arXiv subjects

T. Mertelj

Publications and source records attributed to T. Mertelj.

49 records · Page 3Linked to original sources

arXiv2008

Single-particle and collective mode couplings associated with 1- and 2-directional electronic ordering in metallic RTe$_3$ (R = Ho, Dy, Tb)

The coupling of phonons with collective modes and single-particle gap excitations associated with one (1d) and two-directional (2d) electronically-driven charge-density wave (CDW) ordering in metallic RTe_3 is investigated as a function of rare-earth ion chemical pressure (R=Tb, Dy, Ho) using femtosecond pump-probe spectroscopy. From the T-dependence of the CDW gap Δ_{CDW} and the amplitude mode (AM) we find that while the transition to a 1d-CDW ordered state at T_{c1} initially proceeds in an exemplary mean-field (MF) -like fashion, below T_{c1}, Δ_{CDW} is depressed and departs from the MF behavior. The effect is apparently triggered by resonant mode-mixing of the amplitude mode (AM) with a totally symmetric phonon at 1.75 THz. At low temperatures, when the state evolves into a 2d-CDW ordered state at T_{c2} in the DyTe_3 and HoTe_3, additional much weaker mode mixing is evident but no soft mode is observed.

cond-mat.str-el↗

arXiv2008

Controlled vaporization of the superconducting condensate in cuprate superconductors sheds light on the pairing boson

We use ultrashort intense laser pulses to study superconducting state vaporization dynamics in La(2-x)Sr(x)CuO4 (x=0.1 and 0.15) on the femtosecond timescale. We find that the energy density required to vaporize the superconducting state is 2+- 0.8 K/Cu and 2.6 +- 1 K/Cu for x=0.1 and 0.15 respectively. This is significantly greater than the condensation energy density, indicating that the quasiparticles share a large amount of energy with the boson glue bath on this timescale. Considering in detail both spin and lattice energy relaxation pathways which take place on the relevant timescale of picoseconds, we rule out purely spin-mediated pair-breaking in favor of phonon-mediated mechanisms, effectively ruling out spin-mediated pairing in cuprates as a consequence.

cond-mat.supr-con↗

arXiv2007

Self-organization of charged particles on a 2D lattice subject to anisotropic Jahn-Teller-type interaction and 3D Coulomb repulsion

Self-organization of charged particles on a 2D lattice, subject to an anisotropic Jahn-Teller-type interaction and 3D Coulomb repulsion is investigated. In the mean-field approximation without Coulomb interaction, the system displays a phase transition of first order. In the presence of the Coulomb repulsion the global phase separation becomes unfavorable and the system shows a mesoscopic phase separation, where the size of the charged regions is determined by the competition between the ordering energy and the Coulomb energy. The phase diagram of the system as a function of particle density and temperature is obtained by systematic Monte Carlo simulations. With decreasing temperature a crossover from a disordered state to a state composed from mesoscopic charged clusters is observed. In the phase separated state charged clusters with even number of particles are more stable than those with odd number of particles in a large range of particle densities. With increasing particle density at low temperatures a series of crossovers between states with different cluster sizes is observed. Above half filling in addition to the low temperature clustering another higher temperature scale, which corresponds to orbital ordering of particles, appears. We suggest that the diverse functional behaviour - including superconductivity - observed in transition metal oxides can be thought to arise from the self-organization of this type.

cond-mat.str-el↗

arXiv2005

Charged particles on a 2D plane subject to anisotropic Jahn-Teller interactions

The properties of a system of charged particles on a 2D lattice, subject to an anisotropic Jahn-Teller-type interaction and 3D Coulomb repulsion are investigated. In the mean-field approximation without Coulomb interaction, the system displays a phase transition of first order. When the long range Coulomb interaction is included, Monte Carlo simulations show that the system displays very diverse mesoscopic textures, ranging from spatially disordered pairs to ordered arrays of stripes, or charged clusters, depending only on the ratio of the two interactions (and the particle density). Remarkably, charged objects with even number of particles are more stable than with odd number of particles. We suggest that the diverse functional behaviour - including superconductivity - observed in oxides can be thought to arise from the self-organization of this type.

cond-mat.str-el↗

arXiv2004

Mesoscopic Phase Separation in the Model with Competing Jahn-Teller and Coulomb Interaction

We have derived the interaction between Jahn-Teller centers due to optical and acoustic phonons. Without Coulomb repulsion the model leads to a global phase separation at low temperature. At long distances Coulomb repulsion always dominates the short range attraction leading to phase separation on a short scale. On the basis of Monte-Carlo simulation of microscopic lattice-gas model we have formulated an effective phenomenological model. In the limit of low density of polarons, phase separation takes place in the form of charged bubbles. In the limit of high density, charge segregation occurs on domain walls.

cond-mat.dis-nn↗

arXiv2004

Hole interactions with molecular vibrations on DNA

We report on a study of the interactions between holes and molecular vibrations on dry DNA using photoinduced infrared absorption spectroscopy. Laser photoexcited (PE) holes are found to have a room-temperature lifetime in excess of 1 ms, clearly indicating the presence of localization. However, from a quantitative model analysis of the frequency shifts of vibrational modes caused by the PE holes, we find the holevibrational coupling constant to be relatively small, 0.2. This interaction leads to a change in the conformational energy of 0.015 eV, which is too small to cause selftrapping at room temperature. We conclude that, at least in the dry (A) form, DNA is best understood in terms of a double chain of coupled quantum dots arising from the pseudo-random chain sequence of base pairs, in which Anderson localization prevents the formation of a metallic state.

cond-mat.soft↗

arXiv2003

Phase Transitions in Mesoscopic Superconducting Films

We solve the Ginzburg-Landau equation (GLE) for the mesoscopic superconducting thin film of the square shape in the magnetic field for the wide range of the Ginzburg-Landau parameter $0.05<κ_{eff}<\infty $. We found that the phase with the antivortex exists in the broad range of parameters. When the coherence length decreases the topological phase transition to the phase with the same total vorticity and a reduced symmetry takes place. The giant vortex with the vorticity $m=3$ is found to be unstable for any field, $ξ/a$ and $κ_{eff}\ge 0.1$. Reduction of $ κ_{eff}$ does not make the phase with antivortex more stable contrary to the case of the cylindric sample of the type I superconductor.

cond-mat.supr-con↗

arXiv2003

Ultrafast photoinduced reflectivity transients in $(Nd_{0.5}Sr_{0.5})MnO_3$

The temperature dependence of ultrafast photoinduced reflectivity transients is reported in Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin film. The photoinduced reflectivity shows a complex response with very different temperature dependences on different timescales. The response on the sub-ps timescale appears to be only weakly sensitive to the 270K-metal-insulator phase transition. Below $\sim 160$ K the sub-ps response displays a two component behavior indicating inhomogeneity of the film resulting from the substrate induced strain. On the other hand, the slower response on the 10-100 ps timescale is sensitive only to the metal-insulator phase transition and is in agreement with some previously published results. The difference in the temperature dependences of the responses on nanosecond and $μ$s timescales indicates that thermal equilibrium between the different degrees of fredom is established relatively slowly - on a nanosecond timescale.

cond-mat↗

arXiv2002

Vortex-antivortex configurations and their stability in mesoscopic superconducting square

We solve the Ginzburg-Landau equation (GLE) for the mesoscopic superconducting thin film of the square shape in the magnetic field for the wide range of the Ginzburg-Landau parameter 0.05< kappa. We focus on the region of the field where formation of the antivortex has been reported previously. We found that the phase with the antivortex exists in the broad range of parameters. When the coherence length decreases the topological phase transition to the phase with the same total vorticity and a reduced symmetry takes place. The giant vortex with the vorticity m=3 is found to be unstable for any field, xi/a and kappa. Reduction of kappa does not make the phase with antivortex more stable contrary to the case of the cylindric sample of the type I superconductor.

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

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↗

arXiv1998

a-b plane optical conductivity in YBa_2Cu_3O_{7-delta} above and below T*

Analysis of the a-b plane optical conductivity $σ_{ab}$ for both twinned and untwinned YBa$_{2}$Cu$_{3}$O$_{7-δ}$ as a function of temperature and doping shows that below a well defined temperature T*, a dip in the spectrum systematically appears separating the infrared charge excitation spectrum into two components with distinct energy scales. The change from monotonic behaviour in $σ_{ab}$ is found to be concurrent with the onset of phonon anomalies in Raman and infrared spectra below T*. The optical data are suggested to be evidence for the appearance of an inhomogeneous distribution of carriers rather than the opening of a simple gap for charge excitations below T*, an interpretation wich is consistent with recent angle-resoved photoemission and electronic Raman spectra. We find that the behaviour below T* and the absence of any anomalies at Tc can be interpreted assuming a Bose-Einstein condensation of preformed pairs.

cond-mat.supr-con↗

arXiv1997

A study of photoexcited carrier relaxation in YBa_2Cu_3O_(7-d) by picosecond resonant Raman spectroscopy

The temperature dependence of the energy relaxation of photoexcited (PE) carriers is used as a probe of the electronic structure of YBa_2Cu_3O_(7-d) in the insulating d~0.8 and metallic d~0.1 phases. The energy relaxation rate to phonons is obtained by measuring the non-equilibrium phonon occupation number, n_neq, with pulsed Raman Stokes/anti-Stokes spectroscopy using 1.5 and 70 ps long laser pulses. We can distinguish between relaxation via extended band states and localized states, since theoretically in the former, the relaxation is expected to be virtually T-independent, while in the latter it is strongly T-dependent. From the experiment - which shows strong temperature dependence of n_neq - we deduce that at least part of the PE carrier relaxation proceeds via hopping between localized states and we propose a simple theoretical model of the relaxation process. In addition, we compare the coupling of different vibrational modes to the carriers to find that the apical O vibrational mode is significantly more involved in the energy relaxation process that the in-plane 340 cm^(-1}) mode. This implies that the localized states are mainly (but not entirely) coupled to out-of plane vibrations.

cond-mat.supr-con↗