[show abstract][hide abstract] ABSTRACT: Polarized Raman, IR and time-domain THz spectroscopy of orthorhombic lead
zirconate single crystals yielded a comprehensive picture of
temperature-dependent quasiharmonic frequencies of its low-frequency phonon
modes. It is argued that these modes primarily involve vibration of Pb and/or
oxygen octahedra librations and their relation to particular phonon modes of
the parent cubic phase is proposed. Counts of the observed IR and Raman active
modes belonging to distinct irreducible representations agree quite well with
group-theory predictions. The most remarkable finding is the considerably
enhanced frequency renormalization of the y-polarized polar modes, resulting in
a pronounced low temperature dielectric anisotropy. Results are discussed in
terms of contemporary phenomenological theory of antiferroelectricity.
[show abstract][hide abstract] ABSTRACT: THz-range dielectric spectroscopy and first-principle-based
effective-Hamiltonian molecular dynamics simulations were employed to elucidate
the dielectric response in the paraelectric phase of (Ba,Sr)TiO3 solid
solutions. Analysis of the resulting dielectric spectra suggests the existence
of a crossover between two different regimes: a higher-temperature regime
governed by the soft mode only versus a lower-temperature regime exhibiting a
coupled soft mode/central mode dynamics. Interestingly, a single
phenomenological coupling model can be used to adjust the THz dielectric
response in the entire range of the paraelectric phase (i.e., even at high
temperature). We conclude that the central peak is associated with thermally
activated processes, and that it cannot be discerned anymore in the dielectric
spectra when the rate of these thermally activated processes exceeds certain
characteristic frequency of the system.
[show abstract][hide abstract] ABSTRACT: The integration of ferromagnetism and semiconductors has been studied
extensively, but devices operate well below room temperature. Recent
theoretical and experimental works have opened a new route for
spintronics based on antiferromagnets. Remarkably, high-temperature
antiferromagnetic order is much more compatible with semiconductors than
the ferromagnetic order. In our work we focus on the family of I-Mn-V
antiferromagnetic semiconductor. We report on our synthesis of bulk and
thin-film epilayers of the I-Mn-V compounds and on their basic
electrical and magnetic properties. We will discuss the utility of these
materials for designing antiferromagnetic semiconductor spintronic
devices. [4pt]  Phys. Rev. B 83, 035321 (2010) [0pt] 
[show abstract][hide abstract] ABSTRACT: Terahertz and far-infrared electric and magnetic responses of hexagonal
piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic
resonance is observed in the spectra of magnetic permeability mu_a [H(omega)
oriented within the hexagonal plane] below the Neel temperature T_N. This
excitation softens from 41 to 32 cm-1 on heating and finally disappears above
T_N. An additional weak and heavily-damped excitation is seen in the spectra of
complex dielectric permittivity epsilon_c within the same frequency range. This
excitation contributes to the dielectric spectra in both antiferromagnetic and
paramagnetic phases. Its oscillator strength significantly increases on heating
towards room temperature thus providing evidence of piezomagnetic or
higher-order couplings to polar phonons. Other heavily-damped dielectric
excitations are detected near 100 cm-1 in the paramagnetic phase in both
epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior.
These excitations appearing in the frequency range of magnon branches well
below polar phonons could remind electromagnons; however, their temperature
dependence is quite different. We have used density functional theory for
calculating phonon dispersion branches in the whole Brillouin zone. A detailed
analysis of these results and of previously published magnon dispersion
branches brought us to the conclusion that the observed absorption bands stem
from phonon-phonon and phonon- paramagnon differential absorption processes.
The latter is enabled by a strong short-range in-plane spin correlations in the
[show abstract][hide abstract] ABSTRACT: Terahertz time-resolved conductivity measurements supported by classical simulations of photocarrier motion provide length scales of the charge localization within nanocrystals and their clusters and characterize the conductive coupling between them on the ultrafast time scale.
Lasers and Electro-Optics (CLEO), 2011 Conference on; 06/2011
[show abstract][hide abstract] ABSTRACT: We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional Mn_Ga dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mn-doping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band. Comment: 22 pages, 14 figures
[show abstract][hide abstract] ABSTRACT: Terahertz spectroscopic measurements are usually performed in focused beam geometry while the standard routine for the retrieval of the sample refractive index assumes plane-wave approximation. In this paper we propose a model for the transmission function which accounts for spatially limited Gaussian terahertz beams. We demonstrate experimentally its validity and applicability for an accurate extraction of the refractive index from experimental data.
[show abstract][hide abstract] ABSTRACT: After decades of research, the low Curie temperature of ferromagnetic semiconductors remains the key problem in the development of magnetic semiconductor spintronic technologies. Removing this roadblock might require a change of the field's basic materials paradigm by looking beyond ferromagnets. Recent studies of relativistic magnetic and magnetotransport anisotropy effects, which in principle are equally well present in materials with ferromagnetically and antiferromagnetically ordered spins, have inspired our search for antiferromagnetic semiconductors suitable for high-temperature spintronics. Since these are not found among the magnetic counterparts of common III-V or II-VI semi- conductors, we turn the attention in this paper to high N \'eel temperature I-II-V magnetic compounds whose electronic structure has not been previously identified. Our combined experimental and theoretical work on LiMnAs provides basic prerequisite for the systematic research of this class of materials by demonstrating the feasibility to grow single crystals of group-I alkali metal compounds by molecular beam epitaxy, by demonstrating the semiconducting band structure of the I-Mn-V's, and by analyzing their spin-orbit coupling characteristics favorable for spintronics. Comment: 11 pages, 5 figures
[show abstract][hide abstract] ABSTRACT: We investigate 100 nm thick epitaxial monolayer of SrTiO <sub>3</sub> strained by 1% in biaxial tension and grown on (110) DyScO <sub>3</sub> . Spectra within the whole far-infrared spectral range are obtained as a function of temperature and an electric bias. An appreciable tunability of high frequency polar phonons is observed upon bias. The splitting of TO4 mode clearly observed below 220 K, which is further increased upon bias, is a signature of the ferroelectricity of the film at these temperatures.
Journal of Applied Physics 07/2010; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report an investigation of hexagonal Y_(1-x)Eu_xMnO_3 ceramics with
x=0, 0.1 and 0.2 using infrared and THz spectroscopies in the
temperature range between 5 and 900 K. The temperature dependence of the
THz permittivity reveals a kink near the antiferromagnetic phase
transition temperature T_N ~ 70 K giving evidence of a strong
spin-lattice coupling. Below T_N two absorption peaks were revealed in
the THz spectra close to 43 and 73 cm-1. While the first peak
corresponds to a sharp antiferromagnetic resonance exhibiting softening
on heating towards TN, the second one may be attributed to an impurity
mode or a multiphonon absorption peak. High-temperature THz spectra
measured up to 900 K reveal only small gradual increase of the
permittivity in agreement with a weak phonon softening observed in the
infrared reflectance spectra upon heating. This corresponds to an
improper ferroelectric character of the phase transition proposed from
first principle calculations by Fennie and Rabe [Phys. Rev. B 72 (2005),
[show abstract][hide abstract] ABSTRACT: Charge transport and recombination in nanostructured semiconductors are poorly understood key processes in dye-sensitized solar cells. We have employed time-resolved spectroscopies in the terahertz and visible spectral regions supplemented with Monte Carlo simulations to obtain unique information on these processes. Our results show that charge transport in the active solar cell material can be very different from that in nonsensitized semiconductors, due to strong electrostatic interaction between injected electrons and dye cations at the surface of the semiconductor nanoparticle. For ZnO, this leads to formation of an electron-cation complex which causes fast charge recombination and dramatically decreases the electron mobility even after the dissociation of the complex. Sensitized TiO2 does not suffer from this problem due to its high permittivity efficiently screening the charges.
[show abstract][hide abstract] ABSTRACT: Terahertz time-resolved spectroscopy is used to investigate the transport of photoexcited electrons in nanocrystalline mesoporous TiO <sub>2</sub> films prepared by the recently proposed “brick and mortar” technology [Szeifert etalChem. Mater. 21, 1260 (2009)] with a variable fraction of nanocrystalline titania “bricks” and amorphous titania “mortar.” Both long- and short-range conductivity is significantly enhanced upon calcination. After an ultrafast (subpicosecond) regime where the intrananograin conductivity dominates, the electron conductivity becomes limited by the interaction of electrons with the amorphous mortar. Comparison of the experimental results with Monte Carlo simulations of the electron motion allows us to determine the crystalline grain size after calcination and the yield of mobile photocarriers.
[show abstract][hide abstract] ABSTRACT: We present a design of dielectric metamaterials exhibiting a broad range of negative effective permeability in the terahertz spectral region. The investigated structures consist of an array of high-permittivity rods that exhibit a series of Mie resonances giving rise to the effective magnetic response. The spectral positions of resonances depend on the geometrical parameters of the rods and on their permittivity, which define the resonant confinement of the electromagnetic field within the rods. The electromagnetic coupling between the adjacent rods is negligible. With a suitable aspect ratio of the rods, a broadband magnetic response can be obtained.
[show abstract][hide abstract] ABSTRACT: The lattice dynamics of Pb <sub>1-x</sub> La <sub>x</sub>( Zr <sub>0.9</sub> Ti <sub>0.1</sub>) O <sub>3</sub> (PLZT X/90/10 ) with X=0 , 2, 4, 10% of La was studied by means of Far Infrared, Raman and terahertz spectroscopies in the temperature range of 20–800 K. Infrared active soft phonons with anomalies near the paraelectric-ferroelectric phase transition were found for all the samples. Some Raman active phonons show anomalies ∼200 K below T<sub> C </sub> , due to another phase transition to a ferroelectric state with doubled unit cell. Samples with higher La content (X=4,10) display nonclassical phonon softening described by ω<sup>2</sup>=a(T-T<sub> C </sub>)<sup>γ</sup> with γ=2/3 .
[show abstract][hide abstract] ABSTRACT: Using optical pump - THz probe experiments we studied a series of 1-mum-thick muc-Si:H samples with a variable degree of crystallinity deposited on sapphire substrates. The data provide a full picture of nanoscopic transport of photocarriers on a 100 fs-1 ns timescale.
[show abstract][hide abstract] ABSTRACT: We report on metamaterials which exhibit a tunable interval of negative effective permeability within 0.2-0.36 THz. They consist of a strontium titanate platelet, which shows a high and temperature tunable permittivity, with an array of laser engraved slits. The negative permeability then is due to a resonant confinement of the electromagnetic field within the rods. The range of negative permeability is determined by the geometry and the permittivity of the material used.
[show abstract][hide abstract] ABSTRACT: The dielectric response to infrared waves polarized along the tetragonal axis of a ferroelectric single-domain crystal of BaTiO3 was determined by time-domain THz spectroscopy and Fourier-transform infrared reflectivity techniques. In addition to the three well-known polar lattice modes, the experiment shows an additional mode of the relaxation type in the THz spectral region, which accounts for the Curie-Weiss behavior of the c-axis dielectric constant. A comparison of experimental results with ab initio based effective-Hamiltonian simulations allows us to elucidate its relation to the order-disorder model of Comes, Lambert, and Guinier [Solid State Commun. 6, 715 (1968)10.1016/0038-1098(68)90571-1].
[show abstract][hide abstract] ABSTRACT: The terahertz dielectric properties of several heterostructures based on epitaxial SrTiO<sub>3</sub> and DyScO<sub>3</sub> thin films were characterized without and under applied static or low-frequency electric field. We demonstrate up to 65% variation of the permittivity of SrTiO<sub>3</sub> films and up to 35 % modulation of the power transmission of terahertz waves at 500 GHz and 112 V (75 kV/cm) bias.
Lasers and Electro-Optics, 2008 and 2008 Conference on Quantum Electronics and Laser Science. CLEO/QELS 2008. Conference on; 06/2008
[show abstract][hide abstract] ABSTRACT: Transmittance of SrTiO3/DyScO3 thin film multilayers grown on DyScO3 crystals was studied using THz time-domain spectroscopy. By applying an electrical bias of the order of tens of kV/cm, the transmitted power was shown to vary by tens of per cent in a broad frequency range.