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D. Mihailovic

Publications and source records attributed to D. Mihailovic.

At least 73 records · Page 4Linked to original sources
arXiv2002

Femtosecond quasiparticle relaxation dynamics and probe polarization anisotropy in YBa2Cu4O8

Femtosecond pump probe experiments are reported on quasiparticle relaxation and recombination in YBa2Cu4O8 as a function of temperature and polarization. The data show a 2-component relaxation similar to YBa2Cu3O7-δ, one component being assocated with the superconducting transition, and the other with the pseudogap below T* The relaxation time tau_p associated with the pseudogap is found to be T-independent, while the relaxation time tau_g of the component obseved only below Tc exhibits a clear divergence near Tc. A strong polarisation anisotropy of the picosecond transient is observed below Tc which is attributed to the anisotropy of the probe transition matrix elements.

cond-mat.supr-con↗
arXiv2002

Spatial and temporal probes in inhomogeneous systems: Theory and experiment

The experimental and theoretical challenges posed by the study of dynamically inhomogeneous systems are outlined in the context of cuprates and other oxides. Considering the pitfalls in the single-component approach to the analysis of inhomogeneous systems, the effect of either temporal or spatial averaging by different experiments is discussed. A group-theoretical symmetry analysis of the observed inhomogeneities in real space and k-space observed in the cuprates is shown to lead to a quantitatively verifiable description of the inhomogeneous state, comprising of bound singlet pairs in the ground state and unbound fermions in the excited state. The predicted symmetry breaking associated with pairing is shown to be verifiable experimentally.

cond-mat.supr-con↗
arXiv2002

Femtosecond "snapshots "of gap-forming charge-density-wave correlations in quasi-two-dimensional dichalcogenides 1T-TaS2 and 2H-TaSe2

Time-resolved optical spectroscopy of collective and single-particle excitations of 1T-TaS2 and 2H-TaSe2 reveals the presence of a large gap in the excitation spectrum on the femtosecond timescale, associated with the formation of various degrees of CDW order. In common with superconducting cuprates, excitations with energies less than the full gap show much slower relaxation. This separation of timescales cannot be explained in a quasi-2D Fermi-Liquid picture with an anisotropic gap but rather suggests the formation of a fluctuating spatially inhomogeneous state eventually forming a long-range ordered state at low temperatures.

cond-mat.str-el↗
arXiv2001

Electrons on the double helix: optical experiments on native DNA

Optical experiments on calf thymus DNA films subjected to different buffer environments are reported. The optical conductivity is that of a disordered or lightly doped semiconductor with a well-defined band-gap for charge excitations and low frequency transport determined by a small number of strongly localized electron states.

cond-mat.dis-nn↗
arXiv2001

The attainable superconducting Tc in a model of phase coherence by percolation

The onset of macroscopic phase coherence in superconducting cuprates is considered to be determined by random percolation between mesoscopic Jahn-Teller pairs, stripes or clusters. The model is found to predict the onset of superconductivity near 6% doping, maximum Tc near 15% doping and Tc= T* at optimum doping, and accounts for the destruction of superconductivity by Zn doping near 7%. The model also predicts a relation between the pairing (pseudogap) energy and Tc in terms of experimentally measurable quantities.

cond-mat.supr-con↗
arXiv2000

Quasiparticle dynamics and gap structure in Hg1223 investigated with femtosecond spectroscopy

Measurements of the temperature dependence of the quasiparticle (QP) dynamics in Hg1223 with femtosecond time-resolved optical spectroscopy are reported. From the temperature dependence of the amplitude of the photoinduced reflection, the existence of two gaps is deduced, one temperature dependent Dc that closes at Tc, and another temperature independent ''pseudogap'' Dp. The zero-temperature magnitudes of the two gaps are Dc/kTc = 6 +/- 0.5 and Dp/kTc = 6.4 +/- 0.5 respectively. The quasiparticle lifetime is found to exhibit a divergence as T -> Tc from below, which is attributed to the existence of a superconducting gap which closes at Tc. Above Tc the relaxation time is longer than expected for metallic relaxation, which is attributed to the presence of the ''pseudogap''. The QP relaxation time is found to increase significantly at low temperatures. This behavior is explained assuming that at low temperatures the relaxation of photoexcited quasiparticles is governed by a bi-particle recombination process.

cond-mat.supr-con↗
arXiv2000

Finite-wavevector Jahn-Teller-pairing and superconductivity in the cuprat es

A model interaction is proposed in which pairing is caused by a non-local Jahn-Teller (JT) -like instability due to the coupling between planar O states and knot equal to 0 phonons. Apart from pairing, the interaction is found to naturally allow metallic stripe formation. The consequences of the model for superconductivity in the cuprates are discussed. The model is shown to be consistent with numerous sets of experimental data in quite some detail.

cond-mat.supr-con↗
arXiv2000

Finite-wavevector phonon coupling to degenerate electronic states in LaSrCuO

We consider finite-k electron-phonon coupling appropriate for a superconductor with a very short coherence length and local intersite pairs. From group theoretical analyis we find that phonons with nonzero k can couple to degenerate electronic states in LaSrCuO and a Jahn-Teller-like distortion is found to be operative for E_u and E_g -symmetry O states. Experimentally observed anomalies in inelastic neutron scattering, ESR and EXAFS are found to be consistent with such an interaction of the tau_1 symmetry Sigma point mode with the planar O p_x and p_y states. The proposed interaction is found to naturally facilitate symmetry breaking associated with pairing and/or stripe formation.

cond-mat.supr-con↗
arXiv2000

Photoinduced absorption from localized intra-gap states

A model is developed for photoinduced absorption from localized states observed in femtosecond pump-probe experiments in high-Tc superconductors and other materials. The dynamics of localized carriers are described in terms of phenomenological approach similar to that originaly proposed by Rothwarf and Taylor. Expanding the relaxation rate in powers of the order parameter we have shown that density of localized carriers is sensitive to Tc. From the analysis of the experimental data on YBa2Cu3O(7-x) and K0.3MoO3 we conclude that significant intra-gap density of localized states exists in these materials. Temperature dependence of the density of photoexcited localized carriers in underdoped YBa2Cu3O(7-x) and in K0.3MoO3 is consistent with the observation of the pseudogap above Tc.

cond-mat.supr-con↗
arXiv2000

Quasiparticle relaxation dynamics in Hg-1223 studied by femtosecond time-resolved optical spectroscopy

Following recent progress in understanding the relaxation dynamics of photoexcited carriers in materials exhibiting a small gap in the low-energy excitation spectrum we have performed pump-probe measurements on near optimally doped Hg-1223. We show that the behavior is very similar as in optimally-doped YBCO, where the data can be interpreted with the coexisting presence of two energy gaps: normal state T-independent pseudogap and a mean-field-like collective gap, associated with intrinsic spatially inhomogeneous ground state. An important difference between the two compounds is found in the low temperature relaxation time, which in Hg-1223 is found to be strongly temperature and photoexcitation intensity dependent.

cond-mat.supr-con↗
arXiv2000

Quasiparticle relaxation dynamics in cuprates and lifetimes of low-energy states: Femtosecond data from underdoped to overdoped YBCO and mercury compounds

We show that low-energy spectral features in the cuprates can be separated into different components by the measurement of the recombination dynamics of different low-energy excitations in real-time using femtosecond laser spectroscopy. Quasiparticle (QP) recombination across the gap and intra-gap localized state relaxation processes exhibit qualitatively different time- and temperature-dependences. The relaxation measurements also show the existence of two distinct coexisting energy gaps near optimum doping and in the overdoped region, one more or less temperature independent (which exists above and below Tc) and one which closes at Tc in a mean-field like fashion. Thus systematic studies of QP recombination as a function of doping and temperature suggest that the ground state of the cuprates is a mixed Boson-Fermion system with localised states present over the entire region of the phase diagram.

cond-mat.supr-con↗
arXiv2000

Direct measurement of polaron binding energy in AMnO$_{3}$ as a function of the A site ionic size by photoinduced IR absorption

Photoinduced IR absorption was measured in undoped (LaMn)_{1-δ}O_{3} and (NdMn)_{1-δ}O_{3}. We observe broadening and a ~44% increase of the midinfrared anti-Jahn-Teller polaron peak energy when La^{3+} is replaced with smaller Nd^{3+}. The absence of any concurent large frequency shifts of the observed PI phonon bleaching peaks and the Brillouin-zone-center internal perovskite phonon modes measured by Raman and infrared spectroscopy indicate that the polaron peak energy shift is mainly a consequence of an increase of the electron phonon coupling constant with decreasing ionic radius on the perovskite A site. This indicates that the dynamical lattice effects strongly contribute to the electronic band narrowing with decreasing in doped giant magnetoresistance manganites.

cond-mat↗
arXiv1999

Carrier relaxation dynamics in intra-gap states: the case of the superconductor Y Ba_2 Cu_3O_7-delta and the charge-density-wave semiconductor K_0.3MoO_3

The unusual slow carrier relaxation dynamics - observed in femtosecond pump-probe experiments on high-temperature superconductors and recently also in a charge-density-wave system - is analyzed in terms of a model for relaxation of carriers in intra-gap states. The data on YBa_2Cu_3O_7-delta near optimum doping and K_0.3$MoO_3 are found to be described very well with the model using a BCS-like gap which closes at T_c. From the analysis of the data we conclude that a significant intra-gap density of localized states exists in these materials, which can be clearly distinguished from quasiparticle states by the time-resolved optical experiments because of the different time- and temperature-dependences of the photoinduced transmission or reflection. Localized charges are suggested to be the most likely origin of the intra-gap states, while the similarity of the response in the two materials appear to exclude spin and vortex excitations.

cond-mat.supr-con↗
arXiv1999

Photoinduced IR absorption in (La(1-x)Sr(x)Mn)(1-δ)O3: changes of the anti-Jahn-Teller polaron binding energy with doping

Photoinduced IR absorption was measured in (La(1-x)Sr(x)Mn)(1-δ)O3. A midinfrared peak centered at ~ 5000 cm$^{-1}$ was observed in the x=0 antiferromagnetic sample. The peak diminishes and softens as hole doping is increased. The origin of the photoinduced absorption peak is atributted to the photon assisted hopping of anti-Jahn-Teller polarons formed by photoexcited charge carriers, whose binding energy decreases with increasing hole doping. The shape of the peak indicates that the polarons are small.

cond-mat↗
arXiv1999

Distinct charge and spin gaps in underdoped YBa2Cu3O7-x

A systamatic quantitative comparison of ''pseudogap'' values obtained from the analysis of charge and spin excitation spectroscopies in underdoped YBa2Cu3O7-x using a temperature-independent gap shows two distinct excitations, one visible in spin-flip spectroscopies like NMR and spin polarized neutron scattering, and the other in charge excitation spectoscopies like single particle tunneling and time-resolved quasiparticle relaxation. Both appear to decrease with doping x approximately as 1/x and are T-independent, existing above and below Tc. We suggest that the charge excitation can be attributed to a pair-breaking local gap, while the spin excitation can be explained by an intra-gap local triplet state.

cond-mat.supr-con↗
arXiv1999

Low-energy electronic structure in Y1-xCaxBa2Cu3O7-y comparison of t ime-resolved optical spectroscopy, NMR, neutron and tunneling data

Time-resolved optical measurements give information on the quasiparticle relaxation dynamics in YBCO, from which the evolution of the gap with doping and temperature can be systematically deduced. In this paper these optical charge-channel `pseudogap' data are compared with the `pseudogap' obtained from the NMR Knight shift Ks, spin polarized neutron scattering (SPNS) and single particle tunneling measurements. A simple energy level diagram is proposed to explain the different `gap' magnitudes observed by different spectroscopies in Y1-xCaxBa2Cu3O7-y, whereby the spin gap Delta_s in NMR and SPNS corresponds to a triplet local pair state, while Delta_p in the charge excitation spectrum corresponds to the pair dissociation energy. At optimum doping and in the overdoped state, an additional T-dependent gap becomes evident, which closes at T_c, suggesting a cross-over to a more conventional BCS-like superconductivity scenario.

cond-mat.supr-con↗
arXiv1999

Single particle and collective excitations in the one-dimensional charge density wave solid K0.3MoO3 probed in real time by femtosecond spectroscopy

Ultrafast transient reflectivity changes caused by collective and single particle excitations in the quasi one-dimensional charge-density wave (CDW) semiconductor K0.3MoO3 are investigated with optical pump-probe spectroscopy. The temperature-dependence of non-equilibrium single particle excitations across the CDW gap and their recombination dynamics are reported for the first time. In addition, amplitude mode reflectivity oscillations are observed in real time. A T-dependent overdamped response is also observed which is attributed to relaxation of the phason mode.

cond-mat.str-el↗