arXiv subjects

Spenser Talkington

Publications and source records attributed to Spenser Talkington.

13 recordsLinked to original sources

arXiv2026

Gyrotropy from Extrinsic Geometry in Twisted Materials

Gyrotropy in twisted bilayer graphene can be used as a signature of interlayer electronic coherence. Gyrotropy can emerge in the absence of interlayer coupling in time-reversal symmetric bilayer systems. This gyrotropy originates from the extrinsic geometry associated with the physical geometry of the system and is independent of the structure of the electronic states. We first illustrate this effect for a purely classical bilayer array of one-dimensional wires. Next we study twisted bilayer graphene and show that the gyrotropy is entirely due to interlayer coherence. In doing so we observe that conductivities calculated in the Bistritzer-MacDonald frame differ significantly from conductivities measurable in the lab frame. Finally we consider twisted bilayer MoTe2, first as a pristine model where the gyrotropy exactly vanishes, and then with weak strain and displacement fields where we show that the geometric gyrotropy can dominate the coherent gyrotropy. Our results call attention to the necessity to separate the contribution of extrinsic physical geometry from the contribution of intrinsic electronic states to the properties of twisted materials.

cond-mat.mes-hall↗

arXiv2026

Polarization-Resolved Photon Statistics of Cavity Quantum Materials

By forming hybrid light-matter states, optical cavities offer a route for engineering material properties, however, unambiguously probing the effects of light-matter coupling remains difficult. Here, we show that the polarization-resolved statistics of photons transmitted through a cavity, measurable via $g^{(2)}$, provide one such diagnostic. By relating $g^{(2)}$ to matter correlation functions such as the Raman structure factor, we link photon bunching and antibunching to material properties. By applying this method to the stripy-to-antiferromagnetic transition in the Kitaev-Heisenberg spin model, we find that polarization-dependent patterns of bunching and antibunching encode the magnetic point-group symmetries of each phase and characterize the behavior at the phase boundary. Finally, we predict measuring $g^{(2)}$ for output photon pairs polarized orthogonal to the input field will isolate higher-order light-matter scattering processes that probe higher-order material correlations.

cond-mat.mes-hall↗

arXiv2026

Ultrastrong Coupling Signatures in Photon Statistics from Terahertz Higgs-Polaritons

The ultrastrong coupling regime of cavity photons and quantum materials has emerged as a pathway to modify materials properties, however definitive signatures of ultrastrong coupling remain elusive. Focusing on the quantum photon statistics of light transmitted through a cavity-embedded superconductor, we show that a two-photon Higgs polariton at strong coupling realizes a photonic nonlinearity at the single terahertz photon level. We find that as light-matter coupling increases, the photon statistics show pronounced changes due to the formation of a hybrid photon-matter dark-cavity state with finite photon occupancy, producing testable signatures of ultrastrong coupling. We derive a non-Markovian input output relation and study the cavity-embedded superconductor 2H-NbSe2 as it approaches ultrastrong light-matter coupling. Our results reveal a diagnostic for ultrastrong coupling in the two-photon coincidence statistics that is absent in total counts.

cond-mat.str-el↗

arXiv2025

Weak localization and universal conductance fluctuations in large area twisted bilayer graphene

We study diffusive magnetotransport in highly p-doped large area twisted bilayer graphene in 1{\deg}, 7{\deg}, 9{\deg} and 20{\deg} samples. We report weak localization in twisted bilayer graphene for the first time. All samples exhibit weak localization, from which we extract the phase coherence length and intervalley scattering lengths, and from that determine that dephasing is caused by electron-electron scattering and intervalley scattering is caused by point defects. We observe signatures of universal conductance fluctuations in the 9{\deg} sample, which has high mobility and is near the van Hove singularity. Further improvements in sample quality and applications to large area moire materials will open new avenues to observe quantum interference effects.

cond-mat.mes-hall↗

arXiv2025

Terahertz Landau level spectroscopy of Dirac fermions in millimeter-scale twisted bilayer graphene

Exotic electronic physics including correlated insulating states and fractional Chern insulators have been observed in twisted bilayer graphene in a magnetic field when the Fermi velocity vanishes, however a question remains as to the stability of these states which is controlled by the gap to the first excited state. Free-space terahertz magneto-optics can directly probe the gap to charge excitations which bounds the stability of electronic states, but this measurement has thus-far been inaccessible due to the micron size of twisted bilayer graphene samples, while the wavelength of terahertz light is up to a millimeter. Here we leverage advances in fabrication to create twisted bilayer graphene samples over 5 mm x 5 mm in size with a uniform twist angle and study the magnetic field dependence of the cyclotron resonance by a complex Faraday rotation experiment in p-doped large angle twisted bilayer graphene. These measurements directly probe charge excitations in inter-Landau level transitions and determine the Fermi velocity as a function of twist angle.

cond-mat.mes-hall↗

arXiv2024

Many-Body Photon Blockade and Quantum Light Generation from Cavity Quantum Materials

The strong coupling regime of photons and quantum materials inside optical cavities has emerged as a promising environment for manipulating states of matter with light. Here, in turn, we show that photons bear witness to cavity quantum-electrodynamical modifications of the material, leading to profoundly non-classical properties of light passing through the cavity. By generalizing quantum-optical input-output relations to correlated quantum materials, we study the second-order photon coherence g2(t) and demonstrate that antibunching of transmitted photons serves as direct evidence of light-induced changes to the cavity-embedded material. We show that materials near a quantum critical point can realize a collective many-body photon blockade, enabling the generation of single photons or Einstein-Podolsky-Rosen pairs via leveraging strong matter fluctuations. Our findings provide new routes for interrogating and harnessing cavity-embedded quantum materials as quantum light sources, as a resource for photon-based computation and quantum sensing.

cond-mat.str-el↗

arXiv2024

Linear and Non-Linear Response of Quadratic Lindbladians

Quadratic Lindbladians encompass a rich class of dissipative electronic and bosonic quantum systems, which have been predicted to host new and exotic physics. In this study, we develop a Lindblad-Keldysh spectroscopic response formalism for open quantum systems that elucidates their steady-state response properties and dissipative phase transitions via finite-frequency linear and non-linear probes. As illustrative examples, we utilize this formalism to calculate the (1) density and dynamic spin susceptibilities of a boundary driven XY model at and near criticality, (2) linear and non-linear optical responses in Bernal bilayer graphene coupled to dissipative leads, and (3) steady state susceptibilities in a bosonic optical lattice. We find that the XY model spin density wavelength diverges with critical exponent 1/2, and there are gapless dispersive modes in the dynamic spin response that originate from the underlying spin density wave order; additionally the dispersing modes of the weak and ultra-strong dissipation limits exhibit a striking correspondence since the boundary dissipators couple only weakly to the bulk in both cases. In the optical response of the Bernal bilayer, we find that the diamagnetic response can decrease with increasing occupation, as opposed to in closed systems where the response increases monotonically with occupation; we study the effect of second harmonic generation and shift current and find that these responses, forbidden in centrosymmetric closed systems, can manifest in these open systems as a result of dissipation. We compare this formalism to its equilibrium counterpart and draw analogies between these non-interacting open systems and strongly interacting closed systems.

cond-mat.mes-hall↗

arXiv2024

Floquet-Bloch Theory for Nonperturbative Response to a Static Drive

We develop the Floquet-Bloch theory of noninteracting fermions on a periodic lattice in the presence of a constant electric field. As long as the field lies along a reciprocal lattice vector, time periodicity of the Bloch Hamiltonian is inherited from the evolution of momentum in the Brillouin zone. The corresponding Floquet quasienergies yield the Wannier-Stark ladder with interband couplings included to all orders. These results are compared to perturbative results where the lowest-order interband correction gives the field-induced polarization shift in terms of the electric susceptibility. Additionally, we investigate electronic transport by coupling the system to a bath within the Floquet-Keldysh formalism. We then study the breakdown of the band-projected theory from the onset of interband contributions and Zener resonances in the band-resolved currents. In particular, we consider the transverse quantum-geometric response in two spatial dimensions due to the Berry curvature. In the strong-field regime, the semiclassical theory predicts a plateau of the geometric response as a function of field strength. Here, we scrutinize the conditions under which the semiclassical results continue to hold in the quantum theory.

cond-mat.mes-hall↗

arXiv2023

Terahertz Circular Dichroism in Commensurate Twisted Bilayer Graphene

We report calculations of terahertz ellipticities in large-angle, 21.79$^\circ$ and 38.21$^\circ$, commensurate twisted bilayer graphene, and predict values as high as 1.5 millidegrees in the terahertz region for this non-magnetic material. This terahertz circular dichroism exhibits a magnitude comparable to that of chiral materials in the visible region. At low frequencies, the dichroic response is mediated by strong interlayer hybridization, which allows us to probe the symmetry and strength of these couplings. Crucially, lateral interlayer translation tunes this response, in contrast to small twist angle bilayer graphene's near invariance under under interlayer translation. We examine the magnitude and phase of the interlayer coupling for all structures containing fewer than 400 atoms per unit cell. Finally, we find that the dichroism can be manipulated by applying an electric field or with doping.

cond-mat.mes-hall↗

arXiv2022

Electric Field Tunable Band Gap in Commensurate Twisted Bilayer Graphene

Bernal bilayer graphene exhibits a band gap that is tunable through the infrared with an electric field. We show that sublattice odd commensurate twisted bilayer graphene (C-TBG) exhibits a band gap that is tunable through the terahertz with an electric field. We show that from the perspective of terahertz optics the sublattice odd and even forms of C-TBG are "inflated" versions of Bernal and AA stacked bilayer graphene respectively with energy scales reduced by a factor of 110 for the 21.79 degree commensurate unit cell. This lower energy scale is accompanied by a correspondingly smaller gate voltage, which means that the strong-field regime is more easily accessible than in the Bernal case. Finally, we show that the interlayer coherence energy is a directly accessible experimental quantity through the position of a power-law divergence in the optical conductivity.

cond-mat.mes-hall↗

arXiv2022

Dissipation Induced Flat Bands

Flat bands are an ideal environment to realize unconventional electronic phases. Here, we show that fermionic systems with dissipation governed by a Bloch Lindbladian can realize dispersionless bands for sufficiently strong coupling to an appropriately engineered bath. These flat bands emerge in a "dark space" of the system-environment coupling and are long-lived by virtue of symmetry protection from dissipation. We exhibit the robustness of this mechanism for general one and two band models with and without spin, and discuss conditions for their experimental realization such as in a 2D material on a superconducting substrate.

cond-mat.str-el↗

arXiv2022

Fractional Chern insulators with a non-Landau level continuum limit

Recent developments in fractional quantum Hall (FQH) physics highlight the importance of studying FQH phases of particles partially occupying energy bands that are not Landau levels. FQH phases in the regime of strong lattice effects, called fractional Chern insulators, provide one setting for such studies. As the strength of lattice effects vanishes, the bands of generic lattice models asymptotically approach Landau levels. In this article, we construct lattice models for single-particle bands that are distinct from Landau levels even in this continuum limit. We describe how the distinction between such bands and Landau levels is quantified by band geometry over the magnetic Brillouin zone and reflected in the electromagnetic response. We analyze the localization-delocalization transition in one such model and compute a localization length exponent of 2.57(3). Moreover, we study interactions projected to these bands and find signatures of bosonic and fermionic Laughlin states. Most pertinently, our models allow us to isolate conditions for optimal band geometry and gain further insight into the stability of FQH phases on lattices.

cond-mat.str-el↗

arXiv2020

Efficient unitary method for simulation of driven quantum dot systems

Density matrices evolved according the von Neumann equation are commonly used to simulate the dynamics of driven quantum systems. However, computational methods using density matrices are often too slow to explore the large parameter spaces of solid state quantum systems. Here we develop a unitary computation method to quickly perform simulations for closed quantum systems, where dissipation to the environment can be ignored. We use three techniques to optimize simulations, apply them to six time-dependent pulses for a semiconductor quantum dot qubit system, and predict the dynamic evolutions. We compare computational times between our unitary method and the density matrix method for a variety of image sizes. As an example, we implement our unitary method for a realistic four-state system [Z. Shi, et al., Nat. Commun. 5, 3020 (2014)], and find that it is over two orders of magnitude faster than the corresponding density matrix method implemented in the popular quantum simulation software QuTiP.

physics.comp-ph↗