arXiv subjects

Kirill I. Bolotin

Publications and source records attributed to Kirill I. Bolotin.

At least 19 recordsLinked to original sources

arXiv2026

Tunable Fano Resonance and Frequency Locking in a Graphene-SiNx Hybrid Nanomechanical Resonator

Fano resonances, arising from the interference between discrete and continuum states, are observed across a wide range of quantum and classical systems. Here, we report the experimental observation of Fano resonances in a graphene SiNx hybrid nanomechanical system modeled as coupled oscillators. The broad, low quality factor graphene mode plays the role of the continuum, while the dense comb of sharp, high quality factor SiNx modes provides the discrete states. The inter-mode detuning is tunable via a DC gate voltage, enabling dynamic control of the Fano resonance: we demonstrate gate controlled switching of both the sign and the magnitude of the Fano asymmetry parameter $q$, in quantitative agreement with a coupled oscillator theory that predicts $q=-\cotϕ$, with $ϕ$ the phase of the continuum response. At strong drive, the graphene mode enters the Duffing regime and its jump-down frequency locks to successive SiNx modes, producing a staircase of drive insensitive frequency plateaus; a weak seeding tone deterministically switches the resonator between adjacent locked states. The dense SiNx mode thus acts, in the linear regime, as the discrete states of a tunable Fano interferometer and, in the nonlinear regime, as a frequency ruler that stabilizes and quantizes the graphene oscillation. This platform offers a controllable mechanical realization of Fano interference and opens new avenues for high resolution hybrid resonant sensors and stable nanomechanical frequency references.

cond-mat.mes-hall↗

arXiv2026

Tunable Nonlinear Landscapes in Graphene Nanoelectromechanical Systems

Nonlinear nanomechanical resonators give convenient solid-state access to classical analogs of extreme nonlinear optics and to phononic signal processing. Here we report integer high-harmonic generation and phononic frequency combs in a suspended monolayer graphene drum. A gate voltage breaks the out-of-plane symmetry of the membrane and tunes its fundamental flexural mode onto a 1:2 internal resonance with a higher mode at twice the frequency, where the quadratic coupling between the two modes becomes large. A single drive tone then generates phase-locked integer harmonics in sequence, and at larger drive these fill in to a dense frequency comb. Raising the drive further, we find a reverse period-doubling transition: the comb spacing doubles, the line density halves, and energy flows back into the even-order comb lines. The measured spectra yield the quadratic ($ζ$) and cubic ($β$) nonlinear coefficients of the membrane. These results show how the tunable nonlinear landscape of graphene supports distinct dynamical regimes on demand, allowing a single gated device to act in turn as a frequency multiplier, a broadband comb source, and a chaotic generator.

cond-mat.mes-hall↗

arXiv2026

The Topography Trap: Sifting Interlayer Excitons from Strain-Related Artifacts in Real-World 2D Hetrostructures

Novel excitonic phenomena emerging in transition metal dichalcogenide (TMDC) heterostructures belong to the most exciting topics in contemporary physics of van der Waals materials. Interlayer excitons (IXs) stand out among those due to their long radiative lifetimes and tunability by electric fields, strain, and twist angle. However, many ambiguities persist in the optical identification and manipulation of IXs, highlighting the need for reliable spectroscopic criteria that distinguish interlayer species from spurious signals. Here, we present a decision-tree protocol that evaluates interlayer coupling via intralayer exciton quenching and correlates photoluminescence (PL) with atomic force microscopy (AFM) to correctly assign room-temperature PL features in TMDC-based heterostructures. Applying this protocol, we identify momentum-direct IX between the K valleys of the two layers (KK-IX) in MoS2-MoSe2 and MoS2-WSe2 heterostructures at room temperature. In contrast, our protocol contests the reported bright, momentum-indirect, twist-angle-independent $Γ$K-IX in MoS2-WSe2. Comprehensive experimental data, including infrared and tip-enhanced photoluminescence (TEPL) with sub-diffraction-limited resolution, show no compelling evidence for this excitonic species, despite numerous reports. Instead, the spectroscopic features previously assigned to this $Γ$K-IX originate from locally strained WSe2 at topographical inhomogeneities of the heterostructure interface, underscoring the need for robust, spatially resolved characterization of real-world samples in this highly accessible field and providing a generally applicable framework for identifying interlayer excitons in 2D semiconductor heterostructures.

cond-mat.mes-hall↗

arXiv2025

Tunable magnons in a dual-gated 2D antiferromagnet

The layered antiferromagnet CrSBr features magnons coupled to other quasiparticles, including excitons and polaritons, enabling their easy optical accessibility. In this work, we investigate the tunability of magnons in few-layered devices in response to changes in carrier density and the application of a perpendicular electric field. We demonstrate an on-chip tunability of the in- and out-of-phase magnon frequencies by up to 2 GHz. While the frequencies of both modes increase with the electron density, we observe an asymmetric response with respect to the electric field in a dual-gated trilayer device. To understand the mechanism of this disparity, we develop a layer-resolved macrospin model describing the magnetic dynamics in thin, non-uniformly doped devices. Through this model we establish the doping- and electric-field-dependence of the exchange interaction, magnetic anisotropy, and magnetic moment of individual layers. Our results advance the applications of gate-tunable magnonic devices based on 2D materials.

cond-mat.mes-hall↗

arXiv2025

Observation of tunable discrete time crystalline phases

Discrete time crystals (DTCs) are emergent non-equilibrium phases of periodically driven many-body systems, with potential applications ranging from quantum computing to sensing and metrology. There has been significant recent interest in understanding mechanisms leading to DTC formation and a search for novel DTC phases beyond subharmonic entrainment. Here, we report observation of multiple DTC phases in a nanoelectromechanical system (NEMS) based on coupled graphene and silicon nitride membranes. We confirm the time-crystalline nature of these symmetry broken phases by establishing their many-body characters, long-range time and spatial order, and rigidity against parameter fluctuation or noise. Furthermore, we employ controlled mechanical strain to drive the transitions between phases with different symmetries, thereby mapping the emergent time-crystalline phase diagram. Overall, our work takes a step towards establishing time crystals as a system with complexity rivaling that of solid state crystals.

cond-mat.mes-hall↗

arXiv2025

Strain engineering of valley-polarized hybrid excitons in a 2D semiconductor

Encoding and manipulating digital information in quantum degrees of freedom is one of the major challenges of today's science and technology. The valley indices of excitons in transition metal dichalcogenides (TMDs) are well-suited to address this challenge. Here, we demonstrate a new class of strain-tunable, valley-polarized hybrid excitons in monolayer TMDs, comprising a pair of energy-resonant intra- and intervalley excitons. These states combine the advantages of bright intravalley excitons, where the valley index directly couples to light polarization, and dark intervalley excitons, characterized by low depolarization rates. We demonstrate that the hybridized state of dark KK' intervalley and defect-localized excitons exhibits a degree of circular polarization of emitted photons that is three times higher than that of the constituent species. Moreover, a bright KK intravalley and a dark KQ exciton form a coherently coupled hybrid state under energetic resonance, with their valley depolarization dynamics slowed down a hundredfold. Overall, these valley-polarized hybrid excitons with strain-tunable valley character emerge as prime candidates for valleytronic applications in future quantum and information technology.

cond-mat.mes-hall↗

arXiv2025

Strain activation of localized states in WSe2

Single-photon emission centers generated by controlled atomic force microscopy (AFM) indentation in monolayer WSe$_2$ on a flexible polymer substrate are explored for applications in quantum technologies. Here, we study the response of these emitters to the polymer substrate's strain state, which is controlled by selecting the indentation force and by gradually thermally annealing the samples. In the indented areas, we observe sharp new photoluminescence (PL) peaks in the regions 1.62--1.68 eV and 1.70--1.73 eV, characterized by sublinear power dependence and spectral wandering. We find that these peaks arise only when the indentation force exceeds a few $μ$N and generally redshift as the applied force increases. Conversely, after thermal annealing ($T < 60^{\circ}$C), WSe$_2$ experiences strain relaxation, leading to a blueshift of the peaks' spectral position and their ultimate disappearance. Our analysis of the peaks' positions vs. strain allows us to draw several conclusions about the nature of these emission. Specifically, we elucidate the roles of excitonic confinement and hybridization between free excitons and defect-related states, a process activated by the strain level. Overall, our approach suggests that the energy of localized emitters may be controlled via strain engineering.

physics.app-ph↗

arXiv2024

Excitons under large pseudomagnetic fields

Excitons in Transition Metal Dichalcogenides (TMDs) acquire a spin-like quantum number, a pseudospin, originating from the crystal's discrete rotational symmetry. Here, we break this symmetry using a tunable uniaxial strain, effectively generating a pseudomagnetic field exceeding 40 Tesla. Under this large field, we demonstrate pseudospin analogs of spintronic phenomena such as the Zeeman effect and Larmor precession. Moreover, we determine previously inaccessible fundamental properties of TMDs, including the strength of the depolarizing field responsible for the loss of exciton coherence. Finally, we uncover the bosonic -- as opposed to fermionic -- nature of many-body excitonic species using the pseudomagnetic equivalent of the $g$-factor spectroscopy. Our work is the first step toward establishing this spectroscopy as a universal method for probing correlated many-body states and realizing pseudospin analogs of spintronic devices.

cond-mat.mes-hall↗

arXiv2024

Ultrafast Optical Control of Rashba Interactions in a TMDC Heterostructure

We investigate spin relaxation dynamics of interlayer excitons in a MoSe2/MoS2 heterostructure induced by the Rashba effect. In such a system, Rashba interactions arise from an out-of-plane electric field due to photo-generated interlayer excitons inducing a phonon-assisted intravalley spin relaxation. We develop a theoretical description based on a microscopic approach to quantify the magnitude of Rashba interactions and test these predictions via time-resolved Kerr rotation measurements. In agreement with the calculations, we find that the Rashba-induced intravalley spin mixing becomes the dominating spin relaxation channel above T = 50 K. Our work identifies a previously unexplored spin-depolarization channel in heterostructures which can be used for ultrafast spin manipulation.

cond-mat.mes-hall↗

arXiv2023

Towards tunable graphene phononic crystals

Phononic crystals (PnCs) are artificially patterned media exhibiting bands of allowed and forbidden zones for phonons. Many emerging applications of PnCs from solid-state simulators to quantum memories could benefit from the on-demand tunability of the phononic band structure. Here, we demonstrate the fabrication of suspended graphene PnCs in which the phononic band structure is controlled by mechanical tension applied electrostatically. We show signatures of a mechanically tunable phononic band gap. The experimental data supported by simulation suggest a phononic band gap at 28$-$33 MHz in equilibrium, which upshifts by 9 MHz under a mechanical tension of 3.1 Nm$^{-1}$. This is an essential step towards tunable phononics paving the way for more experiments on phononic systems based on 2D materials.

cond-mat.mes-hall↗

arXiv2023

Mechanically-tunable bandgap closing in 2D graphene phononic crystals

We present a tunable phononic crystal which undergoes a phase transition from mechanically insulating to mechanically transmissive (metallic). Specifically, in our simulations for a phononic lattice under biaxial tension ($σ_{xx} =σ_{yy}$ = 0.01 N$/$m), we find a bandgap for out-of-plane phonons in the range of 48.8 - 56.4 MHz, which we can close by increasing the degree of tension uniaxiality ($σ_{xx} / σ_{yy}$) to 1.7. To manipulate the tension distribution, we design a realistic device of finite size, where $σ_{xx} / σ_{yy}$ is tuned by applying a gate voltage to a phononic crystal made from suspended graphene. We show that the phase transition can be probed via acoustic transmission measurements and that the phononic bandgap persists even after the inclusion surface contaminants and random tension variations present in realistic devices. The proposed system acts as a transistor for phonons with an on/off ratio of $10^5$ (100 dB suppression) and is thus a valuable extension for phonon logic applications. In addition, this mechanical analogue to a metal-insulator transition (mMIT) allows tunable coupling between mechanical entities (e.g. mechanical qubits).

cond-mat.mes-hall↗

arXiv2023

Nanomechanical absorption spectroscopy of 2D materials with femtowatt sensitivity

Nanomechanical spectroscopy (NMS) is a recently developed approach to determine optical absorption spectra of nanoscale materials via mechanical measurements. It is based on measuring changes in the resonance frequency of a membrane resonator vs. the photon energy of incoming light. This method is a direct measurement of absorption, which has practical advantages compared to common optical spectroscopy approaches. In the case of two-dimensional (2D) materials, NMS overcomes limitations inherent to conventional optical methods, such as the complications associated with measurements at high magnetic fields and low temperatures. In this work, we develop a protocol for NMS of 2D materials that yields two orders of magnitude improved sensitivity compared to previous approaches, while being simpler to use. To this end, we use electrical sample actuation, which simplifies the experiment and provides a reliable calibration for greater accuracy. Additionally, the use of low-stress silicon nitride membranes as our substrate reduces the noise-equivalent power to $NEP = 890 fW/\sqrt{Hz}$, comparable to commercial semiconductor photodetectors. We use our approach to spectroscopically characterize a two-dimensional transition metal dichalcogenide (WS$_2$), a layered magnetic semiconductor (CrPS$_4$), and a plasmonic supercrystal consisting of gold nanoparticles.

cond-mat.mes-hall↗

arXiv2023

Impact of dark excitons on Förster type resonant energy transfer between dye molecules and atomically thin semiconductors

Interfaces of dye molecules and two-dimensional transition metal dichalcogenides (TMDCs) combine strong molecular dipole excitations with high carrier mobilities in semiconductors. Förster type energy transfer is one key mechanism for the coupling between both constituents. We report microscopic calculations of a spectrally resolved Förster induced transition rate from dye molecules to a TMDC layer. Our approach is based on microscopic Bloch equations which are solved self-consistently together with Maxwells equations. This approach allows to incorporate the dielectric environment of a TMDC semiconductor, sandwiched between donor molecules and a substrate. Our analysis reveals transfer rates in the meV range for typical dye molecules in closely stacked structures, with a non-trivial dependence of the Förster rate on the molecular transition energy resulting from unique signatures of dark, momentum forbidden TMDC excitons.

cond-mat.mes-hall↗

arXiv2022

Nanomechanical spectroscopy of 2D materials

We introduce a nanomechanical platform for fast and sensitive measurements of the spectrally-resolved optical dielectric function of 2D materials. At the heart of our approach is a suspended 2D material integrated into a nanomechanical resonator illuminated by a wavelength-tunable laser source. From the heating-related frequency shift of the resonator as well as its optical reflection measured as a function of photon energy, we obtain the real and imaginary parts of the dielectric function. Our measurements are unaffected by substrate-related screening and do not require any assumptions on the underling optical constants. This fast ($τ_{rise}$ $\sim$ 135 ns), sensitive (noise-equivalent power = 90 $\frac{pW}{\sqrt{Hz}}$ ), and broadband (1.2 $-$ 3.1 eV, extendable to UV-THz) method provides an attractive alternative to spectroscopic or ellipsometric characterisation techniques.

cond-mat.mes-hall↗

arXiv2022

High power density energy harvesting devices based on the anomalous Nernst effect of Co/Pt magnetic multilayers

The anomalous Nernst effect (ANE) is a thermomagnetic phenomenon with potential applications in thermal energy harvesting. While many recent works studied the approaches to increase the ANE coefficient of materials, relatively little effort was devoted to increasing the power supplied by the effect. Here we demonstrate a nanofabricated device with record power density generated by the ANE. To accomplish this, we fabricate micrometer-sized devices in which the thermal gradient is three orders of magnitude higher than conventional macroscopic devices. In addition, we use Co/Pt multilayers, a system characterized by a high ANE thermopower (~1 microV/K), low electrical resistivity, and perpendicular magnetic anisotropy. These innovations allow us to obtain power densities of around 13 W/cm3. We believe that this design may find uses in harvesting wasted energy in e.g. electronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Strain control of hybridization between dark and localized excitons in a 2D semiconductor

Mechanical strain is a powerful tuning knob for excitons, Coulomb-bound electron-hole complexes dominating optical properties of two-dimensional semiconductors. While the strain response of bright free excitons is broadly understood, the behavior of dark free excitons (long-lived excitations that generally do not couple to light due to spin and momentum conservation) or localized excitons related to defects remains mostly unexplored. Here, we develop a technique capable of straining pristine suspended WSe2 kept at cryogenic temperatures up to 3\% to study the strain behavior of these fragile many-body states. We find that under the application of strain, dark and localized excitons in monolayer WSe2 - a prototypical 2D semiconductor - are brought into energetic resonance, forming a new hybrid state that inherits the properties of the constituent species. The characteristics of the hybridized state, including an order-of-magnitude enhanced light/matter coupling, avoided-crossing energy shifts, and strain tunability of many-body interactions, are all supported by first-principles calculations. The hybridized exciton reported here may play a critical role in the operation of single quantum emitters based on WSe2. Furthermore, the techniques we developed may be used to fingerprint unidentified excitonic states

cond-mat.mtrl-sci↗

arXiv2022

Generating extreme electric fields in 2D materials by dual ionic gating

We demonstrate a new type of dual gate transistor to induce record electric fields through two-dimensional materials (2DMs). At the heart of this device is a 2DM suspended between two volumes of ionic liquid (IL) with independently controlled potentials. The potential difference between the ILs falls across an ultrathin layer consisting of the 2DM and the electrical double layers above and below it, thereby producing an intense electric field across the 2DM. We determine the field strength via i) electrical transport measurements and ii) direct measurements of electrochemical potentials of the ILs using semiconducting 2DM, WSe2. The field strength across the material reaches more than 3.5 V/nm, the largest static electric field through any electronic device to date. We demonstrate that this field is strong enough to close the bandgap of trilayer WSe2 driving a semiconductor-to-metal transition. Our approach grants access to previously-inaccessible phenomena occurring in ultrastrong electric fields.

physics.app-ph↗

arXiv2022

Impact of gigahertz and terahertz transport regimes on spin propagation and conversion in the antiferromagnet IrMn

Control over spin transport in antiferromagnetic systems is essential for future spintronic applications with operational speeds extending to ultrafast time scales. Here, we study the transition from the gigahertz (GHz) to terahertz (THz) regime of spin transport and spin-to-charge current conversion (S2C) in the prototypical antiferromagnet IrMn by employing spin pumping and THz spectroscopy techniques. We reveal a factor of 4 shorter characteristic propagation lengths of the spin current at THz frequencies (~ 0.5 nm) as compared to the GHz regime (~ 2 nm) which may be attributed to the ballistic and diffusive nature of electronic spin transport, respectively. The conclusion is supported by an extraction of sub-picosecond temporal dynamics of the THz spin current. We also report on a significant impact of the S2C originating from the IrMn/non-magnetic metal interface which is much more pronounced in the THz regime and opens the door for optimization of the spin control at ultrafast time scales.

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