Charge-Stripe Order and Superconductivity in Ir1−xPtxTe2

A combined resistivity and hard x-ray diffraction study of superconductivity and charge ordering in Ir Ir1−xPtxTe2, as a function of Pt substitution and externally applied hydrostatic pressure, is presented. Experiments are focused on samples near the critical composition x c ~ 0.045 where competition and switching between charge order and superconductivity is established. We show that charge order as a function of pressure in Ir0.95Pt0.05Te2 is preempted — and hence triggered — by a structural transition. Charge ordering appears uniaxially along the short crystallographic (1, 0, 1) domain axis with a (1/5, 0, 1/5) modulation. Based on these results we draw a charge-order phase diagram and discuss the relation between stripe ordering and superconductivity.

Competing orders in strongly correlated systems studied by transport measurements

Ana Akrap

Competing orders in strongly correlated systems lead to rich phase diagrams comprising many electronic phases, such as superconductivity, charge/spin density wave, charge order, or bad metallicity. These phases are generically sensitive to a variety of parameters, for example temperature, magnetic field, dimensionality, presence of disorder, geometrical frustration. In this thesis, we employ electronic transport measurements under high pressure on few model compounds to gain insight into the complex physics of strongly correlated compounds. The transport coefficients, resistivity and thermoelectric power, shed light onto conduction processes and the thermodynamics. The pressure is a perfect tool to investigate competition of different ground states: by modifying the lattice parameters, it can tune the interactions without introducing disorder. Several representative compounds were chosen for this study. In the first part, we focus on the transport properties of the quasi-one dimensional BaVS3. The main characteristic of this 3d1 system is the coexistence of a broad one-dimensional dz2 electronic band and a narrow isotropic eg band at the Fermi level. The suppression of the insulating phase by high pressure leads to a non-Fermi liquid phase. We showed that magnetic field does not recover the Fermi liquid behavior, and that the disorder pushes the system further into non-Fermi liquid state. This is at variance with what has been observed in other non-Fermi liquid compounds, and confirms the novelty of the mechanism for non-Fermi liquid behavior in BaVS3. To achieve better understanding of the role of the localized electrons, we investigated systematically the influence of disorder. In addition, we studied the properties of the BaVSe3, which due to the reinforced interchain interactions may be considered as the high-pressure counterpart of BaVS3. The system is a metallic ferromagnet, in which the strong interaction of dz2 and eg electrons dictates the behavior of transport coefficients. In the following part we studied the rich physics of quasi-one dimensional β-vanadium bronzes. In the stoichiometric β-SrV6O15, we followed the pressure dependence of the semiconductor-insulator transition by resistivity and thermopower. We found evidence suggesting that the ground state is charge ordered. Under pressure, the changing character of the transport coefficients implied a competition of different ground states. Moreover, we observed resistive switching in the insulating phase. When strontium doping is decreased, in SrxV6O15 and x < 1, the disorder starts governing the physics of the system. The off-stoichiometric compounds are characterized by the absence of phase transition, absence of resistive switching, and possibly by the presence of polarons. We also found resistive switching in another charge ordered transition-metal oxide, Fe2OBO3. This system shows an interplay of commensurate and incommensurate charge order. The switching is restricted to the incommensurate phase, whose origin probably lies in the geometrical frustration of the interactions between iron atoms. With pressure we enhance the Coulomb repulsion, and the incommensurate phase shrinks in temperature. In the final part, we address the high-pressure transport of a superconductor on a geometrically frustrated pyrochlore lattice, ΚOs2O6. The potassium atoms are enclosed in oversized cages and their rattling motion introduces a localized low-energy mode. The transport coefficients in this compound are highly anomalous: the resistivity shows no saturation at low temperatures, and the scythe-shaped thermoelectric power is reminiscent of the one observed in cuprates. We were able to reproduce the temperature and pressure dependence of the transport coefficients within a simple model of the density of states.

EPFL2008

High-pressure infrared spectroscopy: Tuning of the low-energy excitations in correlated electron systems

Helmuth Berger, László Forró, Richard Gaal, Balazs Sipos

We have extended the range of the high-pressure optical spectroscopy to the far-infrared region keeping the accuracy of ambient-pressure experiments. The developed method offers a powerful tool for the study of pressure-induced phase transitions and electronic-structural changes in correlated electron systems as the optical pressure cell, equipped with large free-aperture diamond window, allows the measurement of optical reflectivity down to omega approximate to 20-30 cm(-1) for hydrostatic pressures up to p approximate to 26 kbar. The efficiency of the technique is demonstrated by the investigation of the two-dimensional charge-density-wave 1T-TaS2 whose electronic structure shows high sensitivity to external pressure. The room-temperature semimetallic phase of 1T-TaS2 is effectively extended by application of pressure and stabilized as the ground state above p=14 kbar. The corresponding fully incoherent low-energy optical conductivity is almost temperature independent below T=300 K. For intermediate pressures, the onset of the low-temperature insulating phase is reflected by the sudden drop of the reflectivity and by the emergence of sharp phonon resonances.

2007

Thermopower as a Unique Tool to Probe Quantum Phase Transitions

Stevan Arsenijevic

In my thesis, transport measurements such as resistivity and, more importantly, thermopower S, were used to explore the phase diagram of bad metals. Bad metals are electronically correlated systems whose ground state lies close to a quantum phase transition. By tuning the control parameters, such as temperature (T ), magnetic field (B), hydrostatic pressure (p) or chemical substitution (x), we can induce phase transitions between the various electronic, magnetic and structural phases. Here, the thermopower is presented as a unique tool for probing quantum phase transition because it is a measure of the entropy of conducting electrons. The main part of the thesis is dedicated to the study of Fe-based superconductors (FeSC) discovered in 2008. Their parent compound has an antiferromagnetic (AF) ground state, where the itinerant electrons form a spin-density wave (SDW), a periodic modulation of spin density. This coincides or is preceded by a structural, tetragonal-to-orthorhombic transition. The nesting between the electron and hole Fermi surface is believed to be the driving mechanism for the SDW state. By changing the structural or chemical properties the AF ground state of FeSC is suppressed, giving way to superconductivity (SC). The remaining antiferromagnetic fluctuations above the transition can provide a glue for SC pairing. Here, the analysis of the thermopower S/T of BaFe1−xCoxAs2 (BFCA) in the x-T phase diagram shows the signatures of the spin fluctuation which have a dome-like dependence and follow the trend of superconducting Tc . The logarithmic increase of S/T upon decreasing T is ascribed to the proximity of the spin-density-wave quantum critical point. It can be understood as an increase of entropy due to the incommensurate AF spin fluctuations. We can ascribe the high values of thermopower in BFCA at intermediate- and room-temperatures to the influence of low-T quantum criticality. To probe the response of the electronic system in FeSC to structural changes, we performed measurements under pressure of the parent compound BaFe2As2 (BFA), the SC electron-doped BFCA, and hole-doped Ba1−xKxFe2As2 (BKFA). In the parent compound pressure suppresses the structural/SDW transition, similar to the effect of doping. For doped systems, in order to describe the behavior of thermopower in the high-T range (above 100K) we used a semi-metallic two-band model which was fitted to the data in order to extract the pressure dependence of the band parameters. In both doping cases the effect of pressure was similar, an increase of the band overlap and of the effective number of charge carriers. With this model we can explain the high-T , x and p dependence of thermopower in both electron- and hole-doped BFA. In a structurally simpler Fe-chalcogenide Fe1+yTe1−xSex compound, the excess of Fe has a Kondo-like influence on the charge carriers which dramatically changes the physics of the normal state. To probe the normal state, pressure, doping, magnetic Fe-excess concentration (y) and temperature were used as control parameters. At low-T a characteristic upturn of resistivity (ρmag ) is observed, followed by an increase of thermopower (Smag ), which we identify as the magnetic contribution caused by the spin-flip scattering events. Increasing the y resulted in an increase of ρmag , and a decrease of Smag , which is in agreement with the behavior of canonical Kondo-systems. Pressure suppresses the magnetic contribution to transport, thus increasing the itinerancy of the system. MnSi is another system in which the sensitivity of thermopower to entropy brings new information related to the complex magnetic structure. Pressure was used to drive the system from a helically ordered, canonical Fermi-liquid (FL) phase with T 2-resistivity to the intrinsically disordered, non-Fermi-liquid (NFL) phase above pc with T3/2-dependence. Our contour plot of S/T demonstrated how powerful the thermopower technique is, by reproducing the whole previously established T -p phase diagram. At the phase transition from the magnetically-ordered FL phase to the disordered NFL, the thermopower is dramatically enhanced. We bring useful information about the mysterious partial order (PO) phase inside the NFL phase, previously detected only by neutron scattering. The fluctuating helices scenario can describe the observed increase of entropy/thermopower in the PO phase. At ambient pressure, close to the helical transition of MnSi, a moderate magnetic field can stabilize the skyrmion lattice - the lattice of topological magnetic whirls, vortices. We observe a signature of the skyrmion lattice as a minute drop in thermopower. It is located exactly in the same region of the T − B phase diagram where an increase in magnetoresistance and Hall effect was reported previously. This feature originates from the additional scattering of conducting electrons on magnetic vortices, while the change in S is dominated by the decrease of entropy as the stable skyrmion lattice is formed. Overall, resistivity was used to confirm the established phase diagram, while thermopower, as an interesting and not sufficiently understood technique, was used to probe the sensitive changes of the charge carriers at the Fermi surface. We explored various phases showing how useful thermopower is to probe the entropy of electronic system on the verge of quantum phase transition.