NMR observation of critical dynamics in the (TMTSF)2PF6 organic superconductor

We have studied the organic superconductor (TMTSF)₂PF₆ using ¹H nuclear magnetic resonance. The spin-lattice (T₁) and the spin-spin relaxation time (T₂) measurements manifested a divergence associated with a structural phase transition at 160 K.     

Fractal flux jumps in an organic superconducting crystal

Angle dependant torque magnetization measurements have been carried out on the organic superconductor, κ-(ET)₂Cu(NCS)₂ at extremely low temperatures (25-300 mK). Magneto-thermal instabilities are observed in the form of abrupt magnetization (flux) jumps for magnetic field sweeps of 0-20 T. A fractal analysis of the flux jumps indicate that the instabilities do show a self similar structure with a fractal dimension of varying between 1.15 and 1.6. The fractal structure of the flux jumps in our sample shows a striking similarity to that of MgB₂ thin film samples, in which magneto-optical experiments have recently shown that the small flux jumps are due to the formation of dendritic flux structures. These smaller instabilities act to suppress the critical current density of the thin films. The similarity of the flux jump structure of our samples suggests that we may also observing the dendritic instability, but in a bulk sample rather than a thin film. If true, this is the first observation of the dendritic instability in a bulk superconducting sample, and is likely due to the layered nature of κ-(ET)₂Cu(NCS)₂, which results in a quasi-two dimensional flux structure over the majority it's mixed state phase diagram.     

Some parameters of superconductors with an extreme singularity in the density of state: (s+d) model

We study some parameters of superconductors with δ-function type singularities in the electronic density of states (DOS), exhibiting (s+d)-wave symmetry of the order parameter. Starting with a pure s-wave pairing potential V_s, the critical temperature T_c at first slightly increases with increasing the d-wave interaction potential V_d, being determined by this interaction only for stronger V_d values. The ratio R=2|Δ(0)|/k_BT_c of the mean value of the zero temperature energy gap |Δ(0)| to T_c increases with increasing V_d, reaching a maximum which depends on the mixing interaction term. The maximum values of R are comparable with very high values obtained in some gap measurements. The jump in the specific heat at critical temperature is by a factor 2.4 higher for the extreme singularity of pure s-wave symmetry, as compared with the BCS theory with constant DOS. Such higher values of the jump are in agreement with the experimentally observed values, as well as with the calculations determined by extended saddle points in the electronic bands. By switching the d-wave channel, the value of the jump decreases. The results show the usefulness of calculations with δ-type singularities as a limiting case of very strong singularities in the DOS.     

Porous silicon as efficient surface enhanced Raman scattering (SERS) substrate

Silver nanocrystallites are obtained through immersion of porous silicon samples in AgNO₃ solutions and a successive thermal annealing. The efficiency of nanostructures as surface enhanced Raman scattering (SERS) substrates is checked on cyanine-based dyes and horseradish peroxidase, evidencing detectable concentrations as low as 10⁻⁷ to 10⁻⁸ M. The substrate efficiency is strictly related to the Ag particle morphology, which could yield to either local surface plasmons (LSP) coupled to individual particles or to inter-particle short-range interaction.     

Low integer Landau level filling factors ν and indications for the fractional ν=1/2 in the 2D organic metal κ-(BEDT-TTF)2I3

In the two-dimensional (2D) organic metal κ-(BEDT-TTF)₂I₃ the low integer Landau level filling factors ν=1-4 are observed under specific experimental conditions. In high magnetic fields even the presence of the fractional ν=1/2 is strongly indicated in this multilayer material. These ν are detected by the chemical potential μ, i.e. a thermodynamic quantity, which could be probed under complex fermiological conditions.     

Pinning mechanism in carbon nanotube doping of MgB2 superconductor

MgB_1.9C_0.1 samples are synthesized under the ambient pressure (AP) and high pressure (HP), respectively. The further studies demonstrate different field-dependence of the critical current density J_c(H) in each sample. In the view of two-gap superconductivity in these samples, δT_c pinning (resulting from the spatial fluctuations of the transition temperature) is dominant in the AP sample, while in the HP sample, both δT_c and δl pinning (due to the mean-free-path fluctuations) act together and their contributions vary with temperature. Besides the improvement of H_c2(0), due to the different pinning mechanism, J_c(H) of the HP sample shows a slower decay with the increasing fields than that of the AP sample in high fields, which suggests a possible method of retarding the rapid decay of J_c(H) under elevated fields.     

Resonant Raman scattering in the superconducting cuprates: Frozen-phonon versus perturbational approach

We consider in detail Raman scattering by vibration of the apical oxygen ions in the RBa₂Cu₃O₇ superconducting cuprates. The scattering intensity is very sensitive to the ratio of diagonal and off-diagonal matrix elements of electron-phonon coupling, bandstructure, and carrier concentration. Our results show a large quantitative difference between the results of frozen-phonon and perturbational approach to the Raman process. The discrepancy becomes especially large when interband transitions to the states near the Fermi level are close to resonance with the incident light. The calculation of phonon-induced ion charge fluctuations shows an analogous discrepancy. The reason for these effects is the possibility of carrier redistribution between different parts of the Fermi surface arising in the frozen-phonon approximation. Our results show that Raman scattering in superconducting superlattices is very sensitive to the properties of the states near the Fermi level. For this reason experiments performed on the superlattices can help to resolve the discrepancy. Received 8 December 1999     

First-principles investigation of pressure-induced changes in structural and electronic properties of Y 2C3 superconductor

The structural and electronic properties of Y ₂C₃ superconductor under different external pressures were calculated by employing the first-principles method. This shows that the lattice constants as well as the lengths of C-C dimers decrease with the pressure. Results of band structure calculations indicate that the Fermi level advances to the bonding zone with an increase in pressure; meantime, the valence and conduction bands intersect more deeply with the Fermi level. Moreover, the Fermi level is found to shift from the valley bottom of the density of states (DOS) curve to the shoulder, which means an increase in N(E_F), and therefore the critical temperature, T_c. The calculations verify that the critical temperature is directly related to the electronic structure.     

Lattice Dynamics and Superconductivity of RuB2: A First-Principles Study

We report the first-principles linear response calculations on lattice dynamics and electron-phonon coupling （EPC） of superhard material RuB2. Phonon frequencies and eigenvectors are obtained throughout the whole Brillouin zone. The calculated EPC paracneters for the optical phonon modes at F indicate that the d electrons of transition metal play the most important role in deciding the superconducting behaviour, and there are sizeable contributions from B p electrons to EPC. Our calculated EPC constant is 0.41, and the estimated superconducting transition temperature Tc is 1.6K using the Coulomb pseudopotential u^＊ = 0.12, in excellent agreement with the experimental ones.     

Concerning the first-order phase transition in the low-dimensional organic superconductor (BEDO—TTF)2ReO4 · H2O. Crystal and electronic band structures below the phase transition (T = 170 K)

The effect of the first-order phase transition occurring around 220 K on the crystal and electronic structure of the molecular superconductor (BEDO-TTF) has been studied. The crystal structure at 170 K has been determined and it is found that the main structural change resulting from the phase transition is connected with a rotation of the anions. This rotation leads to changes in the SS and SO intermolecular contacts within the BEDO-TTF donor layers. How these structural changes affect the electronic structure of the salt is studied by comparing the transfer integrals for the different donordonor intermolecular interactions calculated for the room temperature and 170 K crystal structures. It is shown that Fermi surfaces for the 170 K and room temperature structures are very similar, do not exhibit nesting properties, and can be described as resulting from the hybridization of superposing ellipses. Received: 4 September 1997 / Received in final form: 2 December 1997 / Accepted: 3 December 1997     

Structural and electronic properties of a new molecular conductor, α-(BEDT-TTF)2CsCd(SCN)4

Structural and electronic properties of a new BEDT-TTF based radical cation salt, α-(BEDT-TTF)₂CsCd(SCN)₄, are presented. In the measurements of the electrical resistivity and the magnetic susceptibility, this new α-type salt shows metallic behavior down to low temperature. Measurements of the resistivity under in-plane c-axial strain reveal a newly observed insulating phase, suggesting that α-(BEDT-TTF)₂CsCd(SCN)₄ is placed near an insulating phase which might have close relationships with a superconducting phase realized in α-type salts.     

Superconductivity induced phonon anomalies in the Raman spectra of Zn and Ni doped YBa2Cu3O7

Here we present Raman spectra of YBa₂(Cu_1–x Zn _x )₃O₇ and YBa₂(Cu_1–x Ni _x )₃O₇ as a function of temperature and Zn or Ni content. The temperature dependence of two modes at 340 and 440 cm^–1 is analyzed. Similarly to the infrared measurements it is found that Zn substantially suppresses the superconductivity induced phonon softening whereas, Ni does not affect much that effect. Moreover, the superconductivity induced phonon stiffening of the 440 cm^–1 mode completely disappeared with the Zn doping. We find this behaviour might support the model where Zn acts effectively as a magnetic pair breaker.     

Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF) 2 PF 6

The phase diagram of the organic superconductor (TMTSF)₂PF₆has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between T _SDW and T _SC and the superconducting critical current. The onset temperature T _SC is practically constant ( 1.20±0.01 K) in this region where only the SC/SDW domain proportion below T _SC is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with T _SC falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings. Received 3 September 2001 and Received in final form 9 November 2001     

Magnetothermal instabilities in an organic superconductor

We have studied the occurrence of magnetothermal instabilities in a single crystal during field sweep magnetization experiments, equivalent to short time relaxation studies. We find instability behaviour in good agreement with a recent model by Mints, for a non-linear E(J) characteristic. In particular, we find that a decrease of the dynamic relaxation rate, characterizing the effective activation energy, precedes the unstable regime. We point out formal analogies between such instabilities and the general predictions for flux avalanches. Received: 11 December 1997 / Revised: 15 January 1998 / Accepted: 31 March 1998     

Raman study on the pressure-induced disordered state of polystyrene

The pressure-induced disordered state was first reported for iPS, the polymer showing thermal shrinkage. The low-frequency Raman study showed clearly that pressure introduces irregularity into the polymer backbone of iPS. It has also been revealed that there exist two types of vibrational modes, which show different sensitivities on the regularity of the main chain. These modes indicate two distinctive features of dispersion in the ordered form. In the disordered state, the dispersive modes smear out and the dispersionless one survives to be observed. We conclude that pressure causes the ordered-disordered conversion like thermal shrinkage. This conversion will be a common feature of the polymers showing thermal shrinkage and would be able to take place at relatively low pressure.     

Phonon density-of-states in the new superconductor MgB 2

We provide here experimental data on the phonon density-of-states of MgB₂ obtained by the inelastic neutron scattering technique. The measurements were performed for the natural boron-based magnesium diboride with use of a time-of-flight neutron spectrometer. Several phonon bands were observed in the phonon spectrum at energies of about 33, 55, 82 and 99 meV. We show that the cut-off energy of the density-of-states occurs at around 105 meV which is much higher than expected so far from heat-capacity data and partially explains the high T _c value observed for MgB₂. The characteristic phonon energies are indicative of an intermediate coupling regime in this compound. We conclude that a much needed neutron experiment aimed at the study of the isotopic effect in the phonon density-of-states of MgB₂ is conceivable. Received 19 March 2001     

Resonant Raman scattering from superconducting single crystals of (BEDT-TTF)2I3

A study of low energetic resonant Raman scattering of (BEDT-TTF)₂I₃ superconducors was performed. The vanishing of phonon bands accompanied by a decrease of the electronic background was observed belowT _c . We propose a theoretical explanation for this novel effect in terms of the Balseiro-Falicov model of phonon-superconducting amplitude mode interaction.     

Metal-insulator transition in the low-dimensional organic conductor (TMTSF)<Subscript>2</Subscript>FSO<Subscript>3</Subscript> probed by infrared microspectroscopy

We present measurements of the infrared response of the quasi-one-dimensional organic conductor (TMTSF)₂FSO₃ along (E ) and perpendicular (E ) to the stacking axis as a function of temperature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra show a Drude-like response. Below the transition an energy gap of about 1500 cm^-1 (185 meV) opens, leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transition temperature and for the short-range order fluctuations of the lattice modulation above the transition temperature. We also report about a new infrared mode at around 710 cm-1 with a peculiar temperature behavior, which has so far not been observed in any other (TMTSF)₂X salt showing a metal-insulator transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the reorientation of electrical dipole moment of the FSO₃ anion is proposed, in order to explain the anomalous behavior of this new mode.