arXiv subjects

A. Pimenov

Publications and source records attributed to A. Pimenov.

At least 55 records · Page 3Linked to original sources

arXiv2015

Giant directional birefringence in multiferroic ferroborate

Many technological applications are based on electric or magnetic order of materials, for instance magnetic memory. Multiferroics are materials which exhibit electric and magnetic order simultaneously. Due to the coupling of electric and magnetic effects, these materials show a strong potential to control electricity and magnetism and, more generally, the properties and propagation of light. One of the most fascinating and counter-intuitive recent results in multiferroics is directional anisotropy, the asymmetry of light propagation with respect to the direction of propagation. The absorption in the material can be different for forward and backward propagation of light, which in extreme case may lead to complete suppression of absorption in one direction. Another remarkable effect in multiferroics is directional birefringence, i.e. different velocities of light for different directions of propagation. In this paper, we demonstrate giant directional birefringence in a multiferroic samarium ferroborate. The effect is easily observed for linear polarization of light in the range of millimeter-wavelengths, and survives down to very low frequencies. The dispersion and absorption close to the electromagnon resonance can be controlled and fully suppressed in one direction. Therefore, samarium ferroborate is a universal tool for optical control: with a magnetic field as an external parameter it allows switching between two functionalities: polarization rotation and directional anisotropy.

cond-mat.mtrl-sci↗

arXiv2015

Order-disorder type critical behaviour at the magnetoelectric phase transition in multiferroic DyMnO$_3$

We present the results of detailed dielectric investigations of the relaxation dynamics in DyMnO$_3$ multiferroic manganite. Strong low-frequency relaxation process near the paraelectric-ferroelectric phase transition is observed. The high frequency mode is directly to the relaxational motion of multiferroic domain walls. We provide an experimental evidence that this relaxation mode corresponds to a chirality switching of the spin cycloid in DyMnO$_3$. We demonstrate that the relaxation dynamics in DyMnO$_3$ is typical for an order-disorder phase transition and may be understood within a simple model with a double well potential. DyMnO$_3$ follows an order-disorder transition scenario implicating that a short range cycloidal order of Mn-spins exists above $T_C$. These results suggest the interpretation of the paraelectric sinusoidal phase in manganites as a dynamical equilibrium of magnetic cycloids with opposite chiralities.

cond-mat.mtrl-sci↗

arXiv2014

Cavity solitons in vertical-cavity surface-emitting lasers

We investigate a control of the motion of localized structures of light by means of delay feedback in the transverse section of a broad area nonlinear optical system. The delayed feedback is found to induce a spontaneous motion of a solitary localized structure that is stationary and stable in the absence of feedback. We focus our analysis on an experimentally relevant system namely the Vertical-Cavity Surface-Emitting Laser (VCSEL). In the absence of the delay feedback we present experimental evidence of stationary localized structures in a 80 $μ$m aperture VCSEL. The spontaneous formation of localized structures takes place above the lasing threshold and under optical injection. Then, we consider the effect of the time-delayed optical feedback and investigate analytically the role of the phase of the feedback and the carrier lifetime on the self-mobility properties of the localized structures. We show that these two parameters affect strongly the space time dynamics of two-dimensional localized structures. We derive an analytical formula for the threshold associated with drift instability of localized structures and a normal form equation describing the slow time evolution of the speed of the moving structure.

nlin.PS↗

arXiv2014

Timing jitter in passively mode-locked semiconductor lasers

We study the effect of noise on the dynamics of passively mode-locked semiconductor lasers both experimentally and theoretically. A method combining analytical and numerical approaches for estimation of pulse timing jitter is proposed. We investigate how the presence of dynamical features such as wavelength bistability affects timing jitter.

physics.optics↗

arXiv2013

ESR of the quasi-two-dimensional antiferromagnet with a triangular lattice CuCrO2

Using electron-spin-resonance (ESR) technique we investigate the magnetic structure of CuCrO2, quasi-two-dimensional antiferromagnet with weakly distorted triangular lattice. Resonance frequencies and the excitation conditions in CuCrO2 at low temperatures are well described in the frame of cycloidal spin structure, defined by two susceptibilities parallel and perpendicular to the spin plane and by a biaxial crystal-field anisotropy. In agreement with the calculations, the character of the eigenmodes changes drastically at the spin-flop transition. The splitting of the observed modes can be well attributed to the resonances from different domains. The domain structure in CuCrO2 can be controlled by annealing of the sample in magnetic field.

cond-mat.str-el↗

arXiv2013

Delayed feedback control of self-mobile cavity solitons

Control of the motion of cavity solitons is one the central problems in nonlinear optical pattern formation. We report on the impact of the phase of the time-delayed optical feedback and carrier lifetime on the self-mobility of localized structures of light in broad area semiconductor cavities. We show both analytically and numerically that the feedback phase strongly affects the drift instability threshold as well as the velocity of cavity soliton motion above this threshold. In addition we demonstrate that non-instantaneous carrier response in the semiconductor medium is responsible for the increase in critical feedback rate corresponding to the drift instability.

nlin.PS↗

arXiv2012

Terahertz Quantum Hall Effect in a Topological Insulator

Using THz spectroscopy in external magnetic fields we investigate the low-temperature charge dynamics of strained HgTe, a three dimensional topological insulator. From the Faraday rotation angle and ellipticity a complete characterization of the charge carriers is obtained, including the 2D density, the scattering rate and the Fermi velocity. The obtained value of the Fermi velocity provides further evidence for the Dirac character of the carriers in the sample. In resonator experiments, we observe quantum Hall oscillations at THz frequencies. The 2D density estimated from the period of these oscillations agrees well with direct transport experiments on the topological surface state. Our findings open new avenues for the studies of the finite-frequency quantum Hall effect in topological insulators.

cond-mat.str-el↗

arXiv2010

High-Frequency Electromagnon in GdMnO3

We present the results of transmittance experiments on GdMnO3 multiferroic manganite in the far infrared frequency range. The spectra allow to obtain the positions and the intensity of the high frequency electromagnon (~ 75 cm-1) in this compound. We present the comparative analysis of the high and low frequency electromagnons across the phase diagram of GdMnO3. The traces of the electromagnon excitation can be detected even at room temperature, i.e. deeply in the paramagnetic state.

cond-mat.str-el↗

arXiv2010

Soft-mode behavior of electromagnons in multiferroic manganite

The behavior of the low-frequency electromagnon in multiferroic DyMnO3 has been investigated in external magnetic fields and in a magnetically ordered state. Significant softening of the electromagnon frequency is observed for external magnetic fields parallel to the a-axis (BIIa), revealing a number of similarities to a classical soft mode behavior known for ferroelectric phase transitions. The softening of the electromagnon yields an increase of the static dielectric permittivity which follows a similar dependence as predicted by the Lyddane-Sachs-Teller relation. Within the geometry BIIb the increase of the electromagnon intensity does not correspond to the softening of the eigenfrequency. In this case the increase of the static dielectric permittivity seem to be governed by the motion of the domain walls.

cond-mat.str-el↗

arXiv2010

On the room temperature multiferroic BiFeO3: Magnetic, dielectric and thermal properties

Magnetic dc susceptibility between 1.5 and 800 K, ac susceptibility and magnetization, thermodynamic properties, temperature dependence of radio and audio-wave dielectric constants and conductivity, contact-free dielectric constants at mm-wavelengths, as well as ferroelectric polarization are reported for single crystalline BiFeO3. A well developed anomaly in the magnetic susceptibility signals the onset of antiferromagnetic order close to 635 K. Beside this anomaly no further indications of phase or glass transitions are indicated in the magnetic dc and ac susceptibilities down to the lowest temperatures. The heat capacity has been measured from 2 K up to room temperature and significant contributions from magnon excitations have been detected. From the low-temperature heat capacity an anisotropy gap of the magnon modes of the order of 6 meV has been determined. The dielectric constants measured in standard two-point configuration are dominated by Maxwell-Wagner like effects for temperatures T > 300 K and frequencies below 1 MHz. At lower temperatures the temperature dependence of the dielectric constant and loss reveals no anomalies outside the experimental errors, indicating neither phase transitions nor strong spin phonon coupling. The temperature dependence of the dielectric constant was measured contact free at microwave frequencies. At room temperature the dielectric constant has an intrinsic value of 53. The loss is substantial and strongly frequency dependent indicating the predominance of hopping conductivity. Finally, in small thin samples we were able to measure the ferroelectric polarization between 10 and 200 K. The saturation polarization is of the order of 40 microC/cm^2, comparable to reports in literature.

cond-mat.mtrl-sci↗

arXiv2010

Metamaterials proposed as perfect magnetoelectrics

Magnetoelectric susceptibility of a metamaterial built from split ring resonators have been investigated both experimentally and within an equivalent circuit model. The absolute values have been shown to exceed by two orders of magnitude that of classical magnetoelectric materials. The metamaterial investigated reaches the theoretically predicted value of the magnetoelectric susceptibility which is equal to the geometric average of the electric and magnetic susceptibilities.

cond-mat.mtrl-sci↗

arXiv2010

Evidence of electro-active excitation of the spin cycloid in TbMnO3

Terahertz electromagnetic excitations in the multiferroic TbMnO3 at the field-induced magnetic transition are investigated for different orientations of the magnetic cycloid. In addition to the electromagnon along the a-axis, the detailed polarization analysis of the experimental spectra suggests the existence of an electro-active excitation for ac electric fields along the crystallographic c-axis. This excitation is possibly the electro-active eigenmode of the spin cycloid in TbMnO3, which has been predicted within the inverse Dzyaloshinskii-Moriya mechanism of magnetoelectric coupling.

cond-mat.str-el↗

arXiv2009

Magnetic and magnetoelectric excitations in TbMnO$_3$

Magnetic and magnetoelectric excitations in the multiferroic TbMnO_3 have been investigated at terahertz frequencies. Using different experimental geometries we can clearly separate the electro-active excitations (electromagnons) from the magneto-active modes, i.e. antiferromagnetic resonances (AFMR). Two AFMR resonances were found to coincide with electromagnons. This indicates that both excitations belong to the same mode and the electromagnons can be excited by magnetic ac-field as well. In external magnetic fields and at low temperatures distinct fine structure of the electromagnons appears. In spite of the 90^o rotation of the magnetic structure, the electromagnons are observable for electric ac-fields parallel to the a-axis only. Contrary to simple expectations, the response along the c-axis remains purely magnetic in nature.

cond-mat.str-el↗

arXiv2008

Dynamical Dzyaloshinsky-Moriya Interaction in KCuF3

The spin dynamics of the prototypical quasi one-dimensional antiferromagnetic Heisenberg spin S=1/2 chain KCuF3 is investigated by electron spin resonance spectroscopy. Our analysis shows that the peculiarities of the spin dynamics require a new dynamical form of the antisymmetric anisotropic spin-spin interaction. This dynamical Dzyaloshinsky-Moriya interaction is related to strong oscillations of the bridging fuorine ions perpendicular to the crystallographic c axis. This new mechanism allows to resolve consistently the controversies in observation of the magnetic and structural properties of this orbitally ordered perovskite compound.

cond-mat.str-el↗

arXiv2007

Terahertz spectroscopy of electromagnons in Eu_{1-x}Y_xMnO_3

Dielectric permittivity spectra of yttrium-doped EuMnO$_3$ in the composition range 0 =< x =< 0.5 have been investigated in the terahertz frequency range. Magnetoelectric contributions to the permittivity were observed in all compositions for ac electric fields parallel to the crystallographic a-axis. Well defined electromagnons exist for x >= 0.2 close to ν~ 20 cm^{-1} and with dielectric strength strongly increasing on doping. In addition to electromagnons, a broad contribution of magnetoelectric origin is observed for all compositions. For Eu_{0.8}Y_{0.2}MnO_3 the electromagnons can be suppressed by external magnetic fields which induce a canted antiferromagnetic phase. Magnetoelectric effects in the different doping regimes are discussed in detail.

cond-mat.str-el↗

arXiv2006

Structural and magnetic dimers in the spin-gapped system CuTe2O5

We investigated the magnetic properties of the system CuTe2O5 by susceptibility and electron spin resonance measurements. The anisotropy of the effective g-factors and the ESR linewidth indicates that the anticipated structural dimer does not correspond to the singlet-forming magnetic dimer. Moreover, the spin susceptibility of CuTe2O5 can only be described by taking into account interdimer interactions of the same order of magnitude than the intradimer coupling. Analyzing the exchange couplings in the system we identify the strongest magnetic coupling between two Cu ions to be mediated by super-super exchange interaction via a bridging Te ligand, while the superexchange coupling between the Cu ions of the structural dimer only results in the second strongest coupling.

cond-mat.str-el↗

arXiv2006

Spin-driven Phonon Splitting in Bond-frustrated ZnCr2S4

Utilizing magnetic susceptibility, specific heat, thermal expansion and IR spectroscopy we provide experimental evidence that the two subsequent antiferromagnetic transitions in ZnCr_2S_4 at T_N1 = 15 K and T_N2= 8 K are accompanied by significant thermal and phonon anomalies. The anomaly at T_N2 reveals a strong temperature hysteresis typical for a first-order transformation. Due to strong spin-phonon coupling both magnetic phase transitions induce a splitting of phonon modes, where at T_N1 the high-frequency and at T_N2 the low-frequency modes split. The anomalies and phonon splitting observed at T_N2 are strongly suppressed by magnetic field. Regarding the small positive Curie-Weiss temperature Theta= 8 K, we argue that this scenario of two different magnetic phases with concomitant different magneto-elastic couplings results from the strong competition of ferromagnetic and antiferromagnetic exchange of equal strength.

cond-mat.str-el↗

arXiv2006

Coupling of phonons and electromagnons in GdMnO_3

The infrared and Terahertz properties of GdMnO_3 have been investigated as function of temperature and magnetic field, with special emphasis on the phase boundary between the incommensurate and the canted antiferromagnetic structures. The heterogeneous incommensurate phase reveals strong magnetodielectric effects, characterized by significant magnetoelectric contributions to the static dielectric permittivity and by the existence of electrically excited magnons (electromagnons). In the commensurate canted antiferromagnetic phase the magnetoelectric contributions to the dielectric constant and electromagnons are suppressed. The corresponding spectral weight is transferred to the lowest lattice vibration demonstrating the strong coupling of phonons with electromagnons.

cond-mat.str-el↗