G. Mihály

Impurity effects in the organic charge transfer salt Qn(TCNQ)2

Variations in the conductivity of QN(TCNQ)2 depending on sample preparation are correlated with the presence of doubly ionised TCNQ in the samples. Increasing TCNQ2- concentration results in decreasing conductivity and increasing activation energy. The influence of D+ impurities compensating the excess charge is discussed.

Conductance of Pd-H Nanojunctions

Results of an experimental study of palladium nanojunctions in a hydrogen environment are presented. Two new hydrogen-related atomic configurations are found, which have conductances of similar to0.5 and similar to1 quantum unit (2e(2)/h). Phonon spectrum measurements demonstrate that these configurations are situated between electrodes containing dissolved hydrogen. The crucial differences compared to the previously studied Pt-H-2 junctions and the possible microscopic realizations of the new configurations in palladium-hydrogen atomic-sized contacts are discussed.

Magnetic scattering of spin polarized carriers in (In, Mn)Sb dilute magnetic semiconductor.

Magnetoresistance measurements on the magnetic semiconductor (In, Mn)Sb suggest that magnetic scattering in this material is dominated by isolated Mn2+ ions located outside the ferromagnetically ordered regions when the system is below T(c). A model is proposed, based on the p-d exchange between spin-polarized charge carriers and localized Mn2+ ions, which accounts for the observed behavior both below and above the ferromagnetic phase transition. The suggested picture is further verified by high-pressure experiments, in which the degree of magnetic interaction can be varied in a controlled way.

Multicritical end point of the first-order ferromagnetic transition in colossal magnetoresistive manganites

We have studied the bandwidth-temperature-magnetic-field phase diagram of RE0.55Sr0.45MnO3 colossal magnetoresistance manganites with ferromagnetic metal (FM) ground state. The bandwidth was controlled both via chemical substitution and hydrostatic pressure with a focus on the vicinity of the critical pressure p;{*} where the character of the zero-field FM transition changes from first to second order. Below p;{*} the first-order FM transition extends up to a critical magnetic field. It approaches zero on the larger bandwidth side where the surface of the first-order FM phase boundary is terminated by a multicritical end point. The change in the character of the transition and the decrease of the colossal magnetoresistance effect is attributed to the reduced charge-order and orbital-order fluctuations.

Magnetic-order-induced crystal symmetry lowering in ACr2O4 ferrimagnetic spinels.

We demonstrate that the onset of complex spin orders in ACr2O4 spinels with magnetic and Jahn-Teller active A=Fe and Cu ions lowers the lattice symmetry. This is clearly indicated by the emergence of anisotropic lattice dynamics-i.e., by the pronounced phonon splittings-even when experiments probing static distortions fail. The crystal symmetry in the magnetic phase is reduced from tetragonal to orthorhombic for both compounds. The conical spin ordering in FeCr2O4 is also manifested in the hardening of the phonon frequencies. In contrast, the multiferroic CoCr2O4 with no orbital degrees of freedom shows tiny deviations from cubic structure even in its ground state.

Dimensionality and disorder in TTT-I1.6

Abstract DC conductivity and thermoelectric power measurements are presented on the organic charge transfer salt TTT-I 1.6 . It is argued that the metallic low temperature behaviour srises from the joint influence of a finite interchain bandwidth and weak disorder.

Defect dependence of the dielectric permeability of Qn(TCNQ)2

Abstract The conductivity and 9.1 GHz dielectric constant of neutron irradiated Qn(TCNQ) 2 single crystals are strongly affected by defect concentrations less than 1 %. This indicates long range electronic coherence of the order of 100 lattice constants in the non-irradiated material for which the coherence length may still be impurity limited. It is suggested that a collective excitation of electrons is responsible for the temperature dependence of the large dielectric constant.

Pressure induced quantum critical point and non-Fermi-liquid behavior in BaVS3

The phase diagram of BaVS3 is studied under pressure using resistivity measurements. The temperature of the metal to nonmagnetic Mott insulator transition decreases under pressure, and vanishes at the quantum critical point p(cr) = 20 kbar. We find two kinds of anomalous conducting states. The high-pressure metallic phase is a non-Fermi liquid described by Deltarho approximately T(n) where n = 1.2-1.3 at 1<T<40 K. At p<p(cr), the transition is preceded by a wide precursor region with critically increasing resistivity which we ascribe to the opening of a soft Coulomb gap.

Huge negative differential conductance in Au- H2 molecular nanojunctions

Experimental results showing huge negative differential conductance in gold-hydrogen molecular nanojunctions are presented. The results are analyzed in terms of two-level system (TLS) models: It is shown that a simple TLS model cannot produce peaklike structures in the differential conductance curves, whereas an asymmetrically coupled TLS model gives perfect fit to the data. Our analysis implies that the excitation of a bound molecule to a large number of energetically similar loosely bound states is responsible for the peaklike structures. Recent experimental studies showing related features are discussed within the framework of our model.

Single crystal conductivity of bipyridine-TCNQ salts

Abstract The sharp contrast between the conductivity of complex TCNQ salts with symmetric and asymmetric bipyridine donors reveals the role played by the asymmetric molecules in well conducting organic salts. A highly anisotropic strongly correlated semiconductor is obtained when the donor molecule is symmetric, a smeared out “metal insulator” transition is found with the asymmetric donor modification.