Vladimir Gnezdilov

Technische Universität Braunschweig · Institute of condensed Matter Physics

Iron-pnictides Ca(Fe1-xRhx)2As2 (x = 0, 0.035, and 0.19) were studied across the tetragonal-orthorhombic and uncollapsed to collapsed tetragonal phase transitions using Raman spectroscopy. The effect of the phase separation was observed in the high-temperature phase for the first time. Two phases with the low-spin and high-spin states of the Fe2+ i...

We report comprehensive Raman scattering measurements on a single crystal of binary ferroborate Nd0.75Dy0.25Fe3(BO3)4 in the temperature range of 7–295 K with 532 nm (18797 cm⁻¹) laser excitation. The performed analysis of the polarized Raman spectra revealed the bands assigned to phonon, magnetic, and electronic excitations. The temperature evolut...

We present an experimental study of the quasi-elastic Raman scattering (QES) of plane-wave and twisted light by liquid crystals. Depending on their temperature, these crystals can exhibit isotropic, nematic and chiral nematic phases. The question is addressed of how the phase of a crystal and the state of incident light can affect the quasi–elastic...

We report a Raman scattering study of single-crystalline homometallic oxyborate Mn2OBO3, a compound realizing a one-dimensional ribbon-structure. Phonon excitations as a function of temperature and light polarization are compared to lattice dynamical calculations, giving evidence for a strong coupling between lattice and magnetic degrees of freedom...

Fe3+ S=5/2 ions form saw-tooth-like chains along the a axis of the oxoselenite Fe2O(SeO3)2 and an onset of long-range magnetic order is observed for temperatures below TC=105 K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations with small linewidths emerge...

Fe$^{3+}$ $S = 5/2$ ions form saw-tooth like chains along the $a$ axis of the oxo-selenite Fe$_2$O(SeO$_3$)$_2$ and an onset of long-range magnetic order is observed for temperatures below $T_C = 105$ K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations wit...

Vibrational Raman spectra of a single crystal of the coupled Haldane chain compound SrNi2V2O8 with uniaxial anisotropy were investigated in the 10-1000 cm⁻¹ frequency range at temperatures 7-300 K. No structural phase transition was observed. The number of phonon lines observed in the experiment and their intensity were analyzed on the basis of the...

Specific heat C, thermal conductivity κ, dielectric permittivity ε, electric polarization P, and Raman scattering experiments are performed on Cu3Bi(SeO3)2O2X (X=Br,Cl) single crystals. The Cl compound undergoes a structural phase transition at T*∼115K evident in C(T), ɛ(T), and κ(T) and accompanied by the appearance of unique phonon lines in Raman...

Raman scattering in the three-dimensional Dirac semimetal Cd3As2 shows an intricate interplay of electronic and phonon degrees of freedom. We observe resonant phonon scattering due to interband transitions, an anomalous anharmonicity of phonon frequency and intensity, as well as quasielastic (E∼0) electronic scattering. The latter two effects are g...

In the three-dimensional Dirac semimetal Cd3As2 we notice an interplay of electronic and phonon degrees of freedom mediated by topological effects. There exist a pronounced quasielastic scattering and an anomalous anharmonicity of optical phonons. These effects follow a similar temperature dependence with a characteristic temperature T* ~ 100 K. Fu...

We report Raman light scattering in the phase separated superconducting single crystal Rb0.77Fe1.61Se2 with Tc = 32 K. The spectra have been measured in a wide temperature range 3K -500K. The observed phonon lines from the majority vacancy ordered Rb2Fe4Se5 (245) antiferromagnetic phase with TN= 525 K demonstrate modest anomalies in frequency, inte...

We performed Raman studies and a dielectric characterization of the pseudo-kagome Cu3Bi(SeO3)2O2X (X = Cl, Br). These compounds share competing nearest-neighbour ferromagnetic exchange and frustrating next-nearest-neighbour antiferromagnetic exchange as well as highly noncollinear magnetic ground state. However, at low temperature they differ with...

In the giant Rashba semiconductor BiTeI electronic surface scattering with Lorentzian linewidth is observed that shows a strong dependence on surface termination and surface potential shifts. A comparison with the topological insulator Bi2Se3 evidences that surface confined quantum well states are the origin of these processes. We notice an enhance...

In the giant Rashba semiconductor BiTeI electronic surface scattering with Lorentzian linewidth is observed that shows a strong dependence on surface termination and surface potential shifts. A comparison with the topological insulator Bi2Se3 evidences that surface confined quantum well states are the origin of these processes. We notice an enhance...

We report Raman scattering experiments on the strongly correlated electron system NaxCoO2 with x= 0.71 and ordered Na vacancies. In this doping regime, NaxCoO2 exhibits a large and unusual thermopower and becomes superconducting upon hydration. Our Raman scattering data reveal pronounced low energy fluctuations that diverge in intensity at low temp...

We report magnetic-susceptibility, specific-heat, and Raman-scattering investigations of -TeVO 4 contain-ing V-O edge-sharing chains. These chains promote a system of ferromagnetic/antiferromagnetic spin-1/2 Heisenberg alternating exchange chains with pronounced spin frustration. The magnetic susceptibility and Raman scattering evidence a crossover...

In the giant Rashba, polar semiconductor BiTeI phonons as well as spin split bulk and surface states are probed using Raman scattering. The observed bulk states are in good agreement with symmetry considerations and photoemission data of spin splitting. A very large enhancement of the Raman scattering cross section is observed if the laser energy i...

Polarized Raman-scattering spectra of superconducting, single-crystalline FeSe evidence pronounced phonon anomalies with temperature reduction. A large 6.5% hardening of the B_1g(Fe) phonon mode is attributed to the suppression of local fluctuations of the iron spin state with the gradual decrease of the iron paramagnetic moment. The ab-initio latt...

The density functional theory was applied to calculate the electronic structure, ground state energy, phonon frequencies at the center and at the boundary of the Brillouin zone, as well as the distribution of magnetic moment over d-orbitals of iron in the magnetically ordered phase of FeTe as functions of the magnetic moment—spin state of iron. It...

Specific heat studies of a series of RBaCo2O5.5 (R=Y, Gd, and Tb) layered cobaltites are reported. They were aimed at explaining an influence of different rare-earth ions on thermal properties of these compounds and at studying phase transitions appearing in them. The studies were performed over the temperature range from 3 to 395 K, in the magneti...

We present a comparative study of the low-dimensional compounds α- and β-TeVO4. Our data clearly show that the change in the local coordination geometry of V4+ ions between α- and β-TeVO4 leads to drastic differences in their magnetic properties. Despite sharing the same crystal structure, the two compounds manifest different magnetic exchange topo...

We present a comparative study of the low-dimensional compounds α- and β-TeVO 4. Our data clearly show that the change in the local coordination geometry of V 4+ ions comparing α- and β-TeVO 4 leads to a drastic differences in the magnetic properties. Despite sharing the same crystal structure, the two compounds realize different magnetic exchange...

The IR reflectance spectra of the cubic noncentrosymmetric ferrimagnet Cu2OSeO3 (TC ~ 60 K) were investigated in the temperature range 10-80 K. Detailed analysis of the symmetry and composition of vibrational modes was conducted on the basis of Raman scattering data and data obtained in this investigation. 68 out of 69 modes consistent with the cry...

The Raman-active phonons and magnetic excitations in the orbitally ordered, quasi one-dimensional Heisenberg antiferromagnet KCuF3 are studied as a function of temperature in the range between 3 and 290 K in different scattering configurations. The low-energy Eg and B1g phonon modes show an anomalous softening (∼25% and ∼13%, respectively) between...

Lattice dynamics and magnetic excitations are investigated to elucidate the origin of magneto-dielectric effects in the S=1/2 two-dimensional quantum spin compound K$_2$V$_3$O$_8$. We find evidence for lattice instabilities at 110 K and 60 K as optical phonon anomalies and a soft mode at 26 cm$^{-1}$ in A$_1$ symmetry. Two-magnon excitations in B$_...

The low-dimensional s=1/2 compound (NO)[Cu(NO3)3] has recently been suggested to follow the Nersesyan-Tsvelik model of coupled spin chains. Such a system shows unbound spinon excitations and a resonating valence bond ground state due spin frustration. Our Raman scattering study demonstrates phonon anomalies as well as the suppression of a broad mag...

High quality single crystals of Ba1-xKxBiO3 (x=0; 0.15, 0.2, 0.33, 0.42) have been investigated by Raman light scattering in the 5-300 K temperature range. All spectra are interpreted as phonon states that are introduced in Raman scattering by folding back of modes from different points of the zone boundary due to structural distortions. Direct com...

Strong spin-orbit interaction in the two dimensional compound Sr2IrO4 leads to the formation of Jeff=1/2 isospins with unprecedented dynamics. In Raman scattering a continuum attributed to double spin scattering is observed. With higher excitation energy of the incident Laser this signal crosses over to an incoherent background. The characteristic...

We present a comparative study of the low-dimensional compounds alpha- and beta-TeVO4. Our data clearly show that the change in the local coordination geometry of V4+ ions between alpha- and beta-TeVO4 leads to drastic differences in their magnetic properties. Despite sharing the same crystal structure, the two compounds manifest different magnetic...

The topological insulator Bi2Se3 shows a Raman scattering response related to topologically protected surface states amplified by a resonant interband transition. Most significantly this signal has a characteristic Lorentzian lineshape and spin-helical symmetry due to collision dominated scattering of Dirac states at the Fermi level E_F on bulk val...

We investigate the origin of large magnetocapacitive effects in CdCr2S4 single crystals focusing on phonon anomalies in the fluctuation regime T < T* = 130 K and with the onset of ferromagnetic order Tc = 84.4 K. The observation of longitudinal optical and formerly infrared active modes for T < Tc is used to propose a loss of inversion symmetry and...

Polarized Raman-scattering spectra of nonsuperconducting, single-crystalline FeTe are investigated as a function of temperature. We have found a relation between the magnitude of ordered magnetic moments and the linewidth of A1g phonons at low temperatures. This relation is attributed to the intermediate spin state (S = 1) and the orbital degenerac...

We report on an extended fluctuation regime in the spin dimer system Sr3Cr2O8 based on anomalies in Raman active phonons and magnetic scattering. The compound has two characteristic temperatures, TS = 275 K, related to a Jahn-Teller transition with structural distortions and orbital ordering and a second, T*= 150 K, which is due to further changes...

Polarized Raman-scattering spectra of non-superconducting, single-crystalline FeTe are investigated as function of temperature. We have found a relation between the magnitude of ordered magnetic moments and the linewidth of A1g phonons at low temperatures. This relation is attributed to the intermediate spin state (S=1) and the orbital degeneracy o...

sublattices ferrimagnet Cu2OSeO3 with a cubic symmetry and a linear magnetoelectric effect. There is no spectroscopic evidence for structural lattice distortions below TC=60 K, which are expected due to magnetoelectric coupling. Using symmetry arguments we explain this observation by considering a special type of ferrimagnetic ground state which do...

In the orbitally ordered, quasi-one dimensional Heisenberg antiferromagnet KCuF3 the low-energy Eg and B1g phonon modes show an anomalous softening (25% and 13%) between room temperature and the characteristic temperature T_S = 50 K. In this temperature range a freezing-in of F ion dynamic displacements is proposed to occur. In addition, the Eg mod...

Experimental results concerning the temperature and polarization dependence of the one-magnon Raman light scattering in the canted antiferromagnet NiF2 (Néel temperature TN=73 K) are reported. The excitation frequencies, linewidths, and integrated intensities of the lower and upper magnon branches are obtained over the temperature range 5–65 K. A c...

We report on infrared, Raman, magnetic susceptibility, and specific heat measurements on CdCr2O4 and ZnCr2O4 single crystals. We estimate the nearest-neighbor and next-nearest neighbor exchange constants from the magnetic susceptibility and extract the spin-spin correlation functions obtained from the magnetic susceptibility and the magnetic contri...

Raman scattering in the spin-crossover system FepmdH 2 OAuCN 2 2 ·H 2 O reveals a complex three-phase spin-state transition in contrast to earlier observations in magnetization measurements. We observe different spin-state phases as a function of temperature and electromagnetic radiation in the visible spectral range. There exists a fluctuating spi...

The transition metal tellurium oxychloride, Ni5(TeO3)4Cl2, has been investigated by the Raman scattering method at temperatures of 15-300 K. Sixty two phonon modes (of the total 69 Raman-active phonon modes allowed for the monoclinic C6 2h structure) are identified in the spectra, and their behavior is analyzed. Several distinct features are observ...

We report magnetic susceptibility, specific heat, and Raman scattering investigations of alpha-TeVO4 containing V-O edge-sharing chains. These chains promote a system of ferromagnetic/antiferromagnetic spin-1/2 Heisenberg alternating exchange chains with pronounced spin frustration. The magnetic susceptibility and Raman scattering evidence a cross...

We study theoretically and experimentally the dependence of the electron-phonon interaction in NaxCoO2 on the sodium concentration x. For the two oxygen phonon modes found in Raman experiments, A1g and E1g, we calculate the matrix elements of the electron-phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon fr...

Raman scattering and optical birefringence are used in to investigate a low-temperature phase transition in a single crystal of the two-dimensional Na5RbCu4(AsO4)4Cl2. Phonon anomalies point to a first-order nature of the transition. The observed transition is most probably related to an order-disorder transition of the Rb ion positions along the z...

We report inelastic light scattering experiments on FeCr2S4, which shows colossal magnetoresistance (CMR) effects around TC≈170 K. Several distinct features have been observed: (i) anomalies in phonon frequencies and linewidths around TC, (ii) the vanishing of one-magnon excitation close to 0.83 TC, and (iii) the presence of an electronic backgroun...

The magnons in the canted rutile-structure antiferromagnet NiF2 have been studied by Raman scattering. Results are presented for the polarization dependences of the one-magnon spectum over a wide range of temperatures below TN and the two-magnon spectrums at low temperatures. A theoretical analysis of the data, including the important role of the s...

Raman scattering experiments on CdCr2S4 single crystals show pronounced anomalies in intensity and frequency of optical phonon modes with an onset temperature T*=130 K that coincides with the regime of giant magnetocapacitive effects. A loss of inversion symmetry and Cr off-centering are deduced from the observation of longitudinal optical and form...

A giant softening by 30 cm−1 of the 490 and 620 cm−1 Jahn-Teller and breathing optical phonon modes is observed in Raman spectroscopy below the Curie temperature of single crystalline LaMnO3+δ (0.085⩽δ⩽0.125). A pseudogaplike suppression of a continuum and a Fano antiresonance at 144 cm−1 appear below the charge-ordering temperature. Upon going thr...

The crystal structure of the copper(II) tellurium(IV) oxochloride Cu4Te5O12Cl4 (Cu-45124) is composed of weakly coupled tetrahedral Cu clusters and shows crystallographic similarities with the intensively investigated compound Cu2Te2O5X2, with X=Cl,Br (Cu-2252). It differs from the latter by a larger separation of the tetrahedra within the crystall...

Raman scattering experiments on NaxCoO2.yH2O single crystals show a broad electronic continuum with a pronounced peak around 100 cm(-1) and a cutoff at approximately 560 cm(-1) over a wide range of doping levels. The electronic Raman spectra in superconducting and nonsuperconducting samples are similar at room temperature, but evolve in markedly di...

One-and multi-phonon excitations of the single crystalline LaCoO 3 were studied using Raman spectroscopy in the temperature region of 5 K – 300 K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in terms of activated modes due to lattice distortions,...

High quality single crystals of LaMnO3+δ with δ=0.071, 0.085, 0.092, 0.097, 0.125 that extend from antiferromagnetic insulating (δ=0.071, TN=128 K) to ferromagnetic metallic state (δ=0.125, TC=248 K) have been investigated by polarized Raman light scattering. A giant softening up to 30 cm−1 of the 490- and 620-cm−1 phonon modes has been observed be...

Spectroscopic investigation of the crossover behavior in TeVO4

We report an inelastic light scattering study of single-crystalline NaCu2O2, a spin-chain compound known to exhibit a phase with helical magnetic order at low temperatures. Phonon excitations were studied as a function of temperature and light polarization, and the phonon frequencies are compared to the results of ab initio lattice dynamical calcul...

Raman scattering experiments on Na_{x}CoO_2 yH_2O single crystals show a broad electronic continuum with a pronounced peak around 100 cm-1 and a cutoff at approximately 560 cm-1over a wide range of doping levels. The electronic Raman spectra in superconducting and non-superconducting samples are similar at room temperature, but evolve in markedly d...

We report on Raman scattering measurements of single crystalline La1−xSrxMnO3 (x=0, 0.06, 0.09, and 0.125), focusing on the high-frequency regime. We observe multiphonon scattering processes up to fourth order which show distinct features: (i) anomalies in peak energy and its relative intensity and (ii) a pronounced temperature, polarization, and d...

Giant softening by 30 cm$^{-1}$ of the 490- and 620-cm$^{-1}$ modes is observed by Raman scattering measurements below the Curie temperature of single crystalline LaMnO$_{3+\delta}$ ($0.085\leq \delta\leq0.125$). A pseudogap-like electronic continuum and a Fano antiresonance at 144 cm$^{-1}$ appear below the charge ordering temperature. This gives...

We report Raman scattering measurements on the spin ladder compound (C5H12N)2CuBr4. Pronounced multiphonon scattering is observed up to eighth-order, arising from the Franck–Condon process. Furthermore, a two-magnon continuum is seen with frequencies around 18 cm−1 at low temperatures. Its symmetric line shape suggests a substantial hopping of trip...

We present an inelastic light scattering study of single crystalline (La1−yPry)1−xSrxMnO3 (0⩽x⩽0.14, y=0 and x=1∕8, 0⩽y⩽0.5). A giant softening up to 20–30 cm−1 of the Mn-O breathing mode has been observed only for the ferromagnetic insulating (FMI) samples (0.11⩽x⩽0.14) upon cooling below the Curie temperature (TC). With increasing Pr-doping the g...

Raman scattering experiments have been performed on NaxCoO2 as a function of temperature at the composition x=0.5 where a structural instability and a metal–insulator transition have been observed. Three additional phonon modes are observed compared to samples with larger x. An in situ annealing study (Tmax=550K) with the initial presence of water...

Electronic correlation effects in La2−xSrxNiO4(x=1/3 and 0.225) lead to spontaneous phase separation into microscopic spin/charge stripes with commensurate and incommensurate order, respectively. Raman scattering experiments on such single-crystalline materials show a rich phenomenology of phonon and magnon anomalies due to the new, self-organized...

Comparative Raman scattering studies of single crystals GdBaCo2O5+δ with δ~0.35÷0.5, and polycrystalline GdBaCo2O5.5±0.05 in the temperature range 5–295 K have been made. 8Ag+3B1g modes in backscattering XX, and XY configuration from polycrystalline and single crystal samples with δ ~ 0.5 have been observed. The Pmmm space group symmetry of GdBaCo2...

We report a neutron- and Raman-scattering study of a single crystal of La2CuO4.05 prepared by high-temperature electrochemical oxidation. Elastic neutron-scattering measurements show the presence of two phases, corresponding to the two edges of the first miscibility gap, all the way up to 300 K. An additional oxygen redistribution, driven by electr...

We report a drastic change of the temperature dependence of the Mn–O stretching mode at the phase boundary between canted antiferromagnetic insulating (CAF) and ferromagnetic insulating (FI) of La1−xSrxMnO3. The observation of an unusual large softening by 18cm−1 only in the FI phase is ascribed to a change of electronic state and orbital state thr...

We report on a Raman scattering study of the superconducting cobaltite Na(x)CoO(2)(.)yH(2)O as function of Na content and hydration (x approximate to 1/3, 3/4 and y approximate to 0, 2/3, 4/3). The observed scattering intensity is analysed in terms of lattice strain, structural misfit and disorder. Hydration, due to the intercalation of one or two...

The Raman-active phonons in perovskite-like LaMn1-xCoxO3 (x = 0, 0.2, 0.3, 0.4, and 1.0) were studied by measuring Raman spectra at temperatures of 295 and 5 K. The changes in the spectra with Co doping are correlated with the decrease of orthorhombic distortions. Surprisingly more phonon lines than allowed for the rhombohedral LaCoO3 structure wer...

Electronic correlation effects in La2−xSrxNiO4 lead to spontaneous phase separation into microscopic spin/charge stripes with commensurate or incommensurate order. Raman scattering experiments on such single crystalline materials show a rich phenomenology of phonon and magnon anomalies due to the new, self-organized periodicities. These effects are...