arXiv subjects

Z. V. Popovic

Publications and source records attributed to Z. V. Popovic.

16 recordsLinked to original sources

arXiv2022

Evolution of lattice, spin, and charge properties across the phase diagram of FeSe$_{1-x}$S$_x$

A Raman scattering study covering the entire substitution range of the FeSe$_{1-x}$S$_x$ solid solution is presented. Data were taken as a function of sulfur concentration $x$ for $0\le x \le 1$, of temperature and of scattering symmetry. All type of excitations including phonons, spins and charges are analyzed in detail. It is observed that the energy and width of iron-related B$_{1g}$ phonon mode vary continuously across the entire range of sulfur substitution. The A$_{1g}$ chalcogenide mode disappears above $x=0.23$ and reappears at a much higher energy for $x=0.69$. In a similar way the spectral features appearing at finite doping in A$_{1g}$ symmetry vary discontinuously. The magnetic excitation centered at approximately 500 cm$^{-1}$ disappears above $x=0.23$ where the A$_{1g}$ lattice excitations exhibit a discontinuous change in energy. The low-energy mode associated with fluctuations displays maximal intensity at the nemato-structural transition and thus tracks the phase boundary.

cond-mat.supr-con↗

arXiv2020

Self-assembled line network in BiFeO3 thin films

In this work we report on the controlled fabrication of a self-assembled line network in highly epitaxial BiFeO3 thin films on top of LaAlO3 in the kinetically limited grown region by RF sputtering. As previously shown in the case of manganite thin films, the remarkable degree of ordering is achieved using vicinal substrates with well-defined step-terrace morphology. Nanostructured BiFeO3 thin films show mixed-phase morphology. Besides typical formation following (100) and (010) axes, some mixed phase nanodomains are detected also in-between the regular line network. These particular microstructures open a playground for future applications in multiferroic nanomaterials.

cond-mat.mtrl-sci↗

arXiv2016

Insulating and metallic spin glass in K$_{x}$Fe$_{2-δ-y}$Ni$_{y}$Se$_{2}$ (0.06 $\leq$ $y$ $\leq$ 1.44) single crystals

We report electron doping effects by Ni in K$_{x}$Fe$_{2-δ-y}$Ni$_{y}$Se$_{2}$ (0.06 $\leq$ $y$ $\leq$ 1.44) single crystal alloys. A rich ground state phase diagram is observed. Small amount of Ni ($\sim$ 4\%) suppressed superconductivity below 1.8 K, inducing insulating spin glass magnetic ground state for higher Ni content. With further Ni substitution, metallic resistivity is restored. For high Ni concentration in the lattice the unit cell symmetry is high symmetry $I4/mmm$ with no phase separation whereas both $I4/m + I4/mmm$ space groups were detected in the phase separated crystals when concentration of Ni $<$ Fe. The absence of superconductivity coincides with the absence of crystalline Fe vacancy order.

cond-mat.supr-con↗

arXiv2016

Sustained phase separation and spin glass in Co-doped K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals

We present Co substitution effects in K$_{x}$Fe$_{2-y-z}$Co$_{z}$Se$_{2}$ (0.06 $\leq$ $z$ $\leq$ 1.73) single crystal alloys. By 3.5\% of Co doping superconductivity is suppressed whereas phase separation of semiconducting K$_{2}$Fe$_{4}$Se$_{5}$ and superconducting/metallic K$_{x}$Fe$_{2}$Se$_{2}$ is still present. We show that the arrangement and distribution of superconducting phase (stripe phase) is connected with the arrangement of K, Fe and Co atoms. Semiconducting spin glass is found in proximity to superconducting state, persisting for large Co concentrations. At high Co concentrations ferromagnetic metallic state emerges above the spin glass. This is coincident with changes of the unit cell, arrangement and connectivity of stripe conducting phase.

cond-mat.supr-con↗

arXiv2011

Lattice dynamics of FeSb2

The lattice dynamics of FeSb2 is investigated by the first-principles DFT calculations and Raman spectroscopy. All Raman and infra-red active phonon modes are properly assigned. The calculated and measured phonon energies are in good agreement except for the B3g symmetry mode. We have observed strong mixing of the Ag symmetry modes, with the intensity exchange in the temperature range between 210 K and 260 K. The Ag modes repulsion increases by doping FeSb2 with Co. There are no signatures of the electron-phonon interaction for these modes.

cond-mat.mtrl-sci↗

arXiv2006

Raman and infrared studies of La_1-ySr_yMn_1-xM_xO_3 (M=Cr, Co, Cu, Zn, Sc or Ga): Oxygen disorder and local vibrational modes

We present results of our study of polarized Raman scattering and infrared reflectivity of rhombohedral ceramic La_1-ySr_yMn_1-xM_xO_3 manganites in the temperature range between 77 and 320K. In our samples, a part of the Mn atoms is substituted by M = Cr, Co, Cu, Zn, Sc, or Ga with x in the range 0 -- 0.1. The hole concentration was kept at the optimal value of about 32% by tuning the Sr content y. We have monitored the distortions of the oxygen sublattice by the presence of broad bands in the Raman spectra, the increase of d.c. resistivity extracted from the infrared reflectivity, and the change of the critical temperature of the ferromagnetic transition. Our results support the idea, that these properties are mainly determined by the radius of the substituent ion, its electronic and magnetic structure playing only a minor role. Furthermore, the Raman spectra exhibit an additional A_g-like high frequency mode attributed to the local breathing vibration of oxygens surrounding the substituent ion. Its frequency and intensity strongly depend on the type of the substituent. In the Co-substituted sample, the mode anomalously softens when going from 300 to 77K. The frequency of the bulk A_{1g} mode depends linearly on the angle of the rhombohedral distortion.

cond-mat.str-el↗

arXiv2003

Raman scattering study of charge ordering in beta-Ca0.33V2O5

Polarized Raman spectra of the calcium vanadium oxide bronze beta-Ca0.33V2O5 are measured in the temperature range between 300 K and 20 K. The charge ordering phase transition at about 150 K is characterized by the appearance of the new Raman-active modes in the spectra, and by anomalies in the electronic background scattering. The high temperature Raman scattering spectra of beta-Ca0.33V2O5 are in apparent resemblance with those of alpha'-NaV2O5, which suggests that there is a similar charge-phonon dynamics in both compounds. The study of dynamical properties and the symmetry analysis of the Raman modes show that in the mixed valence state of beta-Ca0.33V2O5 the electrons are delocalized into V1-O5-V3 orbitals. We propose that in the charge ordered state below 150 K the d-electrons localize within V1-V3 ladders, either in "zig-zag" fashion like in alpha'-NaV2O5 or in the forms of the double chains like in gamma-LiV2O5.

cond-mat.str-el↗

arXiv2002

Optical properties of NaxV2O5

The optical properties of sodium-deficient NaxV2O5 (0.85 < x <1) single crystals are analyzed in the wide energy range, from 0.012 to 4.5 eV, using ellipsometry, infrared reflectivity, and Raman scattering techniques. The material remains insulating up to the maximal achieved hole concentration of about 15%. In sodium deficient samples the optical absorption peak associated to the fundamental electronic gap develops at about 0.44 eV. It corresponds to the transition between vanadium dxy and the impurity band, which forms in the middle of the pure NaV2O5 gap. Raman spectra measured with incident photon energy larger then 2 eV show strong resonant behavior, due to the presence of the hole-doping activated optical transitions, peaked at 2.8 eV.

cond-mat.str-el↗

arXiv2001

Raman scattering study of phonon and spin modes in (TMTSF)2PF6

We have studied polarized Raman spectra of (TMTSF)2PF6 single crystals in the wide spectral and temperature range using different laser energies. We observed and assigned 23 Raman active modes. The carbon C=C in-phase and out-of-phase stretching modes at 1464 cm-1 and 1602 cm-1 show strong electron-molecular-vibration coupling. The mode at about 1565 cm-1 is temperature independent below 50 K due to the methyl group motion freezing. For (TMTSF)2PF6 in the spin density wave phase for a temperature induced dimensionality 1D (quarter filled dimerized linear chain with correlated electrons) the spin gapless magnon spectra have been calculated. No evidence of spin-lattice coupling or spin-related modes is found in the Raman spectra in the spin density wave phase.

cond-mat.str-el↗

arXiv2001

Optical studies of gap, hopping energies and the Anderson-Hubbard parameter in the zigzag-chain compound SrCuO2

We have investigated the electronic structure of the zigzag ladder (chain) compound SrCuO2 combining polarized optical absorption, reflection, photoreflectance and pseudo-dielectric function measurements with the model calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at 300 K along (perpendicular) to the Cu-O chains. We have found that the lowest energy gap, the correlation gap, is temperature independent. The electronic structure of this oxide is calculated using both the local-spin-density-approximation with gradient correction method, and the tight-binding theory for the correlated electrons. The calculated density of electronic states for non-correlated and correlated electrons shows quasi-one-dimensional character. The correlation gap values of 1.42 eV (indirect transition) and 1.88 eV (direct transition) have been calculated with the electron hopping parameters t = 0.30 eV (along a chain), t_yz = 0.12 eV (between chains) and the Anderson-Hubbard repulsion on copper sites U= 2.0 eV. We concluded that SrCuO_2 belongs to the correlated-gap insulators.

cond-mat.str-el↗

arXiv2000

Raman Scattering from Magnetic Excitations in the Spin Ladder Compounds CaV2O5 and MgV2O5

We present the Raman-scattering spectra of CaV2O5 and MgV2O5. The magnetic contribution in the Raman spectra of CaV2O5 is found in the form of a strong asymmetric line, centered at 2Delta, with a tail on the high-energy side. Our analysis of its spectral shape shows that the magnetic ordering in CaV2O5 can be described using a S=1/2 two-leg ladder Heisenberg antiferromagnetic model with Jparallel/Jperp = 0.1, and a small interladder exchange. The spin gap and exchange constant are estimated to be Delta = 400 cm-1 (570 K) and Jperp = 640 K. No magnon bound states are found. In contrast to CaV2O5 the existence of the spin-gap is not confirmed in MgV2O5, since we found no feature in the spectra which could be associated with the onset of the two-magnon continuum. Instead, we observe two-magnon excitation at 340 cm-1, presumably related to the top of the two-magnon brunch.

cond-mat.str-el↗

arXiv2000

Infrared and Raman spectra of LiV2O5 single crystals

The phonon dynamics of LiV2O5 single crystals is studied using infrared and Raman spectroscopy techniques. The infrared-active phonon frequencies and dielectric constants are obtained by oscillator fitting procedure of the reflectivity data measured at room temperature. The Raman scattering spectra are measured at room temperature and at T=10 K in all nonequivalent polarized configurations. The assignment of the phonons is done by comparing the infrared and Raman spectra of LiV2O5 and NaV2O5. The factor-group-analysis of the LiV2O5 crystal symmetry and of its constituent layers is performed to explain the symmetry properties of the observed modes. We concluded that layer symmetry dominates in the vibrational properties of this compound.

cond-mat.str-el↗

arXiv1999

Effective Dimensionality and Band Structure of alpha-Nav_2o_5 Compound: 1D or 2D?

The AV_nO_{2n+1} mixed valence compounds (n=1, A=Na) are classified as dimerized layered system with strongly interacting d-electrons of vanadium ions. The derived band gaps, energy dispersion relations and density of electronic states are in a good agreement with available experimental and theoretical data. The correlated band gap provides the insulating state of the high-temperature alpha-NaV_2O_5 phase whereas the state, earlier misrepresented as the spin-Peierls state, is governed, in fact, by opening of the Coulomb gap.

cond-mat.str-el↗

arXiv1999

Resonant Raman scattering in NaV2O5 as a probe of its electronic structure

In order to investigate the origin of the phase transition observed in NaV$_2$O$_5$, as well as its electronic structure, we have measured Raman intensities as a function of the laser wavelength above and below the phase transition temperature. In the polarized Raman spectra at room temperature we observe resonant enhancement of the 969 $cm^{-1}$ phonon mode when the laser energy approaches 2.7 eV, presumably related to the (p-d) electron hopping band, $O_3$($p_y$)-V($d_{xy}$), at 3.2 eV. The 969 $cm^{-1}$ mode originates from the stretching vibrations along the c-axis involving the V-$O_1$ bonds. Since an ellipsometric determination of the dielectric function $ε_{cc}$ yields no structure in the 1.7 to 5.5 eV photon energy range, we conclude that plane bonds couple strongly with the apical oxygens leading to a large Raman efficiency. In the low-temperature Raman spectra, almost all modes that become active below the phase transition temperature T$_c$=34 K show resonant behavior. The most interesting ones, those at 66 and 106 $cm^{-1}$, possibly of magnetic origin, exhibit a resonant intensity enhancement, approximately by an order of magnitude, for laser photon energies around 1.85 eV with respect to 2.43 eV. This resonance effect may be associated with a weak absorption band around 2 eV. Finally, a destructive interference between the resonant and the nonresonant contribution to the Raman scattering amplitude (i.e. an antiresonance) is found in the spectra for most of the (bb) low-temperature modes.

cond-mat.str-el↗

arXiv1999

First evidence for charge ordering in NaV$_2$O$_5$ from Raman spectroscopy

We argue on the basis of symmetry selection rules and Raman scattering spectra that NaV$_2$O$_5$ undergoes a charge ordering phase transition at T$_c$=34 K. Such a transition is characterized by the redistribution of the charges at the phase transition and corresponds to the change of the vanadium ions, from uniform V$^{4.5+}$ to two different V$^{4+}$ and V$^{5+}$ states. In the low temperature phase the V$^{4+}$ ions are forming a "zig-zag" ladder structure along the {\bf b}-axis, consistent with the symmetry of the P2/b space group.

cond-mat.str-el↗

arXiv1998

Magnetic excitation spectra of $NaV_2O_5$

We present low temperature Raman spectra of $NaV_2O_5$ single crystal. The assignation of the observed modes is done on the basis of Monte Carlo calculation of excited states of Heisenberg dimerized Hamiltonian, without frustration. We found a good agreement between measured and calculated magnetic excitations for $δ=0.048$ and $J=452 K$. The singlet-triplet, singlet-singlet excitations as well as two-magnon mode are observed to be at 66, 127 and 132 cm^{-1}.

cond-mat↗