Neutron Scattering of Spin Waves in Square Lattice Quantum Antiferromagnets at high and low energies

In the field of quantum magnetism, neutron scattering techniques allows probing of the magnetic properties of many materials, as the neutrons carry spin. Interesting materials are for example the parent compound of a high temperature superconductor La2CuO4 or the band insulator Cu(DCOO)2·4D2O (known as CFTD). Although the electrical properties of these two compounds are different, both show antiferro-magnetic Heisenberg coupling of spin 1/2 in a 2D square lattice geometry, with very weak interplane coupling in the third direction, which implies that the magnetic properties of both La2CuO4 and CFTD present many similarities. In such systems, the excitations of the spins are known as spin waves, and neutron scattering has played a crucial part in better understanding this theory, as it gives the possibility to directly probe the correlation functions describing the system. However, the compounds have also been found to have non classical and highly fluctuating behaviour, due to the quantum nature of spin 1/2 spins and the low dimensionality antiferro-magnetic system.

Order and Dynamics of Model Quantum Antiferromagnets

Martin Mourigal

The spin-1/2 Heisenberg antiferromagnets in one and two dimensions are important models in quantum magnetism in which many-body effects play a crucial role. Their collective excitations are associated with quasiparticles the interaction of which are difficult to describe. Neutron scattering offers a very powerful experimental technique to study these excitations. In addition, it can reveal the nature of exotic ground-states favored by competing frustrated interactions or by an applied magnetic field. This thesis presents neutron scattering studies of four materials, each representing a model magnetic system. • Cu(DCOO)2·4D2O (CFTD) is a model realization of the S =1/2 square-lattice antiferromagnet. It displays Néel order and its long-wavelength excitations are well described by renormalized spin-wave theory. In contrast, the excitations at the magnetic zone-boundary display anomalous dispersion and intensity close to (π, 0). A polarized neutron study of the anomaly additionally reveals the presence of a transverse continuum associated with a lineshape that extends to high energies. Results for the transverse and longitudinal continua are compared with Quantum Monte Carlo and fermionic Resonating Valence Bond theories. • (5CAP)2CuCl4 (CAPCC) is a S = 1/2 square-lattice antiferromagnet with modest saturation magnetic field Hs = 3.6 T and ∼ 20% antiferromagnetic interlayer coupling. Its spin excitations, measured above saturation are used to determine the microscopic exchange parameters of its Hamiltonian, while the presence of spontaneous magnon decay is investigated above the theoretically predicted threshold field H*= 0.76Hs. • CuSO4·5D2O is a model realization of the S = 1/2 antiferromagnetic Heisenberg chain. Its excitations are fully characterized in various regimes of applied magnetic field. In zero field, the presence of two- and four-spinon states is quantitatively confirmed, while the development of long-range Néel order is shown to preserve this picture qualitatively. In applied magnetic field, the excitations are studied with polarized neutrons and the existence of exotic psinons, anti-psinons and string-solutions continua is discussed. • LiCuVO4 is a S = 1/2 frustrated multiferroic chain material with competing ferromagnetic nearest and antiferromagnetic next-nearest neighbor interactions. Its magnetic structure is found to be a perfect circular cycloid the chirality of which can be tuned with an applied electric field. In crossed electric and magnetic fields, the behavior of the cycloid closely matches with that theoretically predicted from the spin supercurrent scenario. A magnetic-field induced transition to short-range longitudinal order is also evidenced with polarized neutrons. The present work was carried out in the Triple-Axis Spectrometers group at the Institut Laue Langevin in Grenoble, France and in the Laboratory for Quantum Mangetism at the EPFL, Switzerland.

EPFL2011

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

Static and dynamic properties of non-collinear quantum antiferromagnets explored by neutron scattering

Luc Testa

This thesis is devoted to the investigation of static and dynamic properties of two different sets of quantum magnets with neutron scattering techniques and the help of linear spin wave theory. Both systems are copper-based with spin-1/2, which makes them ideal to study the interplay between purely quantum and semi-classical effects. I start with the analysis of the antiferromagnet SeCuO3, which has a canted spins structure. Through careful inelastic neutron scattering experiments on thermal and cold triple-axis spectrometers, I demonstrate that this compound exhibits three primary types of excitations that are intrinsically opposite : spin waves (magnons), singlet to triplet excitations (triplons), and fractional spins excitations (spinons). Such a strong coexistence and interdependence of these collective excitations has not been observed yet, thereby the quantification and description of the excitations in SeCuO3 leads the way to further theoretical work on multi-excitation spin systems, as well as the existence of quantum effects in high dimensional systems.\ My second project is on the extraction of the magnetic structure of three members of the A(BO)Cu4(PO4)4 chiral family, namely (A; B) = (Ba; Ti), (Sr; Ti) and (Pb; Ti), from spherical neutron polarimetry measurements. I prove that the first two compounds exhibit a highly non-collinear magnetic structure, with the Cu spins forming clusters of 'two-in--two-out' arrangements on each structural unit. This structure is stabilised by the presence of a strong Dzyaloshinskii-Moriya interaction, and explains the observation of magnetoelectric effects as emerging from quadrupole moments. The analysis of the latter compound did not lead to the confirmation of its magnetic structure due to strong nuclear-magnetic interference. I conclude this thesis by the investigation of the magnetic excitation spectrum of some members of the (A; B) family, probed by inelastic neutron scattering measurements. Indeed, its particular crystallographic structure makes it an ideal playground to study tetramerisation effects on the two dimensional square lattice. Additionally, the aforementioned Dzyaloshinskii-Moriya interaction ensures the presence of a structural gap, which competes with the quantum one emerging from tetramerisation effects. Using linear spin wave theory, I describe (Ba; Ti) as a chequerboard system with almost equal intra- and inter-plaquette couplings, with weak quantum effects. I also provide a qualitative description of (Pb; Ti), which exhibits similar physics, and conclude by presenting the first results on the highly symmetric compound (K; Nb), which shows hints of a strong quantum behaviour.

EPFL2021