Publications (54)117.37 Total impact
 arXiv. 10/2014;
 [Show abstract] [Hide abstract]
ABSTRACT: With xray absorption spectroscopy we investigated the orbital reconstruction and the induced ferromagnetic moment of the interfacial Cu atoms in YBa$_2$Cu$_3$O$_{7}$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (YBCO/LCMO) and La$_{2x}$Sr$_{x}$CuO$_4$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LSCO/LCMO) multilayers. We demonstrate that these electronic and magnetic proximity effects are coupled and are common to these cuprate/manganite multilayers. Moreover, we show that they are closely linked to a specific interface termination with a direct CuOMn bond. We furthermore show that the intrinsic hole doping of the cuprate layers and the local strain due to the lattice mismatch between the cuprate and manganite layers are not of primary importance. These findings underline the central role of the covalent bonding at the cuprate/manganite interface in defining the spinelectronic properties.10/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We report on results of our analysis of published experimental data of the inplane infrared response of two representative underdoped high$T_{\mathrm{c}}$ cuprate superconductors, focusing on a characteristic gap feature in the spectra of the real part of the conductivity and the corresponding structures of the memory function, that develop below a temperature $T^{\mathrm{ons}}$ considerably higher than $T_{\mathrm{c}}$. Several arguments based on comparisons of the data with results of our calculations are provided indicating that the features are due to superconductivity and that $T^{\mathrm{ons}}$ marks the onset of a precursor superconducting phase.01/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We studied the doping and temperature (T) dependence of the infrared (IR) response of Ba(Fe_{1x}Co_{x})_{2}As_{2} single crystals. We show that a weak band around 1000 cm^{−1}, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to lowenergy interband transitions. Specifically, we show that this band exhibits a similar doping and T dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes as a function of doping near the critical point where superconductivity is suppressed in the overdoped regime. Our IR data thus provide bulk specific information (complementary to the surface sensitive ARPES) for a Lifshitz transition. Our IR data also reveal a second lowenergy band around 2300 cm^{−1} which further emphasizes the necessity to consider the multiband nature of these iron arsenides in the analysis of the optical response.Physical Review B 11/2013; 88(18). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The electronic structure of InAs quantum dots covered with the GaAs(1y)Sb(y) strain reducing layer has been studied using the k.p theory. We explain previous experimental observations of the red shift of the photoluminescence emission with increasing y and its blue shift with increasing excitation power. For y>0.19 typeII dots are formed with holes localized in the GaAsSb close to the dot base; two segments at opposite sides of the dot, forming molecularlike states, result from the piezoelectric field. We also propose an experiment that could be used to identify the hole localization using a vertical electric field.Applied Physics Letters 09/2012; 97. · 3.52 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The pumping intensity (I) dependence of the photoluminescence (PL) spectra of perfectly laterally twodimensionally ordered SiGe quantum dots on Si(001) substrates was studied. The PL results from recombinations of holes localized in the SiGe quantum dots and electrons localized due to the strain field in the surrounding Si matrix. The analysis of the spectra revealed several distinct bands, attributed to phononassisted recombination and nophonon recombination of the excitonic ground state and of the excited excitonic states, which all exhibit a linear I dependence of the PL intensity. At approximately I>3W/cm^2, additional bands with a nearly quadratic I dependence appear in the PL spectra, resulting from biexcitonic transitions. These emerging PL contributions shift the composite nophonon PL band of the SiGe quantum dots to higher energies. The experimentally obtained energies of the nophonon transitions are in good agreement with the exciton and biexciton energies calculated using the envelope function approximation and the configuration interaction method.Physical review. B, Condensed matter 09/2012; 86(11). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The formula frequently used to describe the caxis infrared response of the coupled electronphonon system of bilayer cuprate superconductors and providing important insights into the physics of these materials has been originally obtained at the level of the phenomenological multilayer model. Here we derive it using diagrammatic perturbation theory.Physical review. B, Condensed matter 03/2012; 86(1). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that a multilayer analysis of the infrared caxis response of RBa2Cu3O(7δ) (R=Y, Gd, Eu) provides important new information about the anomalous normalstate properties of underdoped cuprate high temperature superconductors. In addition to competing correlations which give rise to a pseudogap that depletes the lowenergy electronic states below T*≫T(c), it enables us to identify the onset of a precursor superconducting state below T(ons)>T(c). We map out the doping phase diagram of T(ons) which reaches a maximum of 180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions.Physical Review Letters 01/2011; 106(4):047006. · 7.73 Impact Factor 
Article: Modelling of electronic states in InAs/GaAs quantum dots with GaAsSb strain reducing overlayer
[Show abstract] [Hide abstract]
ABSTRACT: We present results of our 8band calculations of the emission energy of InAs/GaAs quantum dots (QDs) covered with GaAs1−xSbx strain reducing overlayer (SRO). In agreement with previous experimental observations we find a strong red shift of the emission with increasing Sb content. We explain this effect by: (1) The lowering of the valence band offset between the QD and the SRO with increasing Sb content resulting in the typeII QDs with holes confined in the SRO for Sb concentration above 14%. (2) The reduction of compressive strain inside the QDs. The contributions of these mechanisms to the total red shift are estimated and compared. For realistic shape and size of the QD and a realistic value of the SRO thickness the previously measured photoluminescence data are reproduced with fairly good accuracy.Journal of Physics Conference Series 09/2010; 245(1):012086.  [Show abstract] [Hide abstract]
ABSTRACT: The lowtemperature spectra of the caxis infrared conductivity of bilayer highTc cuprate superconductors (HTCS) exhibit two superconductivityinduced modes [Li Yu et al., Phys. Rev. Lett. 100 (2008) 177004; and references therein]. Both can be understood in terms of a microscopic theory developed recently [J. Chaloupka, C. Bernhard, D. Munzar, Phys. Rev. B 79 (2009) 184513]. Here we summarize the elements of the theory and report on the temperature dependence (TD) of the lowenergy mode and of the total optical spectral weight (SW). The calculated TD of the mode is consistent with experiment but the trends of the SW are not.Physica C Superconductivity 01/2010; · 0.72 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the excitonic structure of pairs of GaAs/AlGaAs quantum dots forming lateral molecules and obtain the entanglement of exciton states. The following advantages of the lateral geometry over the vertical one are found: (1) The energy structures of the dots forming a molecule can be in principle identical. (2) Comparable tunneling of electrons and holes ensures a high entanglement of antisymmetric excitons. A drawback of existing structures are very low tunneling energies, which make the entanglement vulnerable against differences in the sizes and shapes of both dots.Journal of Physics Conference Series 01/2010; 245(1).  [Show abstract] [Hide abstract]
ABSTRACT: We report on results of our theoretical study of the caxis infrared conductivity of bilayer highTc cuprate superconductors using a microscopic model involving the bilayersplit (bonding and antibonding) bands. An emphasis is on the gaugeinvariance of the theory, which turns out to be essential for the physical understanding of the electrodynamics of these compounds. The description of the optical response involves local (intrabilayer and interbilayer) current densities and local conductivities. The local conductivities are obtained using a microscopic theory, where the quasiparticles of the two bands are coupled to spin fluctuations. The coupling leads to superconductivity and is described at the level of generalized Eliashberg theory. Also addressed is the simpler case of quasiparticles coupled by a separable and nonretarded interaction. The gauge invariance of the theory is achieved by including a suitable class of vertex corrections. The resulting response of the model is studied in detail and an interpretation of two superconductivityinduced peaks in the experimental data of the real part of the caxis conductivity is proposed. The peak around 400/cm is attributed to a collective mode of the intrabilayer regions, that is an analogue of the BogolyubovAnderson mode playing a crucial role in the theory of the longitudinal response of superconductors. For small values of the bilayer splitting, its nature is similar to that of the transverse plasmon of the phenomenological Josephson superlattice model. The peak around 1000/cm is interpreted as a pair breakingfeature that is related to the electronic coupling through the spacing layers separating the bilayers.Physical review. B, Condensed matter 03/2009; · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present broadband infrared ellipsometry measurements of the caxis conductivity of underdoped RBa_{2}Cu_{3}O_{7delta} (R=Y, Nd, and La) single crystals. Our data show that separate energy scales are underlying the redistributions of spectral weight due to the normal state pseudogap and the superconducting gap. Furthermore, they provide evidence that these gaps do not share the same electronic states and do not merge on the overdoped side. Accordingly, our data are suggestive of a two gap scenario with a pseudogap that is likely extrinsic with respect to superconductivity.Physical Review Letters 05/2008; 100(17):177004. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present broadband infrared ellipsometry measurements of the caxis dielectric response of underdoped YBa2Cu3O7−d single crystals. Our data provide a clear spectroscopic distinction between the normalstate pseudogap and the superconducting gap. In particular, they establish that different energy scales are underlying the respective redistributions of spectral weight. Furthermore, our data are suggestive of a mutual competition between the two gaps and thus of an extrinsic nature of the pseudogap with respect to superconductivity.Journal of Physics and Chemistry of Solids 01/2008; 69(12):30643069. · 1.53 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report on broadband infrared ellipsometry measurements of the caxis conductivity of underdoped RBa_{2}Cu_{3}O_{7d} (R=Y, Nd, and La) single crystals. Our data provide a detailed account of the spectral weight (SW) redistributions due to the normal state pseudogap (PG) and the superconducting (SC) gap. They show that these phenomena involve different energy scales, exhibit distinct doping dependencies and thus are likely of different origin. In particular, the SW redistribution in the PG state closely resembles the one of a conventional charge or spin density wave (CDW or SDW) system.06/2007;  [Show abstract] [Hide abstract]
ABSTRACT: We report on farinfrared ellipsometric measurements of Si:P with the phosphorus concentration at the metalinsulator (MI) transition, for temperatures from 15 to 300 K in the 50600 cm1 spectral range. Temperature coefficients of the complex conductivity have been measured with high resolution; they reveal a nontrivial evolution of the optical response.04/2007;  [Show abstract] [Hide abstract]
ABSTRACT: Eppenga's sixband k · p model for the description of the valence band structure of GaAs/AlAs quantum wells is modified for the k⟂ = 0 states in GaAs/AlAs superlattices. The model takes into account the coupling between the light holes and the spin—orbit splitoff holes and yields a very good agreement between experimental and calculated values of the splitting between the first heavy and light hole energy levels, which is not obtained in the simple effective mass approximation.Eppenga's sechsBand k · p Modell für die Beschreibung der Valenzbandstruktur der GaAs/AlAs Quantengruben wird für die k⟂ = 0 Zustnde im GaAs/AlAs Supergitter modifiziert. Das Modell zieht die Wechselwirkung zwischen den leichten Löchern und den Spin—Bahnabgespaltenen Löchern in Betracht und gibt ausgezeichnete Übereinstimmung zwischen den gemessenen und berechneten Werten der Aufspaltung zwischen dem ersten Energieniveau der leichten Löcher und dem ersten Energieniveau der schweren Löcher. Diese Übereinstimmung wird im Rahmen des einfachen Modells der effektiven Massen nicht erreicht.physica status solidi (b) 02/2006; 175(2):395  401. · 1.49 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Changes of the 400cm−1 peak in the caxis conductivity of underdoped YBa2Cu3O6.6 upon application of a parallel magnetic field reported by Kojima et al. are shown to be consistent with the model where the peak is due to the superfluid. Results of our calculations of the caxis response of bilayer compounds with well defined bilayer–split bands are presented and discussed. For moderate values of the bilayer splitting (Δε comparable to 2Δmax) the spectra of the superconducting state exhibit an additional mode which is due to the condensate and similar to the one of earlier phenonomenological approaches.Journal of Physics and Chemistry of Solids 01/2006; 67(1):308311. · 1.53 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The inplane infrared response of the high Tc cuprate superconductors was studied using the spinfermion model, where charged quasiparticles of the copperoxygen planes are coupled to spin fluctuations. First, we analyzed structures of the superconductingstate conductivity reflecting the coupling of the quasiparticles to the resonance mode observed by neutron scattering. The conductivity sigma computed with the input spin susceptibility in the simple form of the mode exhibits two prominent features: an onset of the real part of sigma starting around the frequency omega0 of the mode and a maximum of a related function W (omega) , roughly proportional to the second derivative of the scattering rate [1/tau] (omega) , centered approximately at omega= omega0 + Delta0 /ℏ , where Delta0 is the maximum value of the superconducting gap. The two structures are wellknown from earlier studies. Their physical meaning, however, has not been sufficiently elucidated thus far. Our analysis involving quasiparticle spectral functions provides a clear interpretation. Second, we explored the role played by the spinfluctuation continuum, whose spectral weight is known to be much larger than the one of the mode. We have shown that the experimental spectra of 1/tau can be approximately reproduced by augmenting the resonantmode component of the spin susceptibility by a suitable continuum component with a considerably higher spectral weight and with a characteristic width of several hundreds meV. The computed spectra of 1/tau display a new structure in the midinfrared which is related to the finite width of the occupied part of the conduction band. Third, we investigated the temperature dependence (TD) of sigma assuming that the normal state spin susceptibility consists of an overdamped low energy mode and the continuum component. The differences between the experimental normalstate spectra and those of the superconducting state, including some interesting effects at higher frequencies, are reasonably wellreproduced. Motivated by recent experimental (ellipsometric) works by Molegraaf and coworkers [H. J. A. Molegraaf et al., Science 295, 2239 (2002)] and Boris and coworkers [A. V. Boris et al., Science 304, 708 (2004)], we further studied the TDs of the effective kinetic energy (KE) and of the intraband spectral weight IO . Calculations for the trivial case of noninteracting quasiparticles in the normal state and a BCSlike superconducting state reveal a strong sensitivity of the TD of IO to details of the dispersion relation. The TDs of KE and IO in the interacting case, for the set of the values of the input parameters used throughout this work, are similar to those of the trivial case. The physics beyond the changes occurring when going from the normal to the superconducting state, however, is shown to be more complex, involving, besides the formation of the gap, also a feedback effect of the spin fluctuations on the quasiparticles and a significant shift of the chemical potential.Physical review. B, Condensed matter 10/2005; 72(13). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Changes of the 400 cm^{1} peak in the caxis conductivity of underdoped YBa2Cu3O6.6 upon application of a parallel magnetic field reported by Kojima et al. are shown to be consistent with the model where the peak is due to the superfluid. Results of our calculations of the caxis response of bilayer compounds with well defined bilayersplit bands are presented and discussed. For moderate values of the bilayer splitting (Delta E comparable to 2Delta_{max}) the spectra of the superconducting state exhibit an additional mode which is due to the condensate and similar to the one of earlier phenomenological approaches.09/2004;
Publication Stats
401  Citations  
117.37  Total Impact Points  
Top Journals
Institutions

1995–2014

Masaryk University
 • Department of Condensed Matter Physics
 • Faculty of Science
Brünn, South Moravian, Czech Republic


2008–2011

Université de Fribourg
 Département de Physique
Fribourg, FR, Switzerland


1998–2003

Max Planck Institute for Solid State Research
Stuttgart, BadenWürttemberg, Germany


1994–1995

Aarhus University
 Department of Physics and Astronomy
Aars, Region North Jutland, Denmark
