arXiv subjects

Andrey Baydin

Publications and source records attributed to Andrey Baydin.

At least 19 recordsLinked to original sources

arXiv2026

Terahertz anomalous Hall effect in magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$

Time-reversal-symmetry-broken Weyl semimetals are known to have at least two nodes in their electronic band structure, separated in momentum space and acting as sources and sinks of Berry curvature. This gives rise to a transverse Hall conductivity, known as the anomalous Hall effect (AHE), which, in the simplest two-node picture, is proportional to the momentum-space separation between the nodes in the zero frequency limit. In the recently discovered Weyl semimetal $\mathrm{Co_3Sn_2S_2}$, a giant AHE has been observed. However, experimental investigations in the low-energy regime, which directly probe quasiparticle excitations near the Weyl nodes, remain limited. Here, we present a systematic study of the intrinsic low-energy gyrotropic optical response of $\mathrm{Co_3Sn_2S_2}$ using terahertz spectroscopy combined with semianalytical calculations based on a physically intuitive effective model. Our results provide a robust and transparent explanation of the observed magnetooptical phenomena in terms of intrinsic gyrotropy arising from momentum-space separation of the Weyl nodes. Furthermore, quantitative comparison between experiment and theory places stringent constraints on the material parameters.

cond-mat.mes-hall↗

arXiv2026

Nonlinear Diamagnetic Interactions in Ultrastrongly Coupled 2D Electrons

The quantum Hopfield model is widely used to describe ultrastrong light--matter coupling between cavity photons and collective bosonic excitations in solids, where the diamagnetic interaction is conventionally assumed to be a constant. We experimentally demonstrate that the diamagnetic response of Landau polaritons is reduced under strong terahertz field excitation. We show that this behavior originates from field-driven redistribution of electrons into the nonparabolic regime of the conduction band of GaAs, which reduces the plasma frequency and consequently the diamagnetic interaction strength. A microscopic hot-electron model reproduces the observed nonlinear response. Motivated by this microscopic picture, we propose a nonlinear extension of the Hopfield model with a Kerr-like interaction. Our results establish a route toward nonlinear cavity quantum electrodynamics and driven ultrastrong light--matter coupling beyond the conventional linear Hopfield description, which is capable of creating uniquely quantum optical effects such as squeezed light generation.

quant-ph↗

arXiv2026

Floquet Engineering of a Quasiequilibrium Superradiant Phase Transition in Landau Polaritons

Superradiant phase transitions (SRPTs), characterized by photon condensation and macroscopic matter polarization, are forbidden in equilibrium for homogeneous fields by no-go theorems. Here, we show that Floquet driving can circumvent this constraint in a Landau polariton system consisting of a two-dimensional electron gas coupled to a terahertz cavity in a DC magnetic field. An off-resonant AC magnetic field modulates the cyclotron frequency and light--matter coupling strength while leaving the diamagnetic term unchanged, generating an additional DC coupling contribution. This drives the system across a critical threshold into a superradiant phase, characterized by photon condensation and Landau-level polarization in the ground state of the Floquet Hamiltonian. This quasiequilibrium approach offers a route to SRPTs distinct from driven-dissipative schemes.

cond-mat.mes-hall↗

arXiv2026

Single-Crystal, Single-Chirality, Single-Wall Carbon Nanotube Heterostructures for Optoelectronics: An Opinion

The extraordinary one-dimensional properties of carbon nanotubes have captivated scientists and engineers since their discovery in the early 1990s. In particular, semiconducting single-wall carbon nanotubes (SWCNTs) are highly promising for optoelectronic applications because of their diameter-dependent direct band gaps and strong, tunable light-matter interactions. However, the prevalence of structural disorder, misalignment, and chirality heterogeneity in macroscopic assemblies has hindered their practical applications. Recently, advanced assembly methods, combined with post-growth chirality separation techniques, have enabled the fabrication of wafer-scale, nearly crystalline films of highly aligned and densely packed SWCNTs with tailored properties. In this Opinion, we discuss how these films provide a transformative platform for engineering "Single$^3$" heterostructures-assemblies that are simultaneously single-crystal, single-chirality, and single-wall. Stacking these layers with nanometer-scale precision and tunable thicknesses allows for the realization of artificial bilayer junctions, quantum wells, and superlattices. We posit that these architectures will enable a new generation of high-performance devices, including lasers, photodiodes, solar cells, and single-photon emitters.

physics.app-ph↗

arXiv2026

Vacuum-dressed superconductivity in NbN observed in a high-$Q$ terahertz cavity

Emerging theoretical frameworks suggest that physical properties of matter can be altered within an optical cavity by harnessing quantum vacuum electromagnetic fluctuations, even in the total absence of external driving fields. Among the most intriguing predictions is the potential to noninvasively manipulate superconductivity. Here, we experimentally observe modified superconductivity in niobium nitride (NbN) thin films within high-quality-factor ($Q$) terahertz cavities. Using terahertz time-domain spectroscopy, we characterize the NbN response both in free space and within a high-$Q$ photonic-crystal cavity. Our analysis reveals significant cavity-induced modifications to the optical conductivity. A theoretical model indicates that these changes originate from a substantial ($\sim13\,\%$) reduction in the superfluid density and a minor ($\sim2\,\%$) reduction in the superconducting gap, driven by cavity vacuum fluctuations. These results demonstrate a platform for engineering ground states via vacuum--matter coupling, opening frontiers in cavity materials science.

physics.optics↗

arXiv2025

Realization of a Chiral Photonic-Crystal Cavity with Broken Time-Reversal Symmetry

Light-matter interactions in chiral cavities offer a compelling route to manipulate material properties by breaking fundamental symmetries such as time-reversal symmetry. However, only a limited number of chiral cavity implementations exhibiting broken time-reversal symmetry have been demonstrated to date. These typically rely on either the application of strong magnetic fields, circularly polarized Floquet driving, or the hybridization of cavity modes with matter excitations in the ultrastrong coupling regime. Here, we present a one-dimensional terahertz photonic-crystal cavity that exhibits broken time-reversal symmetry. The cavity consists of a high-resistivity silicon wafer sandwiched between lightly n-doped InSb wafers. By exploiting the nonreciprocal response of a terahertz magnetoplasma and the exceptionally low effective mass of electrons in InSb, we demonstrate a circularly polarized cavity mode at 0.67 THz under a modest magnetic field of 0.3 T, with a quality factor exceeding 50. Temperature-, magnetic field-, and polarization-dependent measurements, supported by simulations, confirm the realization of a chiral cavity with broken time-reversal symmetry. This platform offers a robust and accessible approach for exploring chiral light--matter interactions and vacuum dressed quantum condensed matter in the terahertz regime.

physics.optics↗

arXiv2025

Quantum Semiconductor Heterostructures for meV Axion Dark Matter Detection

We propose a novel strategy and a new class of detectors for the direct detection of axion dark matter in the meV mass range, based on resonantly enhanced axion-photon conversion through the inverse Primakoff effect in engineered radiometers composed of quantum semiconductor heterostructures. Semiconductor-Quantum-Well Axion Radiometer Experiments (SQWAREs) are multiple quantum well structures forming magnetoplasmonic cavities, containing high-mobility two-dimensional electron gases, realizing tunable epsilon-near-zero resonances in the terahertz frequency range. By controlling the orientation of the cavity within a strong external magnetic field, both the resonance frequency and the axion-induced current are optimized $\it{in\,situ}$, enabling efficient scanning across a broad mass range without the need for complex mechanical adjustments. The axion-induced electromagnetic signal radiatively emitted from the cavity is then detected by a photodetector. We present the theoretical basis for resonant enhancement, detail the experimental design and benchmarks through extensive simulations, project the sensitivity of an example SQWARE for several realistic configurations, and demonstrate the modularity and flexibility of the design to fit reasonably with any lab's existing capabilities and target unique axion mass ranges. Our results demonstrate that the SQWAREs can probe the well-motivated quantum chromodynamics axion parameter space and close a critical gap in direct searches at meV masses.

hep-ph↗

arXiv2025

Terahertz Chiral Optics with Ordered Carbon Nanotube Architectures

Chiral optical terahertz (THz) devices have significant technological implications for telecommunications, spectroscopy, and sensing. Engineering tunable, broadband, and cost-effective THz chiral materials has long been recognized as a challenging endeavor and has been the bottleneck hindering the full exploitation of the THz spectrum. Here, we present an artificial structure based on aligned carbon nanotube films that exhibit a tunable broadband circular dichroism (CD) up to 2.8 degrees. Its behavior is reciprocal. The developed theoretical simulations are in agreement with the experiments and predict a further increase of CD signal up to ~30 degrees when more CNT layers are added.

physics.optics↗

arXiv2025

Lattice-induced spin dynamics in Dirac magnet CoTiO3

Spin-lattice coupling is crucial for understanding the spin transport and dynamics for spintronics and magnonics applications. Recently, cobalt titanate (CoTiO3), an easy-plane antiferromagnet, has been found to host axial phonons with a large magnetic moment, which may originate from spin-lattice coupling. Here, we investigate the effect of light-driven lattice dynamics on the magnetic properties of CoTiO3 using time-resolved spectroscopy with a THz pump and a magneto-optic probe. We found resonantly driven Raman active phonons, phonon-polariton-induced excitation of the antiferromagnetic magnons, and a slow increase in the polarization rotation of the probe, all indicating symmetry breaking that is not intrinsic to the magnetic space group. The temperature dependence confirmed that the observed spin dynamics is related to the magnetic order, and we suggest surface effects as a possible mechanism. Our results of THz-induced spin-lattice dynamics signify that extrinsic symmetry breaking may contribute strongly and unexpectedly to light-driven phenomena in bulk complex oxides.

cond-mat.mtrl-sci↗

arXiv2025

Quantum Vacuum in Matter

An intriguing consequence of quantum field theory is that vacuum is not empty space; it is full of quantum fluctuating electromagnetic fields, or virtual photons, corresponding to their zero-point energy, even though the average number of photons is zero. These short-lived vacuum fluctuations are behind some of the most fascinating physical processes in the universe, including spontaneous emission, the Lamb shift, and the Casimir force. Recent theory and experiments indicate that the properties of materials placed in photonic cavities may be altered, even in the complete absence of any external fields, through interaction with the fluctuating vacuum electromagnetic fields. Judicious engineering of the quantum vacuum surrounding the matter inside a cavity can lead to significant and nonintuitive modifications of electronic and vibrational states, producing a ``vacuum dressed'' material. These exciting new ideas have stimulated discussions regarding the fundamental physics of vacuum-matter interactions and also broadened the scope of potential applications using zero-point fluctuations to engineer materials. This Perspective will first discuss recent experimental and theoretical developments on vacuum-modified condensed matter systems, which usually require the realization of the so-called ultrastrong light-matter coupling regime. Then, we will overview some of the most promising cavity designs for enhancing vacuum electromagnetic fields in materials with various energy scales. Finally, we will discuss urgent open questions and technical challenges to be solved in this emerging field.

quant-ph↗

arXiv2025

Unconventional Temperature Dependence of Exciton Diamagnetism in 2D Ruddlesden-Popper Lead Halide Perovskites

Layered hybrid perovskites containing larger organic cations have demonstrated superior environmental stability, but the presence of these insulating spacers also strengthens the exciton binding energy, which contributes to reduced carrier separation. The consequences of increased binding energy on device efficiency are still not fully documented, and binding energy measurements are often conducted at cryogenic temperatures where linewidths are decreased and a series of hydrogen-like bound states can be identified, but not under ambient conditions where devices are expected to operate. In contrast to the quenching observed in 3D perovskites such as methylammonium lead iodide, where exciton binding energies are thought to decrease at higher temperatures, we present evidence for a smaller excitonic radius at higher temperatures in the $n=5$ member of butylammonium-spaced methylammonium lead iodide, (BA)$_2$(MA)$_{n-1}$Pb$_n$I$_{3n+1}$. We measured the temperature-dependent diamagnetic shift coefficient in magnetic fields up to 40\,T, which is one-third as large at room temperature as those at cryogenic temperatures. In both the ideal 2D and 3D hydrogen models, this trend would indicate that the exciton binding energy more than triples at room temperature.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz Cavity Phonon Polaritons in Lead Telluride in the Deep-Strong Coupling Regime

Lead telluride is an important thermoelectric material due to its large Seebeck coefficient combined with its unusually low thermal conductivity that is related to the strong anharmonicity of phonons in this material. Here, we have studied the resonant and nonperturbative coupling of transverse optical phonons in lead telluride with cavity photons inside small-mode-volume metallic metasurface cavities that have photonic modes with terahertz frequencies. We observed a giant vacuum Rabi splitting on the order of the bare phonon and cavity frequencies. Through terahertz time-domain spectroscopy experiments, we systematically studied the vacuum Rabi splitting as a function of sample thickness, temperature, and cavity length. Under the strongest light-matter coupling conditions, the strength of coupling exceeded the bare phonon and cavity frequencies, putting the system into the deep-strong coupling regime. These results demonstrate that this uniquely tunable platform is promising for realizing and understanding predicted cavity-vacuum-induced ferroelectric instabilities and exploring applications of light-matter coupling in the ultrastrong and deep-strong coupling regimes in quantum technology.

physics.optics↗

arXiv2024

Terahertz chiral photonic-crystal cavities for Dirac gap engineering in graphene

Strong coupling between matter and vacuum electromagnetic fields in a cavity can induce novel quantum phases in thermal equilibrium via symmetry breaking. Particularly intriguing is the coupling with circularly polarized cavity fields, which can break time-reversal symmetry (TRS) and lead to topological bands. This has spurred significant interest in developing chiral cavities that feature broken TRS, especially in the terahertz (THz) frequency range, where various large-oscillator-strength resonances exist. Here, we present a design for high-quality-factor THz chiral photonic-crystal cavities (PCCs) that achieves broken TRS using a magnetoplasma in a lightly doped semiconductor. We incorporate ab initio density functional theory calculations into the derived microscopic model, allowing a realistic estimate of the vacuum-induced gap in graphene when coupled to our chiral cavity. Our calculations show an enhancement in the light-matter interaction due to Dirac nodes and predict an energy gap on the order of 1 meV. The THz chiral PCCs offer a promising platform for exploring cavity-dressed condensed matter with broken TRS.

physics.optics↗

arXiv2024

Zeeman polaritons as a platform for probing Dicke physics in condensed matter

The interaction of an ensemble of two-level atoms and a quantized electromagnetic field, described by the Dicke Hamiltonian, is an extensively studied problem in quantum optics. However, experimental efforts to explore similar physics in condensed matter typically employ bosonic matter modes (e.g., phonons, magnons, and plasmons) that are describable as simple harmonic oscillators, i.e., an infinite ladder of equally spaced energy levels. Here, we examine ultrastrong coupling between a coherent light mode and an ensemble of paramagnetic spins, a finite-multilevel system, in Gd$_3$Ga$_5$O$_{12}$. The electron paramagnetic resonance of Gd$^{3+}$ ions is tuned by a magnetic field into resonance with a Fabry--P\'erot cavity mode, resulting in the formation of spin--photon hybrid states, or Zeeman polaritons. We observe that the light--matter coupling strength, measured through the vacuum Rabi splitting, decreases with increasing temperature, which can be explained by the temperature-dependent population difference between the lower and higher-energy states, a trait of a finite-level system. This finding demonstrates that a spin--boson system is more compatible with the Dicke model and has advantages over boson--boson systems for pursuing experimental realizations of phenomena predicted for ultrastrongly coupled light--matter hybrids.

quant-ph↗

arXiv2024

Multimode Phonon-Polaritons in Lead-Halide Perovskites in the Ultrastrong Coupling Regime

Phonons play a central role in fundamental solid-state phenomena, including superconductivity, Raman scattering, and symmetry-breaking phases. Harnessing phonons to control these effects and enable quantum technologies is therefore of great interest. However, most existing phonon control strategies rely on external driving fields or anharmonic interactions, limiting their applicability. Here, we realize multimode ultrastrong light--matter coupling and theoretically show the modulation of phonon emission. This regime is realized by coupling two optical phonon modes in lead halide perovskites to a nanoslot array functioning as a single-mode cavity. The small mode volume of the nanoslots enables high coupling strengths in the phonon-polariton system. We show theoretically that the nanoslot resonator mediates an effective interaction between phonon modes, leading to superthermal phonon bunching in thermal equilibrium between distinct modes. Our findings are well described by a multimode Hopfield model. This work establishes a pathway for engineering phononic properties for light-harvesting and light-emitting technologies.

quant-ph↗

arXiv2024

Chip-Scale Aligned Chiral Carbon Nanotubes Exhibiting Giant Second Harmonic Generation

Chiral carbon nanotubes (CNTs) are direct-gap semiconductors with optical properties governed by one-dimensional excitons with enormous oscillator strengths. Each species of chiral CNTs has an enantiomeric pair of left- and right-handed CNTs with nearly identical properties, but enantiomer-dependent phenomena can emerge, especially in nonlinear optical processes. Theoretical studies have predicted strong second-order nonlinearities in chiral CNTs, but no experimental quantitative verification has been reported due to the lack of macroscopically ordered assemblies of single-enantiomer chiral CNTs. Here, we report the synthesis of centimeter-scale, densely packed, aligned single-enantiomer chiral CNT films that are microfabrication-compatible. We observe giant second harmonic generation (SHG) emission from the chiral CNT film, which originates from the intrinsic chirality and inversion symmetry breaking of the atomic structure of chiral CNTs. The observed nonlinear susceptibility of the as-fabricated film reaches $4.9\times 10^2$\,pm/V at a pump wavelength of 1030\,nm, corresponding to the lowest-energy excitonic resonance, indicating $\chi_{xyz} = 1.6\times 10^3$\,pm/V for a perfectly aligned CNT crystal. Our calculations based on many-body theory correctly estimate the spectrum and magnitude of such excitonically enhanced optical nonlinearity. These results are promising for the development of scalable chiral-CNT electronics and nonlinear photonics.

physics.app-ph↗

arXiv2024

Graphene Terahertz Devices for Sensing and Communication

Graphene-based terahertz (THz) devices have emerged as promising platforms for a variety of applications, leveraging graphene's unique optoelectronic properties. This review explores recent advancements in utilizing graphene in THz technology, focusing on two main aspects: THz molecular sensing and THz wave modulation. In molecular sensing, the environment-sensitive THz transmission and emission properties of graphene are utilized for enabling molecular adsorption detection and biomolecular sensing. This capability holds significant potential, from the detection of pesticides to DNA at high sensitivity and selectivity. In THz wave modulation, crucial for next-generation wireless communication systems, graphene demonstrates remarkable potential in absorption modulation when gated. Novel device structures, spectroscopic systems, and metasurface architectures have enabled enhanced absorption and wave modulation. Furthermore, techniques such as spatial phase modulation and polarization manipulation have been explored. From sensing to communication, graphene-based THz devices present a wide array of opportunities for future research and development. Finally, advancements in sensing techniques not only enhance biomolecular analysis but also contribute to optimizing graphene's properties for communication by enabling efficient modulation of electromagnetic waves. Conversely, developments in communication strategies inform and enhance sensing capabilities, establishing a mutually beneficial relationship.

physics.optics↗

arXiv2023

Macroscopically Self-Aligned and Chiralized Carbon Nanotubes: From Filtration to Innovation

Because of their natural one-dimensional (1D) structure combined with intricate chiral variations, carbon nanotubes (CNTs) exhibit various exceptional physical properties, such as ultrahigh electrical and thermal conductivity, exceptional mechanical strength, and chirality-dependent metallicity. These properties make CNTs highly promising for diverse applications, including field-effect transistors, sensors, photodetectors, and thermoelectric devices. While CNTs excel individually at the nanoscale, their 1D and chiral nature can be lost on a macroscopic scale when they are randomly assembled. Therefore, the alignment and organization of CNTs in macroscopic structures is crucial for harnessing their full potential. In this review, we explore recent advancements in understanding CNT alignment mechanisms, improving CNT aligning methods, and demonstrating macroscopically 1D properties of ordered CNT assemblies. We also focus on a newly discovered class of CNT architectures, combining CNT alignment and twisting mechanisms to create artificial radial and chiral CNT films at wafer scales. Finally, we summarize recent developments related to aligned and chiral CNT films in optoelectronics, highlighting their unique roles in solar cells, thermal emitters, and optical modulators.

physics.app-ph↗