Physique de la matière condensée

La physique de la matière condensée est la branche de la physique qui étudie les propriétés microscopiques et macroscopiques de la matière dans un état dit « condensé ». Ce terme doit être entendu par opposition à d'autres états de la matière, plus dilués, tels que l’état gazeux et les plasmas, ou encore par opposition à l’étude des atomes ou molécules isolés ou peu nombreux. Son objet d’étude consiste donc principalement dans les solides, ce qui explique que cette branche de la physique a longtemps été désignée par le terme de « physique des solides ». Un état est dit condensé lorsque la corrélation spatiale des atomes ou molécules qui le constituent reste substantielle, même à grande distance, ce qui n'est pas le cas des gaz ou liquides. Les physiciens de la matière condensée utilisent les lois de la physique, en particulier la mécanique quantique, l'électromagnétisme et la physique statistique. Pour des raisons historiques et méthodologiques, le champ de la discipline est limité au

Charge transport in proximitized graphene within van der Waals heterostructures

Katharina Polyudov

Since their discovery, graphene and other 2D materials have become a subject of intense research in condensed matter physics. Especially the vast possibilities of combining those materials into heterostructures are promising for the discovery of novel physical phenomena. The heterostructures accessible through state-of-the-art techniques have suitably clean interfaces between the layers. Graphene has gained a lot of attention due to its remarkable mechanical stability and extremely high electron mobility. However, the zero bandgap of graphene is a major bottleneck for implementing this 2D material into most electronic applications. The ability to tune the properties of graphene by proximity effects has shifted the focus of graphene research towards the combination of graphene with other van der Waals materials. The main objective of the present thesis was to explore for two different types of graphene-based van der Waals heterostructure whether fingerprints of electronic proximity effects can be traced in their low temperature magnetotransport properties. The first part of the thesis deals with heterostructures of graphene and Bi2Te2Se (BTS). The strong spin-orbit coupling of BTS makes it a three-dimensional topological insulator with topological surface states which are protected by time-reversal symmetry. At the same time, BTS is promising to exert a pronounced spin-orbit proximity effect on graphene, and thereby to open a band gap and/or introduce a spin texture in the latter. The graphene/BTS heterostructures were fabricated by the direct growth of BTS on graphene to ensure a clean interface between both materials and correspondingly a good electronic coupling between them. Analysis of the weak localization effect observed in the magnetoconductivity revealed the presence of enhanced SOC in the proximitized graphene. The second part of the thesis focuses on heterostructures wherein graphene is combined with a-RuCl3 which has recently gained a lot of attention as a potential quantum spin liquid system. Previous studies on graphene/a-RuCl3 heterostructures found an unusual temperature evolution of the quantum oscillation amplitude, whose origin remained unclear. The magnetotransport data collected in this thesis point toward two possible origins for this behavior, namely spin fluctuations associated with the magnetic transition into an antiferromagnetic phase at the Néel temperature of approximately 7 K, and the hybridization-induced formation of heavy (flat) bands, both of which are likely to depend on the a-RuCl3 layer thickness. In addition, heterostructures comprising an a-RuCl3 monolayer were found to display a unusual gate dependence of the quantum oscillations, further corroborating the importance of the a-RuCl3 thickness.

EPFL2020

Nonperturbative Quantum Field Theory in Curved Spacetime

Kamran Salehi Vaziri

Quantum Field Theory(QFT) as one of the most promising frameworks to study high energy and condensed matter physics, has been mostly developed by perturbative methods. However, perturbative methods can only capture a small island of the space of QFTs.QFT in hyperbolic space can be used to link the conformal bootstrap and massive QFT. Conformal boundary correlators also can be studied by their general properties such as unitarity, crossing symmetry and analicity. On the other hand, by sending the curvature radius to infinity we reach to the flat-space limit in hyperbolic space. This allows us to use conformal bootstrap methods to study massive QFT in one higher dimension and calculate observables like scattering amplitudes or finding bounds on the couplings of theory. The main goal of my research during my Ph.D. would be to study QFTs in hyperbolic space to better understand strongly coupled QFTs.Hamiltonian truncation is a numerical method to study strongly coupled QFTs by imposing a UV cutoff. We use this method to study strongly coupled QFT in hyperbolic space background. For simplicity, we start with scalar field theory in 2-dimensional AdS. We expect to extract the spectrum of our theory as a function of AdS curvature and find the boundary correlation functions.

EPFL2022

Order and dynamics in Kagome like compound Cu2OSO4 and other quantum magnets

Virgile Yves Favre

This thesis presents results of studies of novel compounds modeling complex fundamental physics phenomena. Cu2OSO4 is a copper based magnetic Mott Insulator system, where spin half magnetic moments form a new type of lattice. These intrinsically quantum pins are exhibiting atypical magnetic order and spin dynamics. The recent success in the growth of large single crystals of Cu2OSO4 enabled to perform measurements probing its static and fluctuating properties. The peculiarity of this sample is that its atoms are forming layers, with a geometry close to the intensively studied Kagomé lattice, but with a third of its spins replaced by dimers. This quantum magnetism system has been probed in its bulk, by the means of heat capacity and DC-susceptibility measurements, revealing a transition to a magnetically long range ordered state upon cooling, the details of which are revealed by neutron scattering. Single crystal inelastic neutron scattering shed light on the spin-dynamics in the system, with clear spin waves appearing as fluctuations around the peculiar ground state of the system: a 120 degrees spin configuration where the magnetic moment of the spin-dimer causes the sample to be globally ferrimagnetic. The presented results indicate that Cu2OSO4 represents a new type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored. The magnetic excitations of the compound can be modeled by a yet-to-be-understood internal effective mean-field that no simple magnetic coupling seems to reproduce. K2Ni2(SO4)3 is another compound that allows for the existence of non-trivial topological phases. This thesis presents results of the study of the unusual magnetic behavior of K2Ni2(SO4)3. No clear sign of well-established magnetic long range order has been observed down to dilution temperatures. Neutron scattering reveals the details of the competition between frustration and fluctuations that prevent order from settling in. Low temperature spin excitations take the form of a continuum at 500 mK, but also of broad, energy independent continua at higher temperatures. Bulk and neutron scattering measurements are put in perspective and linked together with a view to building up a better understanding of how quantum spin liquids can be stabilized in general, and in particular in this model compound. Finally, the last contribution of this thesis to the field of condensed matter physics regards the establishment of a state-of-the-art technique to fit heat capacity and unit cell volume of samples to try and make the extraction of magnetic information from specific heat measurements more robust. This newly-developed technique consists in modeling lattice contributions with better accuracy by using data from multiple experimentally accessible quantities to consolidate the fitting scheme. This method has been cautiously applied to several compounds at the forefront of research in experimental physics.

EPFL2021