arXiv subjects

Justin C. W. Song

Publications and source records attributed to Justin C. W. Song.

At least 37 records · Page 2Linked to original sources

arXiv2023

Polariton-drag enabled quantum geometric photocurrents in high symmetry materials

Lowered symmetry enables access to a wide set of responses not typically accessible in high symmetry materials. Prime examples are time-reversal forbidden quantum geometric photocurrent responses (e.g., linear injection and circular shift photocurrents) that are thought to vanish in non-magnetic materials. Here we argue that polariton-drag processes enable to unblock such quantum geometric photocurrents even in non-magnetic and centrosymmetric materials. Strikingly, we uncover how a cooperative effect between finite q irradiation and the Fermi surface position leads to a polariton selective photoexcitation (PSP). PSP enables to directly address carriers within tight momentum resolved windows of the Fermi surface to yield giant enhancements of quantum geometric photocurrents. This selectivity enables to directly track momentum resolved quantum geometric quantities along the Fermi surface providing a new tool to interrogate the quantum geometry of high symmetry materials.

cond-mat.mes-hall↗

arXiv2023

Layer photovoltaic effect in van der Waals heterostructures

We argue that the layer electric polarization of noncentrosymmetric layered heterostructures can be generically controlled by light yielding a layer photovoltaic effect (LPE). The LPE possesses a rich phenomenology and can arise from myriad distinct mechanisms displaying strong sensitivity to symmetry (e.g., point group and time-reversal) as well as the presence/absence of a Fermi surface. We systematically classify these and unveil how LPE manifests for a range of light polarizations and even for unpolarized light. These unusual layer photoresponses can be realized in a range of layered heterostructures such as bilayer graphene aligned on hexagonal Boron Nitride and manifest sizeable layer polarization susceptibilities in the terahertz frequency range that can be used as novel means of bulk photodetection.

cond-mat.mes-hall↗

arXiv2023

The Fermi-Dirac staircase occupation of Floquet bands and current rectification inside the optical gap of metals: a rigorous perspective

We consider a model of a Bloch band subjected to an oscillating electric field and coupled to a featureless fermionic heat bath, which can be solved exactly. We demonstrate rigorously that in the limit of vanishing coupling to this bath (so that it acts as an ideal thermodynamic bath) the occupation of the Floquet band is not a simple Fermi-Dirac distribution function of the Floquet energy, but instead it becomes a ``staircase'' version of this distribution. We show that this distribution generically leads to a finite rectified electric current within the optical gap of a metal even in the limit of vanishing carrier relaxation rates, providing a rigorous demonstration that such rectification is generically possible and clarifying previous statements in the optoelectronics literature. We show that this current remains non-zero even up to the leading perturbative second order in the amplitude of electric fields, and that it approaches the standard perturbative expression of the Jerk current obtained from a simpler Boltzmann description within a relaxation time approximation when the frequencies are small compared to the bandwidth.

cond-mat.mes-hall↗

arXiv2022

Photocurrent as a multi-physics diagnostic of quantum materials

The photoexcitation life-cycle from incident photon (and creation of photoexcited electron hole pair) to ultimate extraction of electrical current is a complex multi-physics process spanning across a range of spatio-temporal scales of quantum materials. While often viewed through a device-technology lens, photocurrent is a key observable of the life-cycle that is sensitive to a myriad of physical processes across these scales. As a result, photocurrent is emerging as a versatile probe of electronic states, Bloch band quantum geometry, quantum kinetic processes, and device characteristics of quantum materials. This review outlines the key multi-physics principles of photocurrent diagnostics. In particular, we describe how the fundamental link between light-matter interaction and quantum geometry renders photocurrent a wavefunction-sensitive probe capable of resolving bandstructure and characterizing topological materials. We further highlight the sensitivity of the photoexcitation life-cycle to relaxational processes which in turn enables photocurrent to disentangle distinct types of carrier scattering that can range from femtosecond to nanosecond timescales. We survey the intrinsically nonlocal character of photocurrent collection that allows new types remote sensing protocols and photocurrent nanoscopy. These distinctive features underscore photocurrent diagnostics as a novel multi-physics probe for a growing class of quantum materials with properties governed by physics spanning multiple spatio-temporal scales.

cond-mat.mes-hall↗

arXiv2022

The Berry dipole photovoltaic demon and the thermodynamics of photo-current generation within the optical gap of metals

We dismantle the previously held misconception that it is impossible for bulk rectification mechanisms to induce a net DC electric current when the frequency of the impinging radiation lies within the optical gap of a metal in the limit of small carrier relaxation rates. We argue that generically such in-gap rectification mechanisms are irreversible and accompanied by a continuous exchange of energy with a heat bath and must also be necessarily accompanied by a small but finite absorption of radiation in order to guarantee the positivity of the net entropy production and abide by the second law of thermodynamics. We show, however, that the intra-band non-linear Hall effect arising from the Berry curvature is a special kind of in-gap rectification mechanism that behaves as a ``photo-voltaic demon'', namely it can operate as an ideal reversible and dissipationless conveyor of energy between the radiation and an external circuit. Its reversible nature allows for an interesting mode of operation as an amplifier of circularly polarized light, whose efficiency can approach 100%, and which could be technologically promising especially in the infrared frequency range.

cond-mat.mes-hall↗

arXiv2022

Tunable and giant valley-selective Hall effect in gapped bilayer graphene

Berry curvature is analogous to magnetic field but in momentum space and is commonly present in materials with non-trivial quantum geometry. It endows Bloch electrons with transverse anomalous velocities to produce Hall-like currents even in the absence of a magnetic field. We report the direct observation of in situ tunable valley-selective Hall effect (VSHE), where inversion symmetry, and thus the geometric phase of electrons, is controllable by an out-of-plane electric field. We use high-quality bilayer graphene with an intrinsic and tunable bandgap, illuminated by circularly polarized mid-infrared light and confirm that the observed Hall voltage arises from an optically-induced valley population. Compared with molybdenum disulfide, we find orders of magnitude larger VSHE, attributed to the inverse scaling of the Berry curvature with bandgap. By monitoring the valley-selective Hall conductivity, we study Berry curvature's evolution with bandgap. This in situ manipulation of VSHE paves the way for topological and quantum geometric opto-electronic devices, such as more robust switches and detectors.

cond-mat.mes-hall↗

arXiv2021

Strain-induced large injection current in twisted bilayer graphene

The electronic wavefunctions in moiré materials are highly sensitive to the details of the local atomic configuration enabling Bloch band geometry and topology to be controlled by stacking and strain. Here we predict that large injection currents (under circular polarized irradiation) can develop in strained twisted bilayer graphene (TBG) heterostructures with broken sublattice symmetry; such bulk photovoltaic currents flow even in the absence of a p-n junction and can be controlled by the helicity of incident light. As we argue, large injection current rates proceed from strong and highly peaked interband Berry curvature dipole distributions (arising from the texturing of Bloch wavefunctions in strained TBG heterostructures). Strikingly, we find that TBG injection current displays pronounced responses in the THz regime and can be tuned by chemical potential. These render injection currents a useful photocurrent probe of symmetry breaking in TBG heterostructures and make TBG a promising material for THz technology.

cond-mat.mes-hall↗

arXiv2021

Quantum Nanophotonics in Two-Dimensional Materials

The field of 2D materials-based nanophotonics has been growing at a rapid pace, triggered by the ability to design nanophotonic systems with in situ control, unprecedented degrees of freedom, and to build material heterostructures from bottom up with atomic precision. A wide palette of polaritonic classes have been identified, comprising ultra confined optical fields, even approaching characteristic length scales of a single atom. These advances have been a real boost for the emerging field of quantum nanophotonics, where the quantum mechanical nature of the electrons and-or polaritons and their interactions become relevant. Examples include, quantum nonlocal effects, ultrastrong light matter interactions, Cherenkov radiation, access to forbidden transitions, hydrodynamic effects, single plasmon nonlinearities, polaritonic quantization, topological effects etc. In addition to these intrinsic quantum nanophotonic phenomena, the 2D material system can also be used as a sensitive probe for the quantum properties of the material that carries the nanophotonics modes, or quantum materials in its vicinity. Here, polaritons act as a probe for otherwise invisible excitations, e.g. in superconductors, or as a new tool to monitor the existence of Berry curvature in topological materials and superlattice effects in twisted 2D materials.

physics.optics↗

arXiv2021

Quenched topological boundary modes can persist in a trivial system

Topological boundary modes can occur at the spatial interface between a topological and gapped trivial phase and exhibit a wavefunction that exponentially decays in the gap. Here we argue that this intuition fails for a temporal boundary between a prequench topological phase that possess topological boundary eigenstates and a postquench gapped trivial phase that does not possess any eigenstates in its gap. In particular, we find that characteristics of states (e.g. probability density) prepared in a topologically non-trivial system can persist long after it is quenched into a gapped trivial phase with spatial profiles that appear frozen over long times postquench. After this near-stationary window, topological boundary mode profiles decay albeit, slowly in a power-law fashion. This behavior highlights the unusual features of nonequilibrium protocols enabling quenches to extend and control localized states of both topological and non-topological origins.

cond-mat.mes-hall↗

arXiv2021

Atomic configuration controlled photocurrent in van der Waals homostructures

Conventional photocurrents at a p-n junction depend on macroscopic built-in fields and are typically insensitive to the microscopic details of a crystal's atomic configuration. Here we demonstrate how atomic configuration can control photocurrent in van der Waals (vdW) materials. In particular, we find bulk shift photocurrents (SPC) can display a rich (atomic) configuration dependent phenomenology that range from contrasting SPC currents for different stacking arrangements in a vdW homostructure (e.g., AB vs BA stacking) to a strong light polarization dependence for SPC that align with crystallographic axes. Strikingly, we find that SPC in vdW homostructures can be directed by modest strain, yielding sizeable photocurrent magnitudes under unpolarized light irradiation and manifesting even in the absence of p-n junctions. These demonstrate that SPC are intimately linked to how the Bloch wavefunctions are embedded in real space, and enables a new macroscopic transport probe (photocurrent) of lattice-scale registration in vdW materials.

cond-mat.mes-hall↗

arXiv2020

Stacking enabled strong coupling of atomic motion to interlayer excitons in van der Waals heterojunction photodiodes

We reveal stacking-induced strong coupling between atomic motion and interlayer excitons through photocurrent measurements of WSe$_2$/MoSe$_2$ heterojunction photodiodes. Strong coupling manifests as pronounced periodic sidebands in the photocurrent spectrum in frequency windows close to the interlayer exciton resonances. The sidebands, which repeat over large swathes of the interlayer exciton photocurrent spectrum, occur in energy increments corresponding directly to a prominent vibrational mode of the heterojunction. Such periodic patterns, together with interlayer photoconductance oscillations, vividly demonstrate the emergence of extraordinarily strong exciton-phonon coupling - and its impact on interlayer excitations - in stack-engineered van der Waals heterostructure devices. Our results establish photocurrent spectroscopy as a powerful tool for interrogating vibrational coupling to interlayer excitons and suggest an emerging strategy to control vibronic physics in the solid-state.

cond-mat.mes-hall↗

arXiv2020

Multistable excitonic Stark effect

The optical Stark effect is a tell-tale signature of coherent light-matter interaction in excitonic systems, wherein an irradiating light beam tunes exciton transition frequencies. Here we show that, when excitons are placed in a nanophotonic cavity, the excitonic Stark effect can become highly nonlinear, exhibiting multi-valued and hysteretic Stark shifts that depend on the history of the irradiating light. This multistable Stark effect (MSE) arises from feedback between the cavity mode occupation and excitonic population, mediated by the Stark-induced mutual tuning of the cavity and excitonic resonances. Strikingly, the MSE manifests even for very dilute exciton concentrations and can yield discontinuous Stark shift jumps of order meV. We expect that the MSE can be realized in readily available transition metal dichalcogenide excitonic systems placed in planar photonic cavities, at modest pump intensities. This phenomenon can provide new means to engineer coupled states of light and matter that can persist even in the single exciton limit.

cond-mat.mes-hall↗

arXiv2020

Hall diffusion anomaly and transverse Einstein relation

It is commonly believed that the current response of an electron fluid to a mechanical force (such as an electric field) or to a ``statistical force" (e.g., a gradient of chemical potential) are governed by a single linear transport coefficient - the electric conductivity. We argue that this is not the case in anomalous Hall materials. In particular, we find that transverse (Hall) currents manifest two distinct Hall responses governed by an unconventional {\it transverse} Einstein relation that captures an anomalous relation between the Hall conductivity and the Hall diffusion constant. We give examples of when the Hall diffusion anomaly is prominent, resulting in situations where the transverse diffusion process overwhelms the Hall conductivity and vice versa.

cond-mat.mes-hall↗

arXiv2020

Geometric photon-drag effect and nonlinear shift current in centrosymmetric crystals

The nonlinear shift current, also known as the bulk photovoltaic current generated by linearly polarized light, has long been known to be absent in crystals with inversion symmetry. Here we argue that a non-zero shift current in centrosymmetric crystals can be activated by a photon-drag effect. Photon-drag shift current proceeds from a `shift current dipole' (a geometric quantity characterizing interband transitions) and manifests a purely transverse response in centrosymmetric crystals. This transverse nature proceeds directly from the shift-vector's pseudovector nature under mirror operation and underscores its intrinsic geometric origin. Photon-drag shift current can greatly enhanced by coupling to polaritons and provides a new and sensitive tool to interrogate the subtle interband coherences of materials with inversion symmetry previously thought to be inaccessible via photocurrent probes.

cond-mat.mes-hall↗

arXiv2020

Cooperative orbital moments and edge magnetoresistance in monolayer WTe$_2$

We argue that edge electrons in monolayer WTe$_2$ can possess a "cooperative" orbital moment (COM) that critically impacts its edge magnetoresistance behavior. Arising from the cooperative action of both Rashba and Ising spin orbit coupling, COM quickly achieves large magnitudes (of order few Bohr magnetons) even for relatively small spin-orbit coupling strengths. As we explain, such large COM magnitudes arise from an unconventional cooperative spin canting of edge spins when Rashba and Ising spin orbit coupling act together. Strikingly, COM can compete with spin moments to produce an unusual anisotropic edge magnetoresistance oriented at an oblique angle. In particular, this competition produces a direction along which $\mathbf{B}$ is ineffective at gapping out the edge spectrum leaving it nearly gapless. As a result, large contrasts in gap sizes manifest as $\mathbf{B}$ is rotated granting giant anisotropic magnetoresistance of 0.1-10 million % at 10 T and low temperature.

cond-mat.mes-hall↗

arXiv2019

Shift vector as the geometric origin of beam shifts

Goos-Hanchen (GH) and Imbert-Fedorov (IF) shifts are lateral and transverse displacements of a wavepacket reflecting off a surface. A dramatic real-space manifestation of wavepacket phases, they have traditionally been analyzed in a model dependent fashion. Here we argue that GH and IF shifts admit a general geometrical description and arise from a gauge invariant geometric phase. In particular, we show GH/IF shifts can be naturally captured by a shift vector, analogous to the shift vector from shift currents in the bulk photovoltaic effect. Employing Wilson loops to visualize the scattering processes contributing to the shift vector, we separate the shift into an intrinsic (depends solely on the system bulk) and an extrinsic part. This enables to establish a clear model-independent link between symmetry and the presence/absence of intrinsic and extrinsic GH/IF shifts.

cond-mat.mes-hall↗

arXiv2019

Gate-tunable flat bands in van der Waals patterned dielectric superlattices

Superlattice engineering provides the means to reshape the fabric felt by quasiparticles moving in a material. Here we argue that bandstructure engineering with superlattices can be pushed to the extreme limit by stacking gapped van der Waals (vdW) materials on patterned dielectric substrates. Specifically, we find that high quality vdW patterned dielectric superlattices (PDS) realize a series of robust flat bands that can be directly switched on and off by gate voltage in situ. In contrast to existing superlattice platforms, these flat bands are realized without the need for fine tuning. Instead, the bands become flat as the gate voltage increases in magnitude. The characteristics of PDS flatbands are highly tunable: the type of flatband (single non-degenerate or dirac-cone-like), localization length, and interaction energy are sensitive to the applied gate voltage. As a result, electron-electron interactions in the PDS flatbands can become stronger than both the bandwidth and disorder broadening, providing a setting for correlated behavior such as flatband ferromagnetism. We expect PDS flatbands can be experimentally realized in a range of readily available gapped vdW materials such as monolayer transition metal dichalcogenides, e.g. WSe2.

cond-mat.mes-hall↗

arXiv2019

Non-trivial quantum oscillation geometric phase shift in a trivial band

The accumulation of non-trivial geometric phases in a material's response is often a tell-tale sign of a rich underlying internal structure. Studying quantum oscillations provides one of the ways to determine these geometrical phases, such as Berry's phase, that play a central role in topological quantum materials. We report on magneto-transport measurements in ABA-trilayer graphene, the band structure of which is comprised of a weakly gapped linear Dirac band, nested within a trivial quadratic band. Here we show Shubnikov-de Haas (SdH) oscillations of the quadratic band shifted by a phase that sharply departs from the expected 2$π$ Berry's phase. Our analysis reveals that, surprisingly, the anomalous phase shift is non-trivial and is inherited from the non-trivial Berry's phase of the linear Dirac band due to strong filling-enforced constraints between the linear and quadratic band Fermi surfaces. Given that many topological materials contain multiple bands, our work indicates how additional bands, which are thought to obscure the analysis, can actually be exploited to tease out the subtle effects of Berry's phase.

cond-mat.mes-hall↗