13 research outputs found
Floquet metal to insulator phase transitions in semiconductor nanowires
We study steady-states of semiconductor nanowires subjected to strong
resonant time-periodic drives. The steady-states arise from the balance between
electron-phonon scattering, electron-hole recombination via photo-emission, and
Auger scattering processes. We show that tuning the strength of the driving
field drives a transition between an electron-hole metal (EHM) phase and a
Floquet insulator (FI) phase. We study the critical point controlling this
transition. The EHM-to-FI transition can be observed by monitoring the presence
of peaks in the density-density response function which are associated with the
Fermi momentum of the EHM phase, and are absent in the FI phase. Our results
may help guide future studies towards inducing novel non-equilibrium phases of
matter by periodic driving.Comment: 10 pages including appendice
Crossover between strong and weak measurement in interacting many-body systems
Measurements with variable system-detector interaction strength, ranging from
weak to strong, have been recently reported in a number of electronic
nanosystems. In several such instances many-body effects play a significant
role. Here we consider the weak--to--strong crossover for a setup consisting of
an electronic Mach-Zehnder interferometer, where a second interferometer is
employed as a detector. In the context of a conditional which-path protocol, we
define a generalized conditional value (GCV), and determine its full crossover
between the regimes of weak and strong (projective) measurement. We find that
the GCV has an oscillatory dependence on the system-detector interaction
strength. These oscillations are a genuine many-body effect, and can be
experimentally observed through the voltage dependence of cross current
correlations.Comment: 5 pages, 3 figures, and appendices (9 pages, 3 figures, 2 tables
Steady states and edge state transport in topological Floquet-Bloch systems
We study the open system dynamics and steady states of two dimensional
Floquet topological insulators: systems in which a topological Floquet-Bloch
spectrum is induced by an external periodic drive. We solve for the bulk and
edge state carrier distributions, taking into account energy and momentum
relaxation through radiative recombination and electron-phonon interactions, as
well as coupling to an external lead. We show that the resulting steady state
resembles a topological insulator in the Floquet basis. The particle
distribution in the Floquet edge modes exhibits a sharp feature akin to the
Fermi level in equilibrium systems, while the bulk hosts a small density of
excitations. We discuss two-terminal transport and describe the regimes where
edge-state transport can be observed. Our results show that signatures of the
non-trivial topology persist in the non-equilibrium steady state.Comment: 9 pages, 4 figures + supplementary materia
Generating coherent phonon waves in narrow-band materials: a twisted bilayer graphene phaser
Twisted bilayer graphene (TBG) exhibits extremely low Fermi velocities for
electrons, with the speed of sound surpassing the Fermi velocity. This regime
enables the use of TBG for amplifying vibrational waves of the lattice through
stimulated emission, following the same principles of operation of
free-electron lasers. Our work proposes a lasing mechanism relying on the
slow-electron bands to produce a coherent beam of acoustic phonons. We propose
a device based on undulated electrons in TBG, which we dub the phaser. The
device generates phonon beams in a terahertz (THz) frequency range, which can
then be used to produce THz electromagnetic radiation. The ability to generate
coherent phonons in solids breaks new ground in controlling quantum memories,
probing quantum states, realizing non-equilibrium phases of matter, and
designing new types of THz optical devices.Comment: 5 pages, 3 figures + supplementary materia
Detection of Quantum Interference without an Interference Pattern
Quantum interference is typically detected through the dependence of the interference signal on certain parameters (path length, Aharonov-Bohm flux, etc.), which can be varied in a controlled manner. The destruction of interference by a which-path measurement is a paradigmatic manifestation of quantum effects. Here we report on a novel measurement protocol that realizes two objectives: (i) certifying that a measured signal is the result of interference avoiding the need to vary parameters of the underlying interferometer, and (ii) certifying that the interference signal at hand is of quantum nature. In particular, it yields a null outcome in the case of classical interference. Our protocol comprises measurements of cross-correlations between the readings of which-path weakly coupled detectors positioned at the respective interferometer's arms and the current in one of the interferometer's drains. We discuss its implementation with an experimentally available platform: an electronic Mach-Zehnder interferometer (MZI) coupled electrostatically to "detectors" (quantum point contacts)
How to extract weak values from a mesoscopic electronic system
Weak value (WV) protocols may lead to extreme expectation values that are larger than the corresponding orthodox expectation values. Recent works have proposed to implement this concept in nano-scale electronic systems. Here we address the issue of how one calibrates the setup in question, maximizes the measurement's sensitivity, and extracts distinctly large WVs. Our concrete setup consists of two Mach-Zehnder interferometers (MZIs): the "system" and the "detector". Such setups have already been implemented in experiment
Through the Lens of a Momentum Microscope: Viewing LightâInduced Quantum Phenomena in 2D Materials
Van der Waals (vdW) materials at their 2D limit are diverse, flexible, and unique laboratories to study fundamental quantum phenomena and their future applications. Their novel properties rely on their pronounced Coulomb interactions, variety of crystal symmetries and spin-physics, and the ease of incorporation of different vdW materials to form sophisticated heterostructures. In particular, the excited state properties of many 2D semiconductors and semi-metals are relevant for their technological applications, particularly those that can be induced by light. In this paper, the recent advances made in studying out-of-equilibrium, light-induced, phenomena in these materials are reviewed using powerful, surface-sensitive, time-resolved photoemission-based techniques, with a particular emphasis on the emerging multi-dimensional photoemission spectroscopy technique of time-resolved momentum microscopy. The advances this technique has enabled in studying the nature and dynamics of occupied excited states in these materials are discussed. Then, the future research directions opened by these scientific and instrumental advancements are projected for studying the physics of 2D materials and the opportunities to engineer their band-structure and band-topology by laser fields
Hyperbolic phonon-polariton electroluminescence in graphene-hBN van der Waals heterostructures
Phonon-polaritons are electromagnetic waves resulting from the coherent
coupling of photons with optical phonons in polar dielectrics. Due to their
exceptional ability to confine electric fields to deep subwavelength scales
with low loss, they are uniquely poised to enable a suite of applications
beyond the reach of conventional photonics, such as sub-diffraction imaging and
near-field energy transfer. The conventional approach to exciting
phonon-polaritons through optical methods, however, necessitates costly
mid-infrared and terahertz coherent light sources along with near-field
scanning probes, and generally leads to low excitation efficiency due to the
substantial momentum mismatch between phonon-polaritons and free-space photons.
Here, we demonstrate that under proper conditions, phonon-polaritons can be
excited all-electrically by flowing charge carriers. Specifically, in hexagonal
boron nitride (hBN)/graphene heterostructures, by electrically driving charge
carriers in ultra-high-mobility graphene out of equilibrium, we observe bright
electroluminescence of hBN's hyperbolic phonon-polaritons (HPhPs) at mid-IR
frequencies. The HPhP electroluminescence shows a temperature and carrier
density dependence distinct from black-body or super-Planckian thermal
emission. Moreover, the carrier density dependence of HPhP electroluminescence
spectra reveals that HPhP electroluminescence can arise from both inter-band
transition and intra-band Cherenkov radiation of charge carriers in graphene.
The HPhP electroluminescence offers fundamentally new avenues for realizing
electrically-pumped, tunable mid-IR and THz phonon-polariton lasers, and
efficient cooling of electronic devices.Comment: 8 pages, 4 figures, and supplementary materia