Influence of static correlation on the magnon dynamics of an itinerant ferromagnet with competing exchange interactions: First-principles study of MnBi

We present first-principles calculations of the dynamic susceptibility in strained and doped ferromagnetic MnBi using time-dependent density functional theory. In spite of being a metal, MnBi exhibits signatures of strong correlation and a proper description in the framework of density functional theory requires Hubbard corrections to the Mn d orbitals. To permit calculations of the dynamic susceptibility with Hubbard corrections applied to the ground-state electronic structure, we use a consistent rescaling of the exchange-correlation kernel maintaining the delicate balance between the magnon dispersion and the Stoner continuum. We find excellent agreement with the experimentally observed magnon dispersion for pristine MnBi and show that the material undergoes a phase transition to helical order under application of either doping or strain. The presented methodology paves the way for future linear response time-dependent density functional theory studies of magnetic phase transitions, also for the wide range of materials with pronounced static correlation effects that are not accounted for at the local density approximation level.

Pulsed laser deposition of (SrBa)Nb2O6 thin films and their properties

Anna Infortuna

Strontium barium niobate (SrxBa1-xNb2O6, shortly SBN) is a solid solution system with tetragonal tungsten bronze crystal structure. It exhibits a ferroelectric phase with only one polar axis and a transition temperature depending on the Sr/Ba ratio. This thesis studies the growth of SBN thin films, in order to obtain a ferroelectric thin film model system for the study of 180° domain switching close to the ferroelectric phase transition. SBN has been chosen not only because uniaxial, but because it is possible to tune the temperature of phase transition varying the Sr/Ba ratio. It is thus possible to study electric properties of the phase transition close to room temperature, where conduction phenomena related to defects are normally lower. Thin films of SBN were grown by Pulsed Laser Deposition in a vacuum system constructed during this thesis work. For the formulation of a model it is desirable to have a system as close as possible to a single crystal. Therefore deposition parameters have been optimized to grow SBN thin films with the polar axis oriented perpendicular to the film plane. The films are integrated in parallel plate capacitor structures, in order to measure dielectric properties. The stability of the ferroelectric phase is found to be sensitive to impurities; contaminants with concentration below 1% induce a lowering of the transition temperature of about 100°C. Strain as well has influence on the phase transition; in the case of substrate with thermal expansion coefficients different from the SBN ones, the SBN film is under stress. A 0.6% tensile strain induces a lowering of the transition temperature of about 100°C, this is the case of silicon single crystal substrate. Grown on strontium titanate single crystals (STO), whose thermo-mechanical properties are close to the one of SBN, the films exhibit a phase transition temperature compatible with the one measured on single crystals. The obtained films suffer from leakage that is reduced by thermal treatment in oxygen rich atmosphere. The oxygen vacancies, created during the deposition process, are considered to be responsible of leakage. Vacancies recovery upon annealing does not eliminate the problem, and measurement of polarization at room temperature is difficult. However, piezoelectric measurements performed on individual grains, with atomic force microscope, prove that these have ferroelectric properties; by these experiment has been proved that it is possible to switch the polarization. Films with a disordered structure of grains are much less leaky than the ones with columnar grains spanning the whole cross section. These observations lead to the conclusion that conduction goes trough the grain boundaries. On STO single crystals, SBN grows epitaxial in Volmer-Weber mode. With a suitable preparation of the substrate surface, it is possible to induce the growth of films with different orientations of the polar axis: in the film plane, perpendicular to it or inclined of 45°. A model is proposed, explaining the epitaxial relation with the substrate on the basis of the perovskite kernel contained in the unit cell of the tetragonal tungsten bronze structure. The film grows from the nucleation of the perovskite structure that rules the orientation of the film; the high temperature at which the substrate is kept during the deposition, assure the necessary mobility for the arriving atoms to organize in the SBN structure around these nuclei. The study of literature data allows to state that such a model is valid not only for substrates with perovskite structure, but can be extended to substrates with a cubic crystal structure, like magnesium oxide.

EPFL2005

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.

EPFL2012

Berry phase investigation of spin-S ladders

Natalia Chepiga, Frédéric Alexis Michaud, Frédéric Mila

We investigate the properties of antiferromagnetic spin-S ladders with the help of local Berry phases defined by imposing a twist on one or a few local bonds. In gapped systems with time-reversal symmetry, these Berry phases are quantized, hence able in principle to characterize different phases. In the case of a fully frustrated ladder where the total spin on a rung is a conserved quantity that changes abruptly upon increasing the rung coupling, we show that two Berry phases are relevant to detect such phase transitions: the rung Berry phase defined by imposing a twist on one rung coupling, and the twist Berry phase defined by twisting the boundary conditions along the legs. In the case of nonfrustrated ladders, we have followed the fate of both Berry phases when interpolating between standard ladders and dimerized spin chains, with the surprising conclusion that, at least far enough from dimerized chains, they define different domains in parameter space. A careful investigation of the spin gap and of edge states shows that a change of twist Berry phase is associated with a quantum phase transition at which the bulk gap closes, and at which, with appropriate boundary conditions, edge states appear or disappear, while a change of rung Berry phase is not necessarily associated with a quantum phase transition. The difference is particularly acute for regular ladders, in which the twist Berry phase does not change at all upon increasing the rung coupling from zero to infinity while the rung Berry phase changes 2S times. By analogy with the fully frustrated ladder, these changes are interpreted as crossovers between domains in which the rungs are in different states of total spin from 0 in the strong rung limit to 2S in the weak rung limit. This interpretation is further supported by the observation that these crossovers turn into real phase transitions as a function of rung coupling if one rung is strongly ferromagnetic, or equivalently if one rung is replaced by a spin 2S impurity.

2013