Solid State Commun Research Articles

Pressure-induced structural phase transition in rare-earth trihydrides. Part II. SmH 3 and compressibility systematics

High-pressure studies of samarium trihydride have been performed in a diamond anvil cell at pressures up to 30 GPa at room temperature. As a result, a reversible structural phase transformation from the hexagonal to the cubic phase has been observed. The transition pressure and the lattice parameter follow the tendency of the other lanthanide trihydrides investigated earlier [T. Palasyuk, M. Tkacz, Solid State Commun. 130 (2004) 219; T. Palasyuk, M. Tkacz, Solid State Commun. 133 (2005) 477; T. Palasyuk, M. Tkacz, Solid State Commun. 133 (2005) 481]. The compressibility systematics of lanthanide and yttrium trihydrides has been established.

Electrical transport properties of individual disordered multiwalled carbon nanotubes

The electrical transport properties of individual disordered multiwalled carbon nanotubes (MWCNTs), synthesized with the floating catalyst method, have been investigated. The resistivity of such disordered MWCNTs shows T1∕2 dependence over a temperature range of 4.2–263K. The experimental finding can be interpreted in terms of Al’tshuler-Aronov model [Solid State Commun. 30, 115 (1979); Sov. Phys. JETP 50, 968 (1979); Pis’ma. Zh. Eksp. Teor. Fiz. 27, 700 (1978)], in which strong electron-electron interaction leads to a Coulomb gap in the density of electronic states at the Fermi level. Such a wide fitting range of temperature for T1∕2 dependence has never been reported for other materials, implying an extremely short carrier scattering time of the order of femtoseconds.

Light-induced metastable states in ferroelectric oxides

New Raman scattering lines (at 463cm−1 and at 156cm−1) induced by strong enough optical pumping in nominally pure KTaO3 crystals are manifested. The model of such effect is proposed. This model is based on the light-induced formation of metastable polar clusters constructed from bi-polaronic excitons – Charge Transfer Vibronic Excitons (CTVEs) with their high degree alignment. The CTVEs are caused by photo-carriers with high local concentration which are trapped to local potential wells related with long-range defect fields. CTVE formation are realized in these potential wells due to significant easing of charge transfer fluctuations induced by photo-carrier screening effects. This model is effective also for explanation of giant dielectric constant inducing by strong illumination which was detected recently in KTaO3 and SrTiO3 by Japanese investigators [M. Takesada, T. Yagi, M. Itoh, S. Koshihara, J. Phys. Soc. Jpn. 72 (2003) 37; T. Hasegawa, S. Mouri, Y. Yamada, K. Tanaka, J. Phys. Soc. Jpn. 72 (2003) 41; I. Katayama, Y. Ichikawa, K. Tanaka, Phys. Rev. B 67 (2003) 100102(R)]. Another aspect of the present study was specific recombination luminescence of CTVEs which was investigated here with respect to the influence of additional IR pumping. The present investigation has led to experimental evidence of new, mainly non–linear CTVE with good defined metastable behavior. Such an essentially anharmonic CTVE with respect to charge transfer and lattice displacements was predicted recently in our work [V.S. Vikhnin, Solid State Commun. 127 (2003) 283]. Here, we present experimental evidence of the existence of a new type of exciton state.

Dynamical T‐matrix theory for high‐density excitons in coupled quantum wells

AbstractExcitons in coupled quantum wells open the possibility to reach high densities close to equilibrium. In a recent experiment employing a lateral trap potential, a blue shift and a broadening of the exciton emission line has been seen [D. W. Snoke et al., Solid State Commun. 134, 37 (2005)]. The standard Hartree–Fock treatment can explain the blue shift but fails to give a finite broadening. Starting from the (spin‐dependent) many‐exciton Hamiltonian with direct and exchange potential, we present a dynamical T‐matrix calculation for the single‐exciton Green's function which is directly related to the frequency‐ and angle‐resolved photoluminescence. The calculated spectrum is blue shifted and broadened due to exciton–exciton scattering. At high excitation, both the spectrum and the angular emission are getting narrow. This is a direct manifestation for off‐diagonal long range order and a precursor of condensation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Inhomogeneous laser heating and phonon confinement in silicon nanowires: A micro-Raman scattering study

Results of a systematic set of micro-Raman experiments on the changes in the line shape of the $\ensuremath{\sim}520\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ one-phonon band in Si nanowires with laser flux $\mathrm{\ensuremath{\Phi}}$ are presented. A complicated dependence of the $520\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ Raman band asymmetry $(A)$ with $\mathrm{\ensuremath{\Phi}}$ is observed that depends both on the nanowire diameter and on the thermal anchoring of the wires to an indium foil substrate. With increasing power density in a $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}$ focal spot common to micro-Raman spectroscopy, we see a clear growth in $A$ that has nothing to do with phonon confinement. In fact, we can explain the complex changes in $A(\mathrm{\ensuremath{\Phi}})$ by extending the model [H. Richter, Z. P. Wang, and Y. Ley, Solid State Commun. 39, 625 (1981)] to include an inhomogeneous heating in the Raman volume. The effects we observe in Si nanowires should be common to all semiconducting nanostructures and underscores the importance of demonstrating a flux-independent line shape when studying pure phonon confinement effects by Raman scattering.

Photoluminescence studies of carbon clusters formed by irradiation of Si-based polymer

The formation of C nano structure in silicon-based polymer has been shown as a result of ion irradiation [Pivin et al., 1998. Visible photoluminescence of ion irradiated polysiloxane films. Solid State Commun. 106, 133–138]. In the present work, the photoluminescence (PL) studies of the ion irradiated methyltriethoxysilane gel have been performed to investigate the carbon clusters formed as a result of irradiation. Ions of He (1 MeV), Au (3 MeV) and Ag (100 MeV) were taken at different fluences ranging from 10 11 to 10 15 ions / cm 2 using the accelerator at CSNSM Orsay and 15UD Pelletron at NSC New Delhi. The electronic energy loss is varied from 17 to 602 eV/Å in the present chosen ions and energies. It is observed that PL increased with fluence upto a certain fluence, beyond which the PL decreased. In the PL spectra, we observed two peaks around 470 and 540 nm wavelength regions in the case of 100 MeV Ag ion and 3 MeV Au ion irradiations, whereas irradiation of 1 MeV He ions results in the PL peaks at 530 and 620 nm regions. The PL spectra of irradiated samples give an evidence of formation of carbon clusters. The present study reveals that the maximization of PL occurs, when the product of electronic energy loss and fluence is in the region of 4.9–8.4 eV/atom.

High-dose V+ implantation in ZnO thin film structures

In the last two decades, diluted magnetic semiconductors have attracted great attention as promising materials for spintronics applications. [K. Sato, H. Katyama-Yoshida, Jpn. J. Phys., Part 2 39 (2000) L555] theoretically predicted that ZnO doped with V, Cr, Fe, Co, and Ni can be ferromagnetic. This has been recently confirmed experimentally for vanadium doped ZnO films which were grown on sapphire substrates, using laser deposition technique [H. Saeki, H.N. Tabata, T. Kawai, Solid State Commun. 120 (2001) 439]. In the present work, high-dose vanadium implantation was used to produce Zn1−x VxO (x∼0.10) thin film structures (250nm thick) that had been epitaxially grown on sapphire substrates. Implantation with the dose 2×1016cm−2 was performed to reach a maximum vanadium concentration of 10at%. To avoid ZnO film amorphization due to radiation damage accumulation [S.O. Kucheyev, J.S. Williams, C. Jagadish, J. Zou, C. Evans, A.J. Nelson, A.V. Hamza, Phys. Rev. B 67 (2003) 094115], all implants were done at elevated temperatures 300 and 400°C and ion current density 10μA/cm2. X-ray diffraction, SIMS and photoluminescence techniques were exploited to study the implanted samples. No luminescence was observed in the implanted samples after implantation procedures. However, annealing at 800°C for 30min gave rise to ZnO crystal structure improvement. This implies that healing of implantation induced defects is possible even after heavy-ion bombardment. As a result, the photoluminescence peak at 3.359eV related to the donorbound exiton was detected.

The Second Threshold Field of Charge-Density-Wave ConductorRb0.3MoO3 in High Temperature Range

The switching and threshold properties of quasi-one-dimensionalcharge-density-wave conductor rubidium blue bronzeRb0.3MoO3 single crystals are investigated in acomparative high and large temperature range. Beyond the limittemperature 50 K of Littlewood's theory, even up to about 100 K, typicalsharp switching to negative or zero differential resistance is observedin E–I characteristic curves. Correspondingly, an obvious switchingbetween two conducting states, from a lowly conducting state to ahighly conducting state, is observed in the I–E characteristiccurves in the same temperature range. Temperature dependence of thesecond threshold field ET2 accompanied by this kind of highfield switching behaviour is firstly obtained. These new observationsare discussed in the mechanism of the current inhomogeneity andredistribution due to the existence of transverse energy barrierssuggested by Zhang et al. [Solid State Commun. 85 (1993) 121]

Analysis of scattering by spheres having a negative acoustical refractive index

Electromagnetic waves having oppositely directed phase and group velocities propagate in metamaterials having a negative permeability and negative permittivity [J. B. Pendry and D. R. Smith, Phys. Today 57(6), 37–44 (2004)]. Such materials are predicted to have unusual electromagnetic scattering properties [R. Ruppin, Solid State Commun. 116, 411–415 (2000)]. If it is possible to fabricate acoustical materials having a simultaneously negative effective elastic modulus and density (in a dynamical sense), the mechanical energy flux will have the opposite direction as the wave-vector associated with phase evolution. Rays descriptive of the energy flux refracted by such hypothetical materials at interfaces with ordinary fluids would be characterized by a negative acoustical refractive index. Partial-wave-series calculations of high frequency scattering by fluid spheres having an acoustical refractive index at (or close to) 1 reveal backscattering enhancements associated with glory rays which, unlike ordinary spheres [P. L. Marston and D. S. Langley, J. Acoust. Soc. Am. 73, 1464–1475 (1983)], require only a single internal chord. Generalized Lamb waves on elastic shells having opposite phase and group velocities also cause enhanced backscattering associated with unusual rays [G. Kaduchak, D. H. Hughes, and P. L. Marston, J. Acoust. Soc. Am. 96, 3704–3714 (1994)].

Magnetic ground states of RMn 4Al 8, R=La, Pr, Y

Yamasaki et al. (Solid State Commun. 119 (2001) 415) have suggested that LaMn 4Al 8 is a nearly antiferromagnetic metal with linear chains of Mn atoms. We present evidence from ZF-μSR measurements that both LaMn 4Al 8 and the related compound PrMn 4Al 8 display antiferromagnetic order at low temperatures. In LaMn 4Al 8 the μSR relaxation rate rises to a peak at 4.5 K; below this temperature the asymmetry decreases smoothly. In PrMn 4Al 8 well-defined oscillations appear below 35 K. Neutron crystal field spectroscopy suggests a singlet ground state for the Pr 3+ ion, so it is likely that the internal field seen by the muons arises from ordered Mn moments. YMn 4Al 8 shows no sign of magnetic correlations down to 1.2 K.

Anharmonic metastable charge transfer vibronic exciton in potassium tantalate

Charge Transfer Vibronic Excitons (CTVEs) are bi-polaronic excitons characterized by specific recombination luminescence. The latter has been investigated with respect to the influence of IR pumping and led to the experimental evidence of an non-linear CTVE. Such an essentially anharmonic CTVE with respect to the charge transfer and lattice displacements was predicted recently [Vikhnin, 2003 Solid State Commun. 127 2832], and should manifest a well-defined metastable behavior. It is shown that an IR pumping recalls practically the same CTVE-recombination emission band as that induced initially by a UVpumping. Such an effect is characterized by relaxation time τ after the UV-pumping (for example, τ ∼ 60 sec. at 3 K ). This behavior is explained as a result of occupation of a metastable anharmonic CTVE minima. An additional IR-pumping could transfer the occupation from the anharmonic CTVE state to the harmonic CTVE which is active for recombination luminescence. The temperature dependence of τ for the anharmonic CTVEs with metastable behavior shows that the relaxation time is controlled by resonance as well as by phonon-induced tunneling.

New class of T′-structure cuprate superconductors

High-temperature superconductivity has been discovered in La 2− x Ba x CuO 4 [J.G. Bednorz, K.A. Müller, Z. Phys. B 64 (1986) 189. [1]], a compound that derives from the undoped La 2CuO 4 crystallizing in the perovskite T-structure. In this structure oxygen octahedra surround the copper ions. It is common knowledge that charge carriers induced by doping in such an undoped antiferromagnetic Mott-insulator lead to high-temperature superconductivity [V.J. Emery, Phys. Rev. Lett. 58 (1987) 2794; C.M. Varma, S. Schmitt-Rink, E. Abrahams, Solid State Commun. 62 (1987) 681; E. Dagotto, Rev. Mod. Phys. 66 (1994) 763. [2–4]]. The undoped material La 2CuO 4 is also the basis of the electron-doped cuprate superconductors [Y. Tokura, H. Takagi, S. Uchida, Nature (London) 337 (1989) 345. [5]] of the form La 2− x Ce x CuO 4+ y [M. Naito, M. Hepp, Jpn. J. Appl. Phys. 39 (2000) L485; A. Sawa, M. Kawasaki, H. Takagi, Y. Tokura, Phys. Rev. B 66 (2002) 014531. [6,7]] which, however, crystallize in the so-called T′-structure, i.e. without apical oxygen above or below the copper ions of the CuO 2-plane. It is well known that for La 2− x Ce x CuO 4+ y the undoped T′-structure parent compound cannot be prepared due to the structural phase transition back into the T-structure occurring around x∼0.05. Here, we report that if La is substituted by RE=Y, Lu, Sm, Eu, Gd, or Tb, which have smaller ionic radii but have the same valence as La, nominally undoped La 2− x RE x CuO 4 can be synthesized by molecular beam epitaxy in the T′-structure. The second important result is that all these new T′-compounds are superconductors with fairly high critical temperatures up to 21 K. For this new class of cuprates La 2− x RE x CuO 4, which forms the T′-parent compounds of the La-based electron doped cuprates, we have not been able to obtain the Mott-insulating ground state for small x before the structural phase transition into the T-structure takes place.

Pressure-induced structural phase transition in rare-earth trihydrides. Part I. (GdH 3, HoH 3, LuH 3)

High-pressure X-ray diffraction studies of gadolinum, holmium and lutetium trihydrides have been carried out in a diamond anvil cell up to 30 GPa at room temperature. A reversible structural phase transformation from the hexagonal to cubic phase has been observed for all the hydrides investigated. These results confirm our first discovery of the hexagonal to cubic phase transition in erbium trihydride published recently [T. Palasyuk, M. Tkacz, Solid State Commun. 130 (2004) 219. [1]]. The lattice parameters of the new cubic phases and the volume changes at transition points were determined for SmH 3, GdH 3, and HoH 3. The parameters of the equation of state for all the hexagonal and cubic phases of the investigated compounds have been determined.

Micro-structure and magnetization of the 80-K superconductor, TbSr 2Cu 2.7Mo 0.3O 7+ δ

The results of micro-structural and detailed magnetization studies are reported here for our recently published TbSr 2Cu 2.7Mo 0.3O 7+ δ superconductor with a transition temperature ( T c) at as high as 80 K [V.P.S. Awana, A. Gupta, H. Kishan, E. Takayama-Muromachi, T. Watanabe, M. Karppinen, H. Yamauchi, S.K. Malik, W.B. Yelon, V. Ganesan, A.V. Narlikar, Solid State Commun. 129 (2003) 117]. From XRD and EDX the sample was confirmed to be of single-phase and possess the nominal cation stoichiometry. The SEM images showed well-developed grains for this ceramic high- T c superconductor. Magnetization measurements at various temperatures and fields revealed bulk superconductivity below 80 K. Magnetization versus applied field ( M vs H) loops at various temperatures below the T c exhibited clear full penetration field H P. In normal state (>100 K) the susceptibility follows the Curie–Weiss behavior with an effective paramagnetic moment of 9.83 μ B.

Two-level model and magnetic field effects on the hysteresis inn−GaAs

Efforts are made in this work to interpret the experimentally observed magnetic effects on the hysteretic $I\text{\ensuremath{-}}V$ curve for an $n\text{\ensuremath{-}}\mathrm{GaAs}$ semiconductor through a two-impurity-level model with the assumptions of spatial homogeneity in current flow direction and instantaneous energy balance. We construct the model by considering carefully the Landau level shifts for the electrons in the conduction band, the magnetoresistance property, and the modification on the cross sections of the impact ionization. With the inclusions of the effects from the carrier electron temperature variation and the field-dependent electron mobility, we are able to describe the hysteretic $I\text{\ensuremath{-}}V$ characteristics satisfactorily for the case of applying either a longitudinal or a transverse magnetic field simultaneously within a single model. Our numerical results show that when the applied longitudinal magnetic field $B$ increases, the holding voltage of the hysteresis shifts towards a higher value, while the breakdown voltage remains almost fixed and thus the width of the hysteresis decreases. Above a critical magnetic field intensity $86\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$, the hysteresis vanishes. Under the transverse magnetic field, the breakdown voltage of the hysteresis shifts significantly towards the higher direction with a stronger magnetic field $B$, and therefore a considerably wider hysteresis width. The dynamic behavior of our model has displayed the same features of the experimental observations described by Aoki, Kondo, and Watanabe in Solid State Commun. 77, 91 (1991).

Interpretation of resonant photoemission spectra and electronic structure ofRT2compounds(R=Ce,Pr,Nd;T=Co,Rh,Ir)

Resonant $4f$ photoemission spectra of $R{T}_{2}$ Laves-phase compounds $(R=\mathrm{Ce},\mathrm{Pr},\mathrm{Nd};T=\mathrm{Co},\mathrm{Rh},\mathrm{Ir})$ published in the literature [G. Kalkowski et al., Solid State Commun. 55, 977 (1985); J.-S. Kang et al., Phys. Rev. B 46, 15689 (1992)] are analyzed in the framework of the single-impurity Anderson model. For a given $T$ element the hybridization strength is found to decrease by about $20%$ in going from $\mathrm{Ce}$ to $\mathrm{Nd}$. For given $R$ it increases in going from $\mathrm{Co}$ to $\mathrm{Ir}$. The observed trends are in good agreement with calculated hybridization matrix elements.

Symmetries of the resistance of mesoscopic samples in the quantum Hall regime

The symmetry properties of the resistance of mesoscopic samples in the quantum Hall regime are investigated. In addition to the reciprocity relation, our samples obey new symmetries, that relate resistances measured with different contact configurations. Different kinds of symmetries are identified, depending on whether the magnetic field value is such that the system is above, or below, a quantum Hall transition. Related symmetries have recently been reported for macroscopic samples in the quantum Hall regime by Ponomarenko {\it et al.} (Solid State Commun. {\bf 130}, 705 (2004)), and Karmakar {\it et al.} (Preprint cond-mat/0309694).

Universal explicit expression for quantum Hall conductance at any temperatures and chemical potentials

This paper generalises the theorem already obtained [Solid State Commun. 127 (2003) 505] for the high mobility, dissipationless, integer quantum Hall systems at T=0 K to the T>0 K situations. The results obtained are again suitable at both microscopic and macroscopic scales. In comparison [Solid State Commun. 127 (2003) 505], this generalised form gives a universal explicit expression for the Hall conductance σ xy ( μ, T) between any two points selected in such a system as a function of chemical potential and temperature. Further, thermal deviation Δ σ xy ( μ, T) from the exact quantised values of σ xy ( μ, T) and the minimum slopes of Hall plateaux in the T>0 cases, observed already in experiments, are also derived in theory. Similar to those in the T=0 K case [Solid State Commun. 127 (2003) 505], the overall quantum Hall behaviour of the system can again be obtained from this theory by simply selecting two points on the two Hall contacts.

Direct evidence of phase coexistence in La 0.5Ca 0.5MnO 3

There has been accumulating evidence (Mathur, Solid State Commun. 119 (2001) 271) that a charge ordered antiferromagnetic phase can coexist with a metallic ferromagnetic phase in the mixed valent manganites. We have used electron holography to show that micron sized fully ferromagnetic and non-ferromagnetic phases coexist at 90 K in La 0.5Ca 0.5MnO 3. Dark field imaging and selected area diffraction were used in combination to show that charge order occurs in both non-ferromagnetic and ferromagnetic regions.

Observation of polarization-dependent x-ray absorption spectra arising fromCu3d−F2phybridization in the two-dimensional ferromagnetsA2CuF4(A=K,Cs)

We report on a study of the polarization dependence of the x-ray absorption spectra (XAS) at the Cu ${L}_{2,3}$ edges and the F K edge in single crystals of two-dimensional ferromagnets ${A}_{2}{\mathrm{CuF}}_{4}(A=\mathrm{K},\mathrm{Cs}).$ We successfully observed polarization-dependent XAS at the Cu ${L}_{2,3}$ edges. This provides experimental support of the antiferrodistortive orbital ordering (AFDOO) model proposed by Khomskii and Kugel [D. I. Khomskii and K. I. Kugel, Solid State Commun. 13, 763 (1973)]. We also observed polarization dependence of the XAS at the F K-edge peak at $\ensuremath{\sim}681\mathrm{eV},$ which reveals the F ${2p}_{\ensuremath{\sigma}}$-hole orbitals arising from Cu $3d$-F $2p$ hybridization. We argue its implication in relation to the realization of AFDOO.