Valleytronics is rapidly emerging as an exciting area of basic and applied research. In two-dimen... more Valleytronics is rapidly emerging as an exciting area of basic and applied research. In two-dimensional systems, valley polarization can dramatically modify physical properties through electron-electron interactions as demonstrated by such phenomena as the fractional quantum Hall effect and the metal-insulator transition. Here, we address the electrons' spin alignment in a magnetic field in silicon-on-insulator quantum wells under valley polarization. In stark contrast to expectations from a non-interacting model, we show experimentally that less magnetic field can be required to fully spin polarize a valley-polarized system than a valley-degenerate one. Furthermore, we show that these observations are quantitatively described by parameter-free ab initio quantum Monte Carlo simulations. We interpret the results as a manifestation of the greater stability of the spin-and valley-degenerate system against ferromagnetic instability and Wigner crystalization, which in turn suggests the existence of a new strongly correlated electron liquid at low electron densities.
The quantum-classical crossover from the Fermi liquid towards the Wigner solid is numerically rev... more The quantum-classical crossover from the Fermi liquid towards the Wigner solid is numerically revisited, considering small square lattice models where electrons interact via a Coulomb U/r potential. We review a series of exact numerical results obtained in the presence of weak site disorder for fully polarized electrons (spinless fermions) and when the spin degrees of freedom are included. A novel intermediate regime between the Fermi system of weakly interacting localized particles and the correlated Wigner solid is obtained. A detailed analysis of the non disordered case shows that the intermediate ground state is a solid entangled with an excited liquid. For electrons in two dimensions, this raises the question of the existence of an unnoticed intermediate liquid-solid phase. Using the Coulomb energy to kinetic energy ratio rs ∝ U ∝ n −1/2 s
An intrinsic measure of the quality of a variational wave function is given by its overlap with t... more An intrinsic measure of the quality of a variational wave function is given by its overlap with the ground state of the system. We derive a general formula to compute this overlap when quantum dynamics in imaginary time is accessible. The overlap is simply related to the area under the E(τ ) curve, i.e. the energy as a function of imaginary time. This has important applications to, for example, quantum Monte-Carlo algorithms where the overlap becomes as a simple byproduct of routine simulations. As a result, we find that the practical definition of a good variational wave function for quantum Monte-Carlo simulations, i.e. fast convergence to the ground state, is equivalent to a good overlap with the actual ground state of the system. PACS numbers:
The observation of isolated positive and negative charges, but not isolated magnetic north and so... more The observation of isolated positive and negative charges, but not isolated magnetic north and south poles, is an old puzzle. Instead, evidence of effective magnetic monopoles has been found in the abstract momentum space. Apart from Hall-related effects, few observable consequences of these abstract monopoles are known. Here, we show that it is possible to manipulate the monopoles by external magnetic fields and probe them by universal conductance fluctuation (UCF) measurements in ferromagnets with strong spin-orbit coupling. The observed fluctuations are not noise, but reproducible quasiperiodic oscillations as a function of magnetisation direction, a novel Berry phase fingerprint of the magnetic monopoles.
Using first-principles calculations, we explore the electronic and magnetic properties of graphen... more Using first-principles calculations, we explore the electronic and magnetic properties of graphene nanomesh (GNM), a regular network of large vacancies, produced either by lithography or nanoimprint. When removing an equal number of A and B sites of the graphene bipartite lattice, the nanomesh made mostly of zigzag (armchair) -type edges exhibit antiferromagnetic (spin unpolarized) states. In contrast, in situations of sublattice symmetry breaking, stable ferri(o)magnetic states are obtained. For hydrogen-passivated nanomesh, the formation energy is dramatically decreased, and ground state is found to strongly depend on the vacancies shape and size. For triangular-shaped holes, the obtained net magnetic moments increase with the number difference of removed A and B sites in agreement with Lieb's theorem for even A + B. For odd A + B triangular meshes and all cases of nontriangular nanomeshes, including the one with even A + B, Lieb's theorem does not hold anymore, which can be partially attributed to the introduction of armchair edges. In addition, large triangular-shaped GNMs could be as robust as nontriangular GNMs, providing a possible solution to overcome one of the crucial challenges for the sp magnetism. Finally, significant exchange-splitting values as large as ∼ 0.5 eV can be obtained for highly asymmetric structures evidencing the potential of GNM for room-temperature carbon-based spintronics. These results demonstrate that a turn from zero-dimensional graphene nanoflakes throughout one-dimensional graphene nanoribbons with zigzag edges to GNM breaks localization of unpaired electrons and provides deviation from the rules based on Lieb's theorem. Such delocalization of the electrons leads the switch of the ground state of a system from an antiferromagnetic narrow gap insulator discussed for graphene nanoribons to a ferromagnetic or nonmagnetic metal.
The Fermi liquid-Wigner crystal transition in a two dimensional electronic system is revisited wi... more The Fermi liquid-Wigner crystal transition in a two dimensional electronic system is revisited with a focus on the nature of the fixed node approximation done in quantum Monte Carlo calculations. Recently, we proposed (Phys. Rev. Lett. 94, 046801 (2005) ) that for intermediate densities, a hybrid phase (with the symmetry of the crystal but otherwise liquid like properties) is more stable than both the liquid and the crystal phase. Here we confirm this result both in the thermodynamic and continuum limit. The liquid-hybrid transition takes place at r * s = 31.5 ± 0.5. We find that the stability of the hybrid phase with respect to the crystal one is tightly linked to its delocalized nature. We discuss the implications of our results for various transition scenarii (quantum hexatic phase, supersolid, multiple exchange, microemulsions) proposed in the literature.
Building on the many existing algorithms for calculating the DC transport properties of quantum t... more Building on the many existing algorithms for calculating the DC transport properties of quantum tight-binding models, we develop a systematic approach that expresses finite frequency observables in terms of the stationary Green's function of the system, i.e. the natural output of most DC numerical codes. Our framework allows to extend the simulations capabilities of existing codes to a large class of observables including, for instance, AC conductance, quantum capacitance, quantum pumping, spin pumping or photo-assisted shot noise. The theory is developed within the framework of Keldysh formalism and we provide explicit links with the alternative (and equivalent) scattering approach. We illustrate the formalism with a study of the AC conductance in a quantum point contact and an electronic Mach-Zehnder interferometer in the quantum Hall regime.
Following the recent proposal by Weeks et al., which suggested that indium (or thallium) adatoms ... more Following the recent proposal by Weeks et al., which suggested that indium (or thallium) adatoms deposited on the surface of graphene should turn the latter into a quantum spin Hall (QSH) insulator characterized by a sizeable gap, we perform a systematic study of the transport properties of this system as a function of the density of randomly distributed adatoms. While the samples are, by construction, very disordered, we find that they exhibit an extremely stable QSH phase with no signature of the spatial inhomogeneities of the adatom configuration. We find that a simple rescaling of the spin-orbit coupling parameter allows us to account for the behaviour of the inhomogeneous system using a homogeneous model. This robustness opens the route to a much easier experimental realization of this topological insulator. We additionally find this material to be a very promising candidate for thermopower generation with a target temperature tunable from 1 to 80 K and an efficiency ZT ≈ 1.
PACS. 71.30+h -Metal-insulator transitions and other electronic transitions. PACS. 71.27+a -Stron... more PACS. 71.30+h -Metal-insulator transitions and other electronic transitions. PACS. 71.27+a -Strongly correlated electron systems. PACS. 72.15Rn -Quantum localization.
We consider the spin torque induced by a current flowing ballistically through a magnetic domain ... more We consider the spin torque induced by a current flowing ballistically through a magnetic domain wall. In addition to a global pressure in the direction of the electronic flow, the torque has an internal structure of comparable magnitude due to the precession of the electrons' spins at the "Larmor" frequency. As a result, the profile of the domain wall is expected to get distorted by the current and acquires a periodic sur-structure.
PACS. 71.30+h -Metal-insulator transitions and other electronic transitions. PACS. 72.15Rn -Quant... more PACS. 71.30+h -Metal-insulator transitions and other electronic transitions. PACS. 72.15Rn -Quantum localization. PACS. 71.27+a -Strongly correlated electron systems.
Extending finite size scaling theory to the many body ground state, one finds that Coulomb repuls... more Extending finite size scaling theory to the many body ground state, one finds that Coulomb repulsion can drive a system of spinless fermions in a random potential from the Anderson insulator (non interacting localized states) towards a new metallic phase (Coulomb metal) in dimension d=2. The transition occurs at a Coulomb energy to Fermi energy ratio rs ≈ 4, where a change in the nature of the persistent currents is observed.
For intermediate Coulomb energy to Fermi energy ratios rs, spinless fermions in a two-dimensional... more For intermediate Coulomb energy to Fermi energy ratios rs, spinless fermions in a two-dimensional random potential form a new quantum phase, different from the Fermi glass (weakly interacting Anderson localized states) and the Wigner crystal (regular array of charges pinned by the disorder). The intermediate phase is characterized by an ordered flow of persistent currents with a typical value decreasing with excitation energy. Extending finite size scaling analysis to the many body ground state, we find that electron-electron interactions can drive the Fermi glass towards an intermediate metallic phase (Coulomb metal).
We discuss the current induced magnetization dynamics of spin valves F0|N|SyF where the free laye... more We discuss the current induced magnetization dynamics of spin valves F0|N|SyF where the free layer is a synthetic ferrimagnet SyF made of two ferromagnetic layers F1 and F2 coupled by RKKY exchange coupling. In the interesting situation where the magnetic moment of the outer layer F2 dominates the magnetization of the ferrimagnet, we find that the sign of the effective spin torque exerted on the free middle layer F1 is controlled by the strength of the RKKY coupling: for weak coupling one recovers the usual situation where spin torque tends to, say, anti-align the magnetization of F1 with respect to the pinned layer F0. However for large coupling the situation is reversed and the spin torque tends to align F1 with respect to F0. Careful numerical simulations in the intermediate coupling regime reveal that the competition between these two incompatible limits leads generically to spin torque oscillator (STO) behavior. The STO is found in the absence of magnetic field, with very significant amplitude of oscillations and frequencies up to 50 GHz or higher.
We study the evolution of the two-terminal conductance plateaus with a magnetic field for armchai... more We study the evolution of the two-terminal conductance plateaus with a magnetic field for armchair graphene nanoribbons (GNRs) and graphene nanoconstrictions (GNCs). For GNRs, the conductance plateaus of 2e 2 h at zero magnetic field evolve smoothly to the quantum Hall regime, where the plateaus in conductance at even multiples of 2e 2 h disappear. It is shown that the relation between the energy and magnetic field does not follow the same behavior as in "bulk" graphene, reflecting the different electronic structure of a GNR. For the nanoconstrictions we show that the conductance plateaus do not have the same sharp behavior in zero magnetic field as in a GNR, which reflects the presence of backscattering in such structures. Our results show good agreement with recent experiments on high-quality graphene nanoconstrictions. The behavior with the magnetic field for a GNC shows some resemblance to the one for a GNR but now depends also on the length of the constriction. By analyzing the evolution of the conductance plateaus in the presence of the magnetic field we can obtain the width of the structures studied and show that this is a powerful experimental technique in the study of the electronic and structural properties of narrow structures.
We investigate the transport properties of three-terminal graphene devices, where one terminal is... more We investigate the transport properties of three-terminal graphene devices, where one terminal is superconducting and two are normal metals. The terminals are connected by nanoribbons. Electron transfer (ET) and crossed Andreev reflection (CAR) are identified via the non-local signal between the two normal terminals. Analytical expressions for ET and CAR in symmetric devices are found. We compute ET and CAR numerically for asymmetric devices. ET dominates CAR in symmetric devices, but CAR can dominate ET in asymmetric devices, where only the zero-energy modes of the zigzag nanoribbons contribute to the transport.
We consider non-local transport mediated by Andreev reflection in a two-dimensional electron gas ... more We consider non-local transport mediated by Andreev reflection in a two-dimensional electron gas (2DEG) connected to one superconducting and two normal metal terminals. A robust scheme is presented for observing crossed Andreev reflection (CAR) between the normal metal terminals based on electron focusing by weak perpendicular magnetic fields. At slightly elevated temperatures the CAR signature can be easily distinguished from a background of quantum interference fluctuations. The CAR-induced entanglement between electrons can be switched on and off over large distances by the magnetic field.
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Papers by X. Waintal