect regime. Remarkable multiple-line spin-resonance spectra are attributed to oppositely directed Rashba fields of electrons in counterpropagating quasi-one-dimensional channels on opposite sample edges. Similar effects might lead to ultrafast electric-field control of spins for any spin-polarized topological edge channels.

Resonant Plasmonic Terahertz Detection in Gated Graphene p - i - n Field-Effect Structures Enabled by Nonlinearity from Zener-Klein Tunneling

We propose and analyze the terahertz (THz) detectors based on a gated graphene p-i-n (GPIN) field-effect transistor (FET) structure. The reverse-biased i-region between the gates plays the role of the electrons and holes injectors exhibiting nonlinear $I-V$ characteristics due to the Zener-Klein tunneling. This region enables the THz signal rectification, which provides their detection. The gated regions serve as the electron and hole reservoirs and the THz resonant plasma cavities. The resonant excitation of the electron and hole plasmonic oscillations results in a substantial increase in the THz detector responsivity at the signal frequency close to the plasma frequency and its harmonics. Due to the specifics of the i-region AC conductance frequency dependence, associated with the transit-time effects, the GPIN-FET response at the frequency, corresponding to the excitation of a higher plasmonic mode, can be stronger than for the fundamental mode. The GPIN-FETs can exhibit fairly high responsivity at room temperatures. Lowering of the latter can result in its further enhancement due to weakening of the carrier momentum relaxation.

Limiting capabilities of two-dimensional plasmonics in electromagnetic wave detection

Plasmons in two-dimensional electron systems (2DESs) feature ultrastrong confinement and are expected to efficiently mediate the interactions between light and charge carriers. Despite these expectations, the electromagnetic detectors exploiting 2D plasmon resonance have been so far inferior to their nonresonant counterparts. Here, we theoretically analyze the origin of these failures, and suggest a proper niche for 2D plasmonics in electromagnetic wave detection. We find that a confined 2DES supporting plasmon resonance has an upper limit of absorption cross section, which is identical to that of simple metallic dipole antenna. Small size of plasmonic resonators implies their weak dipole moments and impeded coupling to free-space radiation. Achieving the ``dipole limit'' of the absorption cross section in isolated 2DES is possible either at unrealistically long carrier momentum relaxation times, or at resonant frequencies below units of terahertz. We further show that amendment of even small metal contacts to 2DES promotes the coupling and reduces the fundamental mode frequency. The contacted resonators can still have deep-subwavelength size. They can be merged into compact arrays of detectors responding resonantly to multiple frequencies. Such arrays may find applications in multichannel wireless communications, hyper-spectral imaging, and energy harvesting.

Terahertz scattering-type near-field microscopy quantitatively determines the conductivity and charge carrier density of optically doped and impurity-doped silicon

A terahertz scattering-type scanning near-field optical microscope is used for nano-scale non-invasive conductivity measurements on bulk silicon samples. We first investigate the case where the density of charge carriers is determined by optical interband excitation. We show that the amplitude and phase of the near-field signal are reproduced by simulations based on an established near-field interaction model, which takes the Drude conductivity, ambipolar carrier diffusion, and known recombination properties of photo-excited carrier pairs in Si into account. This study is then extended to impurity-doped Si. We demonstrate that the phase of the near-field signal, which can easily be measured in absolute terms, allows us to quantitatively determine the conductivity of the specimens, from which the carrier density is derived based on the known carrier momentum relaxation time. A measurement at a single properly chosen terahertz frequency is sufficient. The technique proposed here holds promise for the spatially resolved quantitative characterization of micro- and nanoelectronic materials and devices.

Terahertz spectroscopic evidence of electron correlations in SrVO3 epitaxial thin films

Electron correlation in transition metal oxides (TMOs) is an intriguing topic in condensed matter physics, revealing a wide variety of exotic physical properties. Investigating low-energy carrier dynamics by terahertz (THz) spectroscopy is an efficient route to obtain the essential insights into electron correlation. In the present study, THz-time-domain spectroscopy is employed to probe electron correlation in SrVO3 epitaxial thin films. The low energy carrier dynamics of SrVO3 in the range of 0.2–6.0 meV shows a typical metallic behavior as overserved in dc transport measurements. The obtained temperature-dependent optical parameters provide evidence of mass renormalization in the low energy regime and carrier momentum relaxation happens via the electron–electron scattering mechanism. Overall, the frequency and temperature-dependent optical parameters indicate the Fermi liquid ground state in a Mott–Hubbard type correlated metal SrVO3 thin film. Our results provide significant insight on low energy carrier dynamics in the correlated electron system, particularly perovskite-based d 1 TMOs.

Controlling the Spatial and Momentum Distributions of Plasmonic Carriers: Volume vs Surface Effects.

Spatial and momentum distributions of excited charge carriers in nanoplasmonic systems depend sensitively on optical excitation parameters and nanoscale geometry, which therefore control the efficiency and functionality of plasmon-enhanced catalysts, photovoltaics, and nanocathodes. Growing appreciation over the past decade for the different roles of volume- vs surface-mediated excitation in such systems has underscored the need for explicit separation and quantification of these pathways. Toward these ends, we utilize angle-resolved photoelectron velocity map imaging to distinguish these processes in gold nanorods of different aspect ratios down to the spherical limit. Despite coupling to the longitudinal surface plasmon, we find that resonantly excited nanorods always exhibit transverse (sideways) multiphoton photoemission distributions due to photoexcitation within volume field enhancement regions rather than at the tip hot spots. This behavior is accurately reproduced via ballistic Monte Carlo modeling, establishing that volume-excited electrons primarily escape through the nanorod sides. Furthermore, we demonstrate optical control over the photoelectron angular distributions via a screening-induced transition from volume (transverse/side) to surface (longitudinal/tip) photoemission with red detuning of the excitation laser. Frequency-dependent cross sections are separately quantified for these mechanisms by comparison with theoretical calculations, combining volume and surface velocity-resolved photoemission modeling. Based on these results, we identify nanomaterial-specific contributions to the photoemission cross sections and offer general nanoplasmonic design principles for controlling photoexcitation/emission distributions via geometry- and frequency-dependent tuning of the volume vs surface fields.

Continuous angular control over anisotropic photoemission from isotropic gold nanoshells.

A variety of applications rely on the efficient generation of hot carriers within metal nanoparticles and charge transfer to surrounding molecules or materials. The optimization of such processes requires a detailed understanding of excited carrier spatial, temporal, and momentum distributions, which also leads to opportunities for active optical control over hot carrier dynamics on nanometer and femtosecond scales. Such capabilities are emerging in nanoplasmonic systems and typically rely on tuning optical polarization and/or frequency to selectively excite one or more discrete hot spots defined by the particle geometry. Here, we introduce a unique case in which hot electron excitation and emission distributions can instead be continuously controlled via linear laser polarization in the azimuthal plane of a gold nanoshell supported on a substrate. In this configuration, it is the laser field that breaks the azimuthal symmetry of the supported nanoshell and determines the plasmonic field distribution. Using angle-resolved photoelectron velocity map imaging, we find that the hot electrons are predominantly emitted orthogonal to the nanoshell dipolar surface plasmon resonance axis defined by the laser polarization. Furthermore, such anisotropic emission is only observed for nanoshells, while solid gold nanospheres are found to be isotropic emitters. We show that all of these effects are recapitulated via simulation of the plasmonic electric field distributions within the nanoparticle volume and ballistic Monte Carlo modeling of the hot electron dynamics. These results demonstrate a highly predictive level of understanding of the underlying physics and possibilities for ultrafast spatiotemporal control over hot carrier dynamics.

Micron and Nano-Dimensioned Silicon LEDs Emitting at 650 and 750-850 nm Wavelengths in Standard Si Integrated Circuitry

Optical transitions in silicon avalanche mode silcon LEDs were modeled, using the energy band structure, available carrier energy and momentum spreads. Based on previous experimental observations, it was hypothesized that emissions at these wavelengths can be enhanced by using a combination of excitation of carriers in high electric fields and scattering of excited carriers in compensated, high dopant impurity p+n+ environments, optical emissions at 650 and 750-850 nm wavelength emission regimes could be stimulated. A number of p+np+ devices were subsequently designed and realized using a 0.35-micron RF bipolar fabrication process that provided control over both carrier energy and carrier balancing. The optical emission characteristics of the devices was analyzed using an optical fiber lensed probe spectrophotometer and high-resolution optical microscopy. Clear evidence is obtained that 650 nm emissions can be enhanced in nano dimensioned emission spots by using these technologies. The devices operated in the 5-10V and 0.08–10mA regime. External emission intensities of up to 100 nW $\mu \text{m}^{2}$ were observed. Furthermore, using the same modeling, light emission was observed at nano-dimensioned regions emitting in the 750 - 850 nm emission region by placing high dopant impurity regions next to lowly doped high E fleld regions, and relaxing and scattering carriers in these zones. The 750 - 850 nm emissions may find application in both short haul and long haul integrated Si AMLEDs coupled to silicon nitride waveguide structures.

Bulk Fermi surface of the layered superconductor TaSe3 with three-dimensional strong topological state

High magnetic field transport measurements and ab initio calculations on the layered superconductor TaSe3 have provided compelling evidences for the existence of a three-dimensional strong topological insulator state. Longitudinal magnetotransport measurements up to ~ 33 T unveiled striking Shubnikov-de Hass oscillations with two fundamental frequencies at 100 T and 175 T corresponding to a nontrivial electron Fermi pocket at the B point and a nontrivial hole Fermi pocket at the {\Gamma} point respectively in the Brillouin zone. However, calculations revealed one more electron pocket at the B point, which was not detected by the magnetotransport measurements, presumably due to the limited carrier momentum relaxation time. Angle dependent quantum oscillations by rotating the sample with respect to the magnetic field revealed clear changes in the two fundamental frequencies, indicating anisotropic electronic Fermi pockets. The ab initio calculations gave the topological Z2 invariants of (1; 100) and revealed a single Dirac cone on the (1 0 -1) surface at the X point with helical spin texture at a constant-energy contour, suggesting a strong topological insulator state. The results demonstrate TaSe3 an excellent platform to study the interplay between topological phase and superconductivity and a promising system for the exploration of topological superconductivity.

Carrier Transport Engineering in Carbon Nanotubes by Chirality-Induced Spin Polarization.

Carbon nanotubes (CNTs), helically wrapped with single-stranded DNA, have recently emerged as a spin-filtering material. The inversion asymmetric helical potential of DNA creates a spin-filtering effect (commonly known as "chirality-induced spin selectivity" or CISS), which polarizes carrier spins in the nanotube. Thus, tuning of the DNA-CNT interaction is expected to affect carrier spins in nanotubes. The CISS effect induces spin polarization, which is coupled with the carrier's momentum direction, and therefore, in one-dimensional systems, such as nanotubes, momentum flip must be accompanied by a simultaneous spin flip. This spin momentum locking can have a profound impact on charge transport in nanotubes as backscattering due to phonons and disorder will be suppressed as these mechanisms are spin-independent. Here, we report DNA-CNT spin filters in which CNTs have been functionalized with two different classes of sequences, exhibiting different degrees of interaction with the CNT. They induce different degrees of spin polarization in the channel, with significant impact on temperature-dependent charge transport and interference phenomena arising from carrier backscattering. This work raises the intriguing possibility of engineering charge transport in nanotubes via CISS-induced spin polarization by tailor-made DNA sequences.

Negative and positive terahertz and infrared photoconductivity in uncooled graphene

We develop the model for the terahertz (THz) and infrared (IR) photoconductivity of graphene layers (GLs) at room temperature. The model accounts for the linear GL energy spectrum and the features of the energy relaxation and generation-recombination mechanisms inherent at room temperature, namely, the optical phonon absorption and emission and the Auger interband processes. Using the developed model, we calculate the spectral dependences of the THz and IR photoconductivity of the GLs. We show that the GL photoconductivity can change sign depending on the photon frequency, the GL doping and the dominant mechanism of the carrier momentum relaxation. We also evaluate the responsivity of the THz and IR photodetectors using the GL photoconductivity. The obtained results along with the relevant experimental data might reveal the microscopic processes in GLs, and the developed model could be used for the optimization of the GL-based photodetectors.

Application of the Locally Nonequilibrium Diffusion-Drift Cattaneo–Vernotte Model to the Calculation of Photocurrent Relaxation in Diode Structures under Subpicosecond Pulses of Ionizing Radiation

The excitation-relaxation process in electron–hole plasma upon exposure to ionizing radiation for a time shorter than the relaxation time of the mobile carrier energy and momentum is considered. By the example of the calculation of transient ionization processes in a silicon hyperhigh-frequency Schottky diode, local-equilibrium and local-nonequilibrium carrier transport models are compared. It is shown that the local-nonequilibrium model features a wider field of application for describing fast relaxation processes.

Electrical Detection of Charge-to-spin and Spin-to-Charge Conversion in a Topological Insulator Bi2Te3 Using BN/Al2O3 Hybrid Tunnel Barrier

One of the most striking properties of three-dimensional topological insulators (TIs) is spin-momentum locking, where the spin is locked at right angles to momentum and hence an unpolarized charge current creates a net spin polarization. Alternatively, if a net spin is injected into the TI surface state system, it is distinctively associated with a unique carrier momentum and hence should generate a charge accumulation, as in the so-called inverse Edelstein effect. Here using a Fe/Al2O3/BN tunnel barrier, we demonstrate both effects in a single device in Bi2Te3: the electrical detection of the spin accumulation generated by an unpolarized current flowing through the surface states, and that of the charge accumulation generated by spins injected into the surface state system. This work is the first to utilize BN as part of a hybrid tunnel barrier on TI, where we observed a high spin polarization of 93% for the TI surfaces states. The reverse spin-to-charge measurement is an independent confirmation that spin and momentum are locked in the surface states of TI, and offers additional avenues for spin manipulation. It further demonstrates the robustness and versatility of electrical access to the spin system within TI surface states, an important step towards its utilization in TI-based spintronics devices.

Generation-recombination Models in the Matrix Kinetic Approach to Spintronics

A new model, based on an asymptotic procedure for solving the spinor kinetic equations of carriers and phonons is proposed, which gives naturally the displaced Fermi-Dirac distribution function at the leading order. The balance equations for the carrier numbers, energy and momentum densities, plus the Poisson's equation, constitute now a system of nine equations. Moreover two equations for the evolution of the spin densities are added, which account fora general dispersion relation. Direct and Schockley-Reed-Hall (SRH) generation-recombination (gr) models are finally introduced, in order to calculate the gr rates.

Critical de Broglie wavelength in superconductors

There are growing numbers of experimental evidences that the self-field critical currents, [Formula: see text], are a new instructive tool to investigate fundamental properties of superconductors ranging from atomically thin films [M. Liao et al., Nat. Phys. 6 (2018), https://doi.org/10.1038/s41567-017-0031-6 ; E. F. Talantsev et al., 2D Mater. 4 (2017) 025072; A. Fete et al., Appl. Phys. Lett. 109 (2016) 192601] to millimeter-scale samples [E. F. Talantsev et al., Sci. Rep. 7 (2017) 10010]. The basic empirical equation which quantitatively accurately described experimental [Formula: see text] was proposed by Talantsev and Tallon [Nat. Commun. 6 (2015) 7820] and it was the relevant critical field (i.e. thermodynamic field, [Formula: see text], for type-I and lower critical field, [Formula: see text], for type-II superconductors) divided by the London penetration depth, [Formula: see text]. In this paper, we report new findings relating to this empirical equation. It is that the critical wavelength of the de Broglie wave, [Formula: see text], of the superconducting charge carrier which within a numerical pre-factor is equal to the largest of two characteristic lengths of Ginzburg–Landau theory, i.e. the coherence length, [Formula: see text], for type-I superconductors or the London penetration depth, [Formula: see text], for type-II superconductors. We also formulate a microscopic criterion for the onset of dissipative transport current flow: [Formula: see text], where [Formula: see text] is the charge carrier momentum, [Formula: see text] is Planck’s constant and the inequality sign “[Formula: see text]” is reserved for the dissipation-free flow.

Experimental evidence for an anisotropic Berry-phase effect on the anomalous Hall effect in MnAs films

By varying both the carrier momentum and spin orientation separately, the anisotropic Berry-phase effect in MnAs films is explicitly investigated. Specifically, the Berry-phase effect is characterized by the intrinsic anomalous Hall conductivity \ensuremath{\gamma}, which is extracted via treating various scattering sources independently. More than 50% change of \ensuremath{\gamma} is observed when changing the electric current direction from MnAs [2-1-10] to [0001], while \ensuremath{\gamma} varies moderately for the cases of applying the current along two orthogonal directions in the close-packed crystal plane. In addition, the magnetization direction induced variation of \ensuremath{\gamma} is also relatively small. The above phenomena can be explained under the framework of anisotropic spin-orbit interaction, which provides a unified picture for understanding the critical role played by electric current and magnetization directions. Our results pave an avenue for getting deep insights into the anisotropic Berry-phase effect and its contribution to the anomalous Hall effect.

Применение локально-неравновесной диффузионно-дрейфовой модели Каттанео-Вернотта для описания релаксации фототока в диодных структурах при воздействии субпикосекундных импульсов ионизирующих излучений

AbstractThe excitation-relaxation process in electron–hole plasma upon exposure to ionizing radiation for a time shorter than the relaxation time of the mobile carrier energy and momentum is considered. By the example of the calculation of transient ionization processes in a silicon hyperhigh-frequency Schottky diode, local-equilibrium and local-nonequilibrium carrier transport models are compared. It is shown that the local-nonequilibrium model features a wider field of application for describing fast relaxation processes.

Hot electron inelastic scattering and transmission across graphene surfaces

Inelastic scattering and transmission of externally injected hot carriers across graphene layers are considered as a function of graphene carrier density, temperature, and surrounding dielectric media. A finite temperature dynamic dielectric function for graphene for an arbitrary momentum q and frequency ω is found under the random phase approximation and a generalized scattering lifetime formalism is used to calculate the scattering and transmission rates. Unusual trends in scattering are found, including declining rates as graphene carrier density increases and interband transition excitations, which highlights the difference with out-of-plane as compared to in-plane transport. The results also show strong temperature dependence with a drastic increase in scattering at room temperature. The calculated scattering rate at T = 300 K shows a wide variation from 0.2 to 10 fs−1 depending on graphene carrier density, incident carrier momentum, and surrounding dielectrics. The analysis suggests that a transmission rate greater than 0.9 for a carrier with kinetic energy over 1 eV is achievable by carefully controlling the graphene carrier density in conjunction with the use of high-κ dielectric materials. Potential applications to electronic and electro-optical devices are also discussed.

Role of Edge Engineering in Photoconductivity of Graphene Nanoribbons.

The effect of edge engineering of graphene nanoribbons (GNRs) on their ultrafast photoconductivity is investigated. Three different GNRs were fabricated by bottom-up synthesis in the liquid phase, where structure, width, and edge planarity could be controlled chemically at the atomic level. The charge carrier transport in the fabricated GNRs was studied on the ultrafast, sub-picosecond time scale using time-resolved terahertz spectroscopy, giving access to the elementary parameters of carrier conduction. While the variation of the side chains does not alter the photoconductive properties of GNRs, the edge structure has a strong impact on the carrier mobility in GNRs by affecting the carrier momentum scattering rate. Calculations of the ribbon electronic structure and theoretical transport studies show that phonon scattering plays a significant role in microscopic conduction in GNRs with different edge structures. A comparison between theory and experiment indicates that the mean free path of charge carriers in the nanoribbons amounts to typically ∼20 nm.