arXiv subjects

A. Pimenov

Publications and source records attributed to A. Pimenov.

At least 19 recordsLinked to original sources

arXiv2026

Terahertz optical activity near crystal field transitions of Tm3+ ions in magnetoelectric alumoborates

Crystal field (CF) excitations in the ground multiplet $^3H_6$ of Tm$^{3+}$ ions were investigated using terahertz transmission spectra of magnetoelectric TmAl$_3$(BO$_3$)$_4$ and Tm$_{0.05}$Yb$_{0.1}$Y$_{0.85}$Al$_3$(BO$_3$)$_4$. These excitations were identified as mainly magnetic dipole transitions from the ground singlet A$_1$ to the next excited doublet E, split by the crystal field of the D$_3$ symmetry. The fine structure of the modes was resolved at low temperatures. It manifested differently in lightly doped and in pure Tm borates, consistent with different distortions of the local crystal field with the D$_3$ symmetry. Strong natural optical activity was observed near the CF transitions resulting in a polarization plane rotation up to 25 degrees. The optical activity is quantitatively described by contributions of magnetic and electric dipole transitions to dynamic magnetoelectric susceptibility and taking into account the classification of local distortions.

cond-mat.mtrl-sci↗

arXiv2025

Discovery of Slot Plasma Excitations in a AlGaN/GaN Plasmonic Crystal

We experimentally investigate the terahertz spectrum of plasma excitations in a plasmonic crystal based on AlGaN/GaN two-dimensional electron system (2DES). While screened plasmon modes with linear dispersion are readily observed in the plasmonic crystals, the existence of unscreened modes localized in the slots between the gates has remained unobserved until now. We discover this slot plasma excitation exhibiting square-root dispersion. It turned out that these slot plasmons follow an unconventional wave-vector quantization rule, $q_u=(N + 1/4) \times \pi/l_u$ for even integers $N$, and require the condition for excitation $q_u h \ll 1$, where $h$ is the gate-to-2DES distance and $l_u$ is the slot width. We develop an analytical model that accurately captures the found dispersion and relaxation, revealing a non-trivial $-\pi/4$ phase shift upon plasmon reflection at the gate edge. Experiments demonstrate that the slot plasmons persist up to room temperature, thereby enabling a broad range of opportunities for the advancement of plasmonic devices.

cond-mat.mes-hall↗

arXiv2025

Novel SuperLattice Plasmon Mode in a Grating of 2D Electron Strips

We investigate GaAs/AlGaAs heterostructure membranes with a metasurface made up of a grating of two-dimensional electron system (2DES) strips. Experiments have revealed a strong plasma resonance in the transmission of the metasurface. We have found that a collective effect from the superlattice, along with lateral screening between the strips, leads to the emergence of a new plasmon mode in the metasurface under study. Furthermore, we develop an analytical approach that accurately describes the behavior of the discovered superlattice plasmon mode, providing new insights into the fundamental physics of plasmonic metasurface systems.

cond-mat.mes-hall↗

arXiv2025

On the Nature of the Fundamental Plasma Excitation in a Plasmonic Crystal

We report on the experimental study of the spectrum of plasma excitations in a plasmonic crystal fabricated from the two-dimensional electron system in an AlGaAs/GaAs semiconductor heterostructure. We perform a comprehensive research on the mode frequency and relaxation as a function of the gate width across different plasmonic crystal periods. Importantly, we develop an analytical approach that accurately describes the behavior of plasma excitations in plasmonic crystals, providing new insights into the fundamental physics of plasmonic systems.

cond-mat.mes-hall↗

arXiv2025

Universal transparency and fine band structure near the Dirac point in HgTe quantum wells

Spin-orbit coupling in thin HgTe quantum wells results in a relativistic-like electron band structure, making it a versatile solid state platform to observe and control non-trivial electrodynamic phenomena. Here we report an observation of universal terahertz (THz) transparency determined by fine-structure constant $\alpha \approx 1/137$ in 6.5 nm-thick HgTe layer, close to the critical thickness separating phases with topologically different electronic band structure. Using THz spectroscopy in magnetic field we obtain direct evidence of asymmetric spin splitting of the Dirac cone. This particle-hole asymmetry facilitates optical control of edge spin currents in the quantum wells.

cond-mat.mes-hall↗

arXiv2024

Two-dimensional topological Anderson insulator in a HgTe-based semimetal

We report the experimental observation of Anderson localization in two-dimensional (2D) electrons and holes in the bulk of HgTe quantum wells with a semimetallic spectrum and under strong disorder. In contrast, the one-dimensional (1D) edge channels, arising from the spectrum's inversion, demonstrate remarkable robustness against disorder due to topological protection. Strong disorder induces a mobility gap in the bulk, enabling access to the 1D edge states and thereby realizing the two-dimensional topological Anderson insulator (TAI) state. Nonlocal transport measurements confirm the emergence of topologically protected edge channels. The TAI state appears to be very sensitive to an external magnetic field applied perpendicular to the sample. Firstly, a small magnetic field of 30mT breaks the topological protection of 1D edge channels, thus turning the system into an ordinary Anderson insulator. Secondly, the magnetic field of 0.5T delocalizes 2D bulk electrons, transforming the system into a quantum Hall liquid.

cond-mat.mes-hall↗

arXiv2024

Quantum Hall effect and zero plateau in bulk HgTe

The quantum Hall effect, which exhibits a number of unusual properties, is studied in a gated 1000-nm-thick HgTe film, nominally a three-dimensional system. A weak zero plateau of Hall resistance, accompanied by a relatively small value of Rxx of the order of h/e^2, is found around the point of charge neutrality. It is shown that the zero plateau is formed by the counter-propagating chiral electron-hole edge channels, the scattering between which is suppressed. So, phenomenologically, the quantum spin Hall effect is reproduced, but with preserved ballisticity on macroscopic scales (larger than 1mm). It is shown that the formation of the QHE occurs in a two-dimensional (2D) accumulation layer near the gate, while the bulk carriers play the role of an electron reservoir. Due to the exchange of carriers between the reservoir and the 2D layer, an anomalous scaling of the QHE is observed not with respect to the CNP, but with respect to the first electron plateau.

cond-mat.mes-hall↗

arXiv2024

The effect of local distortions on magnetic and magnetoelectric properties of paramagnetic Pr3Ga5SiO14 langasite

Magnetic field-induced electric polarization has been observed in trigonal non-centrosymmetric paramagnetic Pr3Ga5SiO14 langasite. We detected quadratic electric polarization along the a-axis in the basal ab plane for various magnetic-field orientations. Electric polarization along the c-axis is only evident starting from the fourth power of magnetic field, in accordance with the trigonal symmetry. The magnetic properties of Pr3Ga5SiO14 primarily stem from the local anisotropic magnetic moment of the two lowest Pr3+ singlets (quasi-doublet) in the crystal electric field. The random distribution of Ga/Si in the 2d positions leads to a local distortion of C2 symmetry and to a splitting distribution of the quasi-doublet. By considering the interactions of local moments among different Pr3+ positions within a phenomenological approach for the allowed magnetoelectric coupling, we derive the electric polarization in terms of symmetry-allowed combinations of local magnetic susceptibilities and field components. The magnetic field dependence of electric polarization in the basal plane, P_{a,b*}, is mainly determined by the accumulation of effective local susceptibilities, exhibiting similar behavior in low fields, while polarization along the c-axis, P_c, arises from the non-equivalence of local effective magnetic susceptibilities in different Pr3+ positions. Our findings suggest that the temperature dependencies of magnetic and magnetoelectric susceptibilities are highly sensitive to the distribution of the quasi-doublet splitting, which reflects the local symmetry breaking.

cond-mat.mtrl-sci↗

arXiv2024

Resolving the local distortions of Ising-like moments in magnetoelectric Ho-doped langasite

The magnetic properties of Ho-doped langasites (La:Ho)$_3$Ga$_5$SiO$_{14}$ are dominated by the Ising-like magnetic moments of the Ho$^{3+}$ ions. In their saturated regime, the induced magnetic state breaks both time and space inversion symmetries, leading to a novel linear magnetoelectric effect. However, due to distortions induced by a shared Ga/Si occupancy of the 2d sites, resolving the microscopic nature of the magnetic configuration remains a difficult task. Here we combine polarized neutron diffraction and angular dependent magnetization experiments to determine the local distortions of the Ho$^{3+}$ magnetic moments in doped langasites (La$_{1-x}$Ho$_x$)$_3$Ga$_5$SiO$_{14}$ with $x \approx 0.015$ and $x \approx 0.045$. We propose a model for a field-induced magnetic configuration with arbitrary orientations of the local Ising axis of Ho$^{3+}$ in distorted positions. The operations of broken local $C_2$ symmetry and rotations around the trigonal $C_3$ axis connect different sites, restoring the global P321 symmetry of the crystal and simplifying the description of the magnetic properties. The superposition of two distorted Ho$^{3+}$ positions connected by $C_2$ symmetry determines the local magnetic susceptibility tensor, which no longer appears Ising-like at low fields.

cond-mat.mtrl-sci↗

arXiv2024

Optical realization of magneto-intersubband oscillations

We report on the optical realization of the magneto-intersubband oscillations that have been measured in the sub-terahertz transmittance of a GaAs quantum well with two subbands occupied. Following their dc analogue, the oscillations are periodic in the inverse magnetic field with the period governed by the subband gap. Their magnitude and polarization dependence accurately follow the presented simplified version of the dynamic magneto-intersubband oscillations equation that naturally combines dc magneto-intersabband oscillations with microwave-induced resistance oscillations (MIRO). Simultaneously measured photoresistance also reveals its strong sensitivity to the sign of the circular polarization, proving the used theoretical modeling.

cond-mat.mes-hall↗

arXiv2024

Optical Shubnikov - de Haas oscillations in 2D electron systems

We report on dynamic Shubnikov - de Haas (SdH) oscillations that are measured in the optical response, sub - terahertz transmittance of two-dimensional systems, and reveal two distinct types of oscillation nodes: "universal" nodes at integer ratios of radiation and cyclotron frequencies and "tunable" nodes at positions sensitive to all parameters of the structure. The nodes in both real and imaginary parts of the measured complex transmittance are analyzed using a dynamic version of the static Lifshitz-Kosevich formula. These results demonstrate that the node structure of the dynamic SdH oscillations provides an all-optical access to quantization- and interaction-induced renormalization effects, in addition to parameters one can obtain from the static SdH oscillations.

cond-mat.mes-hall↗

arXiv2022

Demonstration of high sensitivity of microwave-induced resistance oscillations to circular polarization

We demonstrate that long-debated immunity of microwave-induced resistance oscillations (MIRO) to the sense of circular polarization is not a generic property of this phenomenon in solid-state two-dimensional electron systems. Using a large-area GaAs-based heterostructure we detect up to 30 times larger MIRO signal for the cyclotron resonance (CR) active helicity, fully consistent with the concurrently measured transmission and the deduced CR shape of the Drude absorption. We further elaborate conditions to avoid extrinsic factors capable of producing an apparent immunity of the photoresponse.

cond-mat.mes-hall↗

arXiv2020

Magnetic equivalent of electric superradiance: radiative damping in yttrium-iron-garnet films

A dense system of independent oscillators, connected only by their interaction with the same cavity excitation mode, will radiate coherently, which effect is termed superradiance. In several cases, especially if the density of oscillators is high, the superradiance may dominate the intrinsic relaxation processes. This limit can be achieved, e.g., with cyclotron resonance in two-dimensional electron gases. In those experiments, the cyclotron resonance is coupled to the electric field of light, while the oscillator density can be easily controlled by varying the gate voltage. However, in the case of magnetic oscillators, to achieve the dominance of superradiance is more tricky, as material parameters limit the oscillator density, and the magnetic coupling to the light wave is rather small. Here we present quasi-optical magnetic resonance experiments on thin films of yttrium iron garnet. Due to the simplicity of experimental geometry, the intrinsic damping and the superradiance can be easily separated in the transmission spectra. We show that with increasing film thickness, the losses due to coherent radiation prevail the system's internal broadening.

cond-mat.mtrl-sci↗

arXiv2020

Band structure of a HgTe-based three-dimensional topological insulator

From the analysis of the cyclotron resonance, we experimentally obtain the band structure of the three-dimensional topological insulator based on a HgTe thin film. Top gating was used to shift the Fermi level in the film, allowing us to detect separate resonance modes corresponding to the surface states at two opposite film interfaces, the bulk conduction band, and the valence band. The experimental band structure agrees reasonably well with the predictions of the $\mathbf{k\cdot p}$ model. Due to the strong hybridization of the surface and bulk bands, the dispersion of the surface states is close to parabolic in the broad range of the electron energies.

cond-mat.mes-hall↗

arXiv2020

Magnetic anisotropy and exchange paths for octa- and tetrahedrally coordinated Mn$^{2+}$ ions in the honeycomb multiferroic Mn$_2$Mo$_3$O$_8$

We investigated the static and dynamic magnetic properties of the polar ferrimagnet Mn$_2$Mo$_3$O$_8$ in three magnetically ordered phases via magnetization, magnetic torque, and THz absorption spectroscopy measurements. The observed magnetic field dependence of the spin-wave resonances, including Brillouin zone-center and zone-boundary excitations, magnetization, and torque, are well described by an extended two-sublattice antiferromagnetic classical mean-field model. In this orbitally quenched system, the competing weak easy-plane and easy-axis single-ion anisotropies of the two crystallographic sites are determined from the model and assigned to the tetra- and octahedral sites, respectively, by ab initio calculations.

cond-mat.str-el↗

arXiv2020

Observation of High Harmonics of the Cyclotron Resonance in Microwave Transmission of a High-Mobility Two-Dimensional Electron System

We report an observation of magnetooscillations of the microwave power transmitted through the high mobility two-dimensional electron system hosted by a GaAs quantum well. The oscillations reflect an enhanced absorption of radiation at high harmonics of the cyclotron resonance and follow simultaneously measured microwave-induced resistance oscillations (MIRO) in the dc transport. While the relative amplitude (up to 1%) of the transmittance oscillations appears to be small, they represent a significant (>50%) modulation of the absorption coefficient. The analysis of obtained results demonstrates that the low-B decay, magnitude, and polarization dependence of the transmittance oscillations accurately follow the theory describing photon-assisted scattering between distant disorder-broadened Landau levels. The extracted sample parameters reasonably well describe the concurrently measured MIRO. Our results provide an insight into the MIRO polarization immunity problem and pave the way to probe diverse high-frequency transport properties of high-mobility systems using precise transmission measurements.

cond-mat.mes-hall↗

arXiv2020

Unusual magnetoelectric effect in paramagnetic rare-earth langasite

Violation of time reversal and spatial inversion symmetries has profound consequences for elementary particles and cosmology. Spontaneous breaking of these symmetries at phase transitions gives rise to unconventional physical phenomena in condensed matter systems, such as ferroelectricity induced by magnetic spirals, electromagnons, non-reciprocal propagation of light and spin waves, and the linear magnetoelectric (ME) effect - the electric polarization proportional to the applied magnetic field and the magnetization induced by the electric field. Here, we report the experimental study of the holmium-doped langasite, Ho$_{x}$La$_{3-x}$Ga$_5$SiO$_{14}$, showing a puzzling combination of linear and highly non-linear ME responses in the disordered paramagnetic state: its electric polarization grows linearly with the magnetic field but oscillates many times upon rotation of the magnetic field vector. We propose a simple phenomenological Hamiltonian describing this unusual behavior and derive it microscopically using the coupling of magnetic multipoles of the rare-earth ions to the electric field.

cond-mat.mtrl-sci↗

arXiv2020

Bifurcation structure of a swept source laser

We numerically analyze a delay differential equation model of a short-cavity semiconductor laser with an intracavity frequency swept filter and reveal a complex bifurcation structure responsible for the asymmetry of the output characteristics of this laser. We show that depending on the direction of the frequency sweep of a narrowband filter, there exist two bursting cycles determined by different parts of a continuous-wave solutions branch.

physics.optics↗