Electronic and electronic-vibrational phenomena in the new organic conductor ϰ-(ET)2[Hg(SCN)2Cl] with a metal-insulator transition

The reflection spectra and optical conductivity spectra of the new organic conductor ϰ-(ET)₂[Hg(SCN)₂Cl] with a metal-insulator transition in the spectral regions 700–5500 and 9000–40 000 cm⁻¹ have been studied in polarized light at 300 K. A comparisonis made between the spectra obtained and the corresponding spectra of related isostructural conductors based on the ET molecule, and also the properties of the crystal structure of the investigated compounds. An electronic transition between the ET molecules of the dimer (ET) ₂ ⁺ in the spectral region 700–5500 cm⁻¹ has been identified, as have the features of the electronic-vibrational structure arising as a consequence of the interaction of this transition with the completely symmetric intramolecular vibrations of the ET molecule. It is found that the conductor with the stronger dimer interaction between the ET molecules has the higher the transition temperature. Fiz. Tverd. Tela (St. Petersburg) 39, 1313–1319 (August 1997)     

Surface impedance of <Emphasis Type="Italic">k</Emphasis>-(BEDT-TTF)<Subscript>2</Subscript>Cu[N(CN)<Subscript>2</Subscript>]Br crystals

Precision measurements of the real and imaginary parts of the microwave surface impedance Z _ac(T) = R _ac(T) + iX _ac(T) of the conducting ac layers of the k-(BEDT-TTF)₂Cu[N(CN)₂]Br crystals in the temperature interval of 0.5 < T < 100 K have demonstrated a series of features: (i) the temperature course of the field penetration depth is close to linear Δλ_ac(T)∞ΔX _ac(T) in the superconducting state at T T _c ∼ 11.5 K; (ii) the curves R _ac(T) = X _ac(T) coincide at T _c < T < 40 K; (iii) the X _ac(T) value at T > 40 K increases in comparison with R _ac(T); (iv) the dependence R _ac(T) at T > 40 K is nonmonotonic in thin crystals. These features of the impedance Z _ac(T) with increasing T are interpreted in terms of (i) the d-type symmetry of the superconducting order parameter, (ii) normal skin effect, (iii) manifestations of the antiferromagnetic fluctuations, and (iv) the size effect. The electrodynamic parameters of k-(BEDT-TTF)₂Cu[N(CN)₂]Br have been determined.     

Knight shift tensors and π-spin densities in the organic metals αt-(BEDT-TTF)2I3 and (BEDT-TTF)2Cu(NCS)2

¹³C-MASS spectra of pure BEDT-TTF and of the organic metals α_t-(BEDT-TTF)₂I₃ and (BEDT-TTF)₂Cu(NCS)₂ were recorded atν _L = 68 MHz. Isotropic shifts and the principal components of the shift tensors were determined, respectively, from the center and spinning side bands. For pure BEDT-TTF which is a diamagnetic insolator, the measured shifts arechemical shifts while for the organic metals they are the sum of chemical and Knight shifts. In each of the compounds the shifts are assigned ingroups to theinner, middle andouter carbons of the BEDT-TTF molecule. For the organic metals the separation of the experimental shifts into chemical and Knight shifts is discussed. From the anisotropic part of the Knight shift tensors the π-spin densities at the carbon and sulphur positions of the BEDT-TTF molecule are inferred. The result is that the π-spin density of the unpaired hole is concentrated on the center part of the BEDT-TTF molecule, i.e., on the inner and middle carbons, and on the inner sulphurs. It is argued that the current density is concentrated on this part of the BEDT-TTF molecule as well.     

Magnetic quantum oscillations in the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

The interlayer magnetoresistance of the organic superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂)Br is measured at ambient pressure and under pressures of up to 12.5 kbar. In addition to the slow Shubnikov-de Haas oscillations with a frequency of ≃150 T observed at P⩾5 kbar, rapid oscillations attributed to the magnetic breakdown orbit enveloping an area equal to 100% of the area of the Brillouin zone are found to emerge above B=20 T. The latter oscillations are observed at ambient pressure as well as under pressures of up to 9 kbar. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 190–194 (10 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Raman Spectra and Solvent-Solute Interaction of Mercury(II)-Thiocyanate Complexes in Some Aprotic Donor Solvents

Raman spectra of the species Hg(SCN)₂, [Hg(SCN)₃], and [Hg(SCN)₄]^2- in solution of ten aprotic donor solvents have been investigated in the region of the Hg-S vibration. Observed frequency values of measured band of Hg(SCN)₂ and [Hg(SCN)₃] in different solutions correlate well with donor strength of the solvents. There is a linearity between Hg-S frequency decreasing and increasing of the interaction of the solvent molecules with the mercury (II) ion in the thiocyanate complexes. No significant frequency changes have be found for [Hg(SCN)₄]^2-. Evidence based on the frequency shifts and depolarization ratios in various solvents supports the view that the Hg(SCN)₂ in solution undergos departure from linearity as a result of increasing solvent coordination to the mercury (II) ion. Almost constant frequencies of Hg-S vibration of [Hg(SCN)₄]^2- in all investigated solvents suggest a regular tetrahedral structure of the ion in solution having much larger radius than corresponding HgX₄ (X=Cl,Br,I) ions.     

Thermopower in quasi-two-dimensional (BEDT-TTF) m X n organic conductors

Thermopower of (BEDT-TTF) _m X _n organic conductors has been studied using a dedicated measurement technique in the temperature range of 4.2 to 300 K. It turned out that some features of the thermopower in quasi-two-dimensional metals, namely the presence of a peak in the thermopower of α-(BEDT-TTF)₂MHg(SCN)₄ and a plateau in κ-(BEDT-TTF)₂Cu(NCS)₂ in the temperature interval between 10 and 50 K, are probably due to the phonon drag effect. Similar temperature dependences of the Seebeck coefficient can be satisfactorily interpreted in terms of a simple model taking into account the real experimental curve of the phonon heat capacity versus temperature, C ∝ T ², which is not described by the Debye formula. One feature distinguishing organic superconductors from magnetically ordered metals is a stronger temperature dependence of the characteristic electron-phonon scattering time τ _e-ph(T). Phonon drag effects also determine the behavior of the thermopower in the (BEDT-TTF)₃Cl₂·2H₂O organic conductor, which is characterized by a metal-insulator transition at T∼150 K. An analysis of measurements of the conductivity and thermopower vs. temperature taken together indicates that the transition in this compound has a complex nature: first (at T∼150 K) a metal-insulator transition occurs, which produces an energy gap in the band spectrum, then at a lower temperature (T∼20 K) a transition to a charge-density wave state takes place. Zh. éksp. Teor. Fiz. 113, 323–338 (January 1998)     

Analysis of electronic vibration spectra of tetrabenzoporphin molecules on the basis of calculations of their normal vibrations

Quasi-line fluorescence and fluorescence-excitation spectra of tetrabenzoporphin (H₂TBP) and its deuterated derivative (D₂TBP) in n-octane at 77 K were measured and analyzed. A calculation of normal vibrations of the H₂TBP and D₂TBP molecules has been performed. The normal vibrations of these molecules were calculated with the use of the valence force fields of porphin and phenanthrene (the direct spectral problem was solved). Good agreement between the calculated and measured frequencies of the investigated normal vibrations has been obtained without correction of the transfered force constants. A detailed interpretation of the fluorescence spectra obtained is presented. It is shown that on deuteration of H₂TBP, the behavior of the quasi-lines in the fluorescence-excitation spectra in the region of resonance with the S₂←S₀ transition becomes more complex. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 375–382, May–June, 1999.     

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.     

Nuclear-spin-lattice relaxation in the organic superconductor (BEDT-TTF)2Cu[N(CN)2]Br0.8Cl0.2

The behavior of the spin-lattice relaxation rate of ¹H nuclei in the organic superconductor (BEDT-TTF)₂Cu[N(CN)₂]Br_0.8Cl_0.2 in the temperature interval 12–319 K is investigated. A relaxation rate maximum, which depends on the resonance frequency and is associated with thermally activated vibrations of the end ethylene groups in the BEDT-TTF molecules, is observed in the vicinity of 270 K. The relaxation rate is observed to deviate from Korringa behavior at T<100 K. These deviations can be attributed to the existence of strong antiferromagnetic spin correlations. Fiz. Tverd. Tela (St. Petersburg) 39, 39–41 (January 1997)     

Triple differential cross-sections for near threshold (e, 2e) process for hydrogen

The resonance Raman (RR) spectra of nickel octaethyl porphyrin, Ni(OEP), in CH₂Cl₂ (solvent) at different excitations such as 514.5, 488.0, 441.6 and 406.7 nm are recorded and analysed. The results of the theory of distortion-induced RR intensity is applied to the observed spectra to determine the excited electronic state symmetry of porphyrin in Ni(OEP). It is concluded that the porphyrin molecule (D_4h structure) attains a non-polar distorted structure of D₂ symmetry rather than S₄ symmetry in CH₂Cl₂ solution.     

A Double Isotopic Labelling Study of the Infrared Spectra of the Linkage Isomers [Pd(bipy)(SCN)2], [Pd(bipy)(NCS)2] and Related Complexes

Abstract

The IR spectra of the linkage isomers [Pd(bipy)(SCN)₂] and [Pd(bipy)(NCS)₂] have been determined in the C≡N stretching region (2200–2000 cm^?1) and below 500 cm^?1. The band shifts resulting from deuteration of the 2,2′-bipyridine (bipy) ring and ¹⁵NCS-labelling are shown to provide a ready means for distinguishing between the internal ligand modes, the μPd-N(bipy) and μPd-SCN/μPd-NCS vibrations. The assignment technique has been further extended to the complexes [Pt(bipy)(SCN)₂] and [Pd(phen)(SCN)₂] (phen = 1,10-phenanthroline). Finally, a comparison between the IR spectra of [Pd(bipy)(NCO)₂], [Pd(bipy)(NCS)₂] and [Pd(bipy)(SCN)₂] reveals that the frequencies μM-NCO, μM-NCS and μM-SCN decrease in the sequence NCO > NCS > SCN.     

Spin dynamics of charge carriers in the process of their localization in α’-(BEDT-TTF)<Subscript>2</Subscript>IBr<Subscript>2</Subscript> single crystals

Sharp changes in the integral intensity and linewidth of the ESR spectrum that accompany the localization of the charge carriers have been revealed in α’-(BEDT-TTF)₂IBr₂ crystals. It has been found that the types of localization in two compounds under investigation are different: charge carriers in β″-(BEDT-TTF)₄NH₄[Cr(C₂O₄)₃] are localized on irregular defects of the crystal lattice, whereas charge carriers in α’-(BEDT-TTF)₂IBr₂ are localized at the regular positions of the unit cell. The exchange narrowing of the ESR line and a sharp decrease in the dc and ac magnetic susceptibilities are observed in α’-(BEDT-TTF)₂IBr₂ at low temperatures T < 50 K. The dc and ac magnetic susceptibilities observed in α’-(BEDT-TTF)₂IBr₂ at high temperatures T > 50 K differ from each other, because the thermally activated hopping frequency of the charge carriers is higher than the frequency of the measuring UHF field of an ESR spectrometer.     

Spectra of the action of photoinduced NH(ND)-rearrangement of cyanosubstituted isobacteriochlorin

Based on the measured spectra of the action of the process of cis-trans-NH(ND)-isomerization of 5-cyano-2,2,8,8,12,13,17, 18-octamethylisobacteriochlorin (H₂-COMiBCh) and its deuteroderivative (D₂-COMiBCh) in polycrystalline matrices of n-octane and r hexale at 4.2 K, it is established that the character of embedding of COMiBCh in the matrices of n-alkanes has a substantial effect on the barrier of dark rearrangement and the rate of photoinduced NH(ND)-rearrangement, while deuteration of the center of a molecule shifts the dynamic equilibrium toward a trans-form in photoexcitation. To whom correspondence should be addressed. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina, Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 555–559, July–August, 1999     

Wave shape of de Haas-van Alphen oscillations and effective mass in the two-dimensional organic conductor α-(BEDT-TTF)2KHg(SCN)4

De Haas-van Alphen (dHvA) oscillations in the two dimensional organic conductor α-(BEDT-TTF)₂KHg(SCN)₄ have been measured by a canti-lever technique at high magnetic fields up to 30 T. The wave shape of the dHvA oscillations undergoes a drastic change around the transition field B_K (24 T). The effective mass shifts 1.5 m₀ in the density wave phase (B<B_K) to 1.65 m₀ in the normal metallic phase (B>B_K). Based on the simulation, we have determined that the primary cause of the characteristic field dependence of the wave shape is the change in the effective mass.     

Theoretical study of the dynamics for the H + Li H ( v = 0, j = 0) $\to$ H2 + Li reaction and its isotopic variants

Quasi-classical trajectory (QCT) method is carried out to calculate the dynamics of the H + LiH (v = 0, j = 0) ?\to H₂ + Li reaction and its isotopic variants based on the potential energy surface of the lowest ²A￠^2A' electronic state reported by Prudente et al. [Chem. Phys. Lett. 474, 18 (2009)]. The reaction cross-section, product rotational alignment parameter áP₂\langle P_2 ([(j￠)\vec]\vec{j'} ·\cdot [(k)\vec])?\vec{k})\rangle and one generalized polarization-dependent differential cross-section (2π/σ)(ds₀₀d\sigma_{00}/d w_t\omega_t) are calculated. We found that different collision energies and mass factors show driving influence on the process of the reactions and product molecules H₂ (HD, D₂) polarization distribution, and the trend of the isotopic effects in the high collision energy range is different to that in the low collision energy range. The calculations are also interpreted in relation to the features of the underlying potential energy surface. A comparison between the title reactions and a barrier-less reaction F + HBr ?\to FH + Br has been discussed in detail.     

Quantum and semiclassical oscillations in the organic metal (BEDO-TTF)2Clx(H2O)y

We study quantum (Shubnikov-de Haas and de Haas-van Alphen) oscillations and angular oscillations of the reluctance in the organic quasi-two-dimensional metal (BO)₂Cl_x(H₂O)_y. We show that the Fermi surface in this compound consists of a slightly corrugated cylinder with its axis perpendicular to the conducting plane. The cross section of the cylinder in this plane is a perfect circle of radius k _F≃3×10⁷ cm⁻¹. The effective carrier mass associated with this cylinder is m*=(1.65–2.0) m ₀ in the conducting plane, while the Dingle temperature is T _D=3–4 K. Zh. éksp. Teor. Fiz. 114, 1137–1146 (September 1998)     

Infrared spectra of BeSO4.4H2O and its deuterated analogue in 4000–1200 cm−1 region

IR spectra of BeSO₄.4H₂O and its deuterated analogue at ∼300 K and ∼110 K are reported in the region 4000–1200 cm⁻¹ using thin film and nujol mull techniques. The observed bands have been assigned as the internal modes of the water and the overtones and combinations of various modes using the recently revised assignments of SO₄ ²⁻ and Be(aq)₄ fundamentals in the region 1200–250 cm⁻¹ (Srivastavaet al 1976). The splitting of the internal modes of water has been discussed in the light of the effects of deuteration and cooling and it is shown that all the water molecules in a unit cell are asymmetric but crystallographically equivalent.     

Raman scattering in YBa2Cu3O7 single crystals: Anisotropy in normal and superconductivity states

Precise temperature and polarization dependences of Raman spectra have been investigated for fully oxygenated twin-free YBa₂Cu₃O₇ single crystals. We have found a striking superconductivity-induced x−y anisotropy in the temperature behavior of the 340 cm⁻¹ line: the magnitudes of the softening and broadening are quite different in the xx-and yy-polarizations. This anisotropy suggests a contribution of the CuO-chain superconductivity with a pairing symmetry different from that for the CuO₂ plane, or indicates that the superconducting gap amplitudes are different in the k _x and k _y directions. The d+s gap symmetry is the only realistic symmetry in the case of Δ_x≠Δ_y. Fiz. Tverd. Tela (St. Petersburg) 40, 403–412 (March 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Influence of magnetic field on the electronic specific heat of the organic metal (BEDT-TTF)2KHg(SCN)4

Specific heat measurements of a single crystal of the organic metal (BEDT-TTF)₂KHg(SCN)₄ have been carried out at low temperatures and under a magnetic field of up to 14 T. A jump in the specific heat of about 0.1 J/mol·K, which corresponds to the antiferromagnetic phase transition, has been observed. The magnetic field is found to decrease the transition temperature at any field orientation. The strongest effect was found to take place in the field direction along the highly conducting ac plane. Zh. éksp. Teor. Fiz. 113, 1058–1063 (March 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Reflectivity spectra of new organic metal (BEDO-TTF)5[CsHg(SCN)4]2

The polarized reflectivity and optical conductivity spectra of microcrystals of the new organic conductor (BEDO-TTF)₅[CsHg(SCN)₄]₂ based on the donor molecule bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) have been studied in the spectral ranges 600–6500 and 9000–40000 cm^?1 at 300 K for three principal lattice directions. The optical evidence for the quasi-two-dimensional character of the conducting electronic system is obtained. The conclusion is made that the studied crystal is the quasi-two-dimensional semimetal with overlapping electron energy bands. The basic parameters of the electronic system of the crystal are determined in the framework of the Drude model. It is found that the allowed electron energy bands of the crystal are somewhat narrower than those of the previously studied structurally allied superconductor based on the same molecule. The features of vibrational structure are identified in the σ(ω) spectra for the specified three polarizations.