arXiv subjects

Benoît Grémaud

Publications and source records attributed to Benoît Grémaud.

At least 19 recordsLinked to original sources

arXiv2026

Room-temperature antiferromagnetic resonance in NaMnAs

We report on antiferromagnetic resonance experiments in bulk tetragonal NaMnAs -- a room-temperature antiferromagnetic semiconductor. Our results corroborate previous ab initio studies, which propose that NaMnAs is an easy-axis antiferromagnet with the Néel vector oriented along the tetragonal axis. At $ B = 0 $, we find a single antiferromagnetic resonance line at 7 meV and associate it with a doubly degenerate ($ k = 0 $) magnon mode. Its energy softens considerably with increasing $ T $, but remains clearly visible in the data up to room temperature. From the experimental data, we estimate the single-ion anisotropy of the Mn ions in NaMnAs in the range 0.1-0.2 meV, a value that is relatively large compared to other manganese-based antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2026

Effect of inter-edge interaction in a quantum Hall collider

Fractional quantum Hall (FQH) colliders measure anyon exchange phases via time-domain braiding, but the $ν=2/5$ state exhibits an intriguing negative Fano factor, challenging theoretical predictions. Here, we study the effect of inter-edge interactions in a multi-mode FQH collider. We demonstrate that the resulting fractionalization into eigenmodes causes the anyon beam to decompose into correlated and uncorrelated components, which have very distinct behavior in terms of time-domain braiding. We show that the uncorrelated part dominates in the long-junction limit, reversing the tunneling current sign and reproducing the observed negative Fano factor at $ν=2/5$. Our results highlight the role of interactions and provide a robust interpretation of anyonic braiding in multi-mode systems.

cond-mat.mes-hall↗

arXiv2026

Charging energy effects on a single-edge anyon braiding detector

We investigate the influence of capacitive coupling on the detection of anyon braiding in a single-edge interferometer realized in the fractional quantum Hall regime. In this setup, a quantum point contact bends a single edge into a loop, where tunneling occurs at the open end and is controlled by the QPC voltage. In contrast with previously studied two-edge geometries, the weak backscattering regime is dominated by the first-order perturbative term, allowing quantum transport quantities to factorize into a non-universal prefactor and a braiding-induced contribution that provides direct access to the universal statistical angle $πλ$. While previous analyses neglected edge-to-edge capacitance, we show that capacitive effects, which are known to play a crucial role in mesoscopic capacitors, modify both the current and the current cross-correlations. Using a two-point Green's function formalism augmented by Dyson's equation to include the charging energy, we quantify how the fluctuations of the cross-correlations depend simultaneously on $λ$ and on the capacitance of the loop. Our results indicate that a reliable extraction of the statistical angle requires a parallel measurement of the loop capacitance, which can be implemented via a charged gate coupled to the junction.

cond-mat.mes-hall↗

arXiv2025

Colloquium: A critique on van der Waals and two-dimensional magnets

Magnetic two-dimensional (2D) crystals were isolated about a decade ago, triggering a tremendous research activity worldwide. This colloquium raises a stiff question: what is really new about them? At first sight, they seem to be purer implementations of 2D spin models than traditional systems such as ultra-thin films. Yet, they partly realized their promises so far, and whether they give fresh perspectives on long-standing predictions in statistical physics is still an open question. Undoubtedly, they are uniquely amenable to electric-field effect, susceptible to mechanical deformation, and sensitive to moirés, for example. They represent interesting platforms for exploring, challenging, or simply revisiting a wide range of phenomena in condensed matter magnetism. This colloquium intends to offer a critical, yet not necessarily skeptical, overview of the field, clarifying what we believe could be unique with 2D magnets, related quasi-2D van der Waals magnets, and their heterostructures.

cond-mat.mtrl-sci↗

arXiv2025

Disorder and the Robustness of Superconductivity on the Flat Band

We study the interplay between on-site disorder and fermion pairing on the quasi one-dimensional flat band Creutz lattice. Both disorder and flat bands localize particles, but an attractive interaction results in pair formation and delocalization giving rise to superconductivity. In this work, we examine the attractive Hubbard model on the Creutz lattice to study the competition between these two effects and elucidate the properties of the superconducting phase and the localization quantum phase transition as the disorder strength is increased. Our main result is that flat band superconductivity is robust against disorder: The critical disorder strength, $W_c$, required to localize the fermion pairs and destroy superconductivity, is finite at any interaction strength, $U$, and is proportional to the superconducting weight, $D_s$, of the clean system. Using large scale density matrix renormalization group computations, we show that this transition is of the BKT form. In addition, even at very small interaction strength, the localization is not due to single fermion localization but to pair localization. For completeness, we briefly study this disorder-induced localization with mean field theory and show that $W_c$ can be accurately determined by using an appropriate scaling function.

cond-mat.supr-con↗

arXiv2025

Probing anyon statistics on a single-edge loop in the fractional quantum Hall regime

We propose a setup to directly measure the anyonic statistical angle on a single edge of a fractional quantum Hall system, without requiring independent knowledge of non-universal parameters. We consider a Laughlin edge state bent into a closed loop geometry, where tunneling processes are controllably induced between the endpoints of the loop. To illustrate the underlying physical mechanism, we compute the time-dependent current generated by the injection of multiple anyons, and show that its behavior exhibits distinctive features governed by the anyonic statistical angle. The measured current reflects quantum interference effects due to the time-resolved braiding of anyons at the junction. To establish experimental relevance, we introduce a protocol where anyons are probabilistically injected upstream of the loop via a quantum point contact (QPC) source. Unlike in Fabry-Perot interferometers, where phase jumps occur spontaneously due to stochastic quasi-particle motion, here the phase jumps are deliberately induced by source injections. These events imprint measurable signatures in the cross-correlation noise, enabling a controlled statistical analysis of the braiding phase. We further show that, by varying the magnetic field while remaining within the same fractional quantum Hall plateau, the statistical angle can be extracted without relying on the knowledge of other non-universal system parameters. Our results provide a minimal and accessible platform for probing anyonic statistics using a single chiral edge.

cond-mat.mes-hall↗

arXiv2025

Photo-assisted shot noise probes multiple charge carriers in quantum Hall edges

Fractional charges in the fractional quantum Hall effect were first observed via DC shot noise measurements of anyons tunneling at a quantum point contact (QPC). However, in scenarios with simultaneous tunneling of different types of charges at the QPC, the connection between DC shot noise and tunneling charge is less transparent. Photo-assisted shot noise (PASN), induced by periodic AC voltage, offers a promising alternative. Here, we investigate PASN in the hierarchical states of the fractional quantum Hall effect, where different types of charges are expected to tunnel concurrently at QPCs. In the particular case of the fractional quantum Hall state $ν= 2/3$, our analysis demonstrates that PASN can be employed as a robust tool to detect different tunneling charges, even when the tunneling amplitude of one type is significantly smaller compared to the other. We show that the features predicted by our calculations are still visible for typical values of temperature and frequency achieved in state-of-the-art experiments. Our general formalism can be used to compute PASN for general Abelian quantum Hall systems with multiple edge modes and charge types.

cond-mat.mes-hall↗

arXiv2025

Periodic source of energy-entangled electrons in helical states coupled to a BCS superconductor

We propose a source of purely electronic energy-entangled states implemented in a solid-state system with potential applications in quantum information protocols based on electron flying qubits. The proposed device relies on the standard tools of Electron Quantum Optics (EQO) and exploits entanglement of the Cooper pairs of a BCS superconductor. The latter is coupled via an adjustable quantum point contact to two opposite spin polarized electron wave-guides, which are driven by trains of Lorentzian pulses. This specific choice for the drive is crucial to inject purely electronic entangled-states devoid of spurious electron-hole pairs. In the Andreev regime, a perturbative calculation in the tunnel coupling confirms that entangled electrons states (EES) are generated at the output of the normal side. We introduce a quantity related to charge current cross-correlations which allows one to verify experimentally the entangled nature of the emitted state.

cond-mat.mes-hall↗

arXiv2024

Current and shot noise in a normal metal-superconductor junction driven by spin-dependent periodic pulse sequence

Andreev reflection is a fundamental transport process occurring at the junction between a normal metal and a superconductor (a N-S junction), when an incident electron from the normal side can only be transmitted in the superconductor as a Cooper pair, with the reflection of a hole in the normal metal. As a consequence of the spin singlet nature of the BCS Cooper pairs, the current due to Andreev reflection at a N-S junction is always symmetric in spin. Using a Keldysh Nambu Floquet approach, combining analytical and numerical calculations, we study in details the AC transport at a N-S junction, when the two spin components in the normal metal are driven by different periodic drives. We show that, in the Andreev regime, i.e. when the superconducting gap is much larger than the frequency of the drives, the spin-resolved photo-assisted currents are always equal even if the two drives are different. In addition, we show that in this regime the excess noise depends only on the sum of the periodic drives, and we consider in particular the case of Lorentzian pulses (Levitons). We also show how these properties get modified when going beyond the Andreev regime. Finally we give a simple analytical proof of the special properties of the Andreev regime using an exact mapping to a particular N-N junction.

cond-mat.supr-con↗

arXiv2023

Correlated two-Leviton states in the fractional quantum Hall regime

We consider a two-dimensional electron system in the Laughlin sequence of the fractional quantum Hall regime to investigate the effect of strong correlations on the mutual interaction between two Levitons, single-electron excitations generated by trains of quantized Lorentzian pulses. We focus on two-Leviton states injected in a single period with a time separation $Δt$. In the presence of a quantum point contact operating in the weak-backscattering regime, we compute the backscattered charge by means of the Keldysh technique. In the limit of an infinite period and zero temperature, we show that the backscattered charge for a two-Leviton state is not equal to twice the backscattered charge for a single Leviton. We present an interpretation for this result in terms of the wave-packet formalism for Levitons, thus proposing that an effective interaction between the two Levitons is induced by the strongly-correlated background. Finally, we perform numerical calculations in the periodic case by using the Floquet formalism for photo-assisted transport. By varying the system parameters such as pulse width, filling factor and temperature we show that the value of the backscattered charge for two-Leviton states is strongly dependent on the pulse separation, thus opening scenarios where the effective interaction between Levitons can be controllably tuned.

cond-mat.mes-hall↗

arXiv2023

An on-demand source of non-local energy-entangled quantum states using Levitons

We propose a device where Electronic Quantum Optics (EQO) is combined with a BCS superconductor -- a reservoir of Cooper pairs. With spin polarized wave guides, this version of the Cooper pair beam splitter is driven by an AC drive, and observables such as period-averaged noise are computed using a Keldysh-Nambu-Floquet formalism. When the (low frequency) voltage applied to the superconductor consists of a train of periodic Lorentzian pulses and the Andreev regime is specified (large gap limit), this allows to propose an on-demand source of non-local energy-entangled states (which we characterise with perturbation theory) operating in the AC regime. We study realistic experimental parameters for our device in order to examine its feasibility.

cond-mat.supr-con↗

arXiv2022

Toward a measurement of the effective gauge field and the Born-Huang potential with atoms in chip traps

We study magnetic traps with very high trap frequencies where the spin is coupled to the motion of the atom. This allows us to investigate how the Born-Oppenheimer approximation fails and how effective magnetic and electric fields appear as the consequence of the non-adiabatic dynamics. The results are based on exact numerical diagonalization of the full Hamiltonian describing the coupling between the internal and external degrees of freedom. The position in energy and the decay rate of the trapping states correspond to the imaginary part of the resonances of this Hamiltonian and are computed using the complex rotation method.

quant-ph↗

arXiv2022

Quantum boomerang effect in systems without time reversal symmetry

In an Anderson localized system, a quantum particle with a nonzero initial velocity returns, on average, to its origin. This recently discovered behavior is known as the quantum boomerang effect. Time reversal invariance was initially thought to be a necessary condition for the existence of this phenomenon. We theoretically analyze the impact of the symmetry breaking on the phenomenon using a one-dimensional system with a spin-orbit coupling and show that the time reversal invariance is not necessary for the boomerang effect to occur.

cond-mat.dis-nn↗

arXiv2022

Momentum Signatures of Site Percolation in Disordered 2D Ferromagnets

In this work, we consider a two-dimensional square lattice of pinned magnetic spins with nearest-neighbour interactions and we randomly replace a fixed proportion of spins with nonmagnetic defects carrying no spin. We focus on the linear spin-wave regime and address the propagation of a spin-wave excitation with initial momentum $k_0$. We compute the disorder-averaged momentum distribution obtained at time $t$ and show that the system exhibits two regimes. At low defect density, typical disorder configurations only involve a single percolating magnetic cluster interspersed with single defects essentially and the physics is driven by Anderson localization. In this case, the momentum distribution features the emergence of two known emblematic signatures of coherent transport, namely the coherent backscattering (CBS) peak located at $-k_0$ and the coherent forward scattering (CFS) peak located at $k_0$. At long times, the momentum distribution becomes stationary. However, when increasing the defect density, site percolation starts to set in and typical disorder configurations display more and more disconnected clusters of different sizes and shapes. At the same time, the CFS peak starts to oscillate in time with well defined frequencies. These oscillation frequencies represent eigenenergy differences in the regular, disorder-immune, part of the Hamiltonian spectrum. This regular spectrum originates from the small-size magnetic clusters and its weight grows as the system undergoes site percolation and small clusters proliferate. Our system offers a unique spectroscopic signature of cluster formation in site percolation problems.

cond-mat.dis-nn↗

arXiv2019

$SU(3)$ Topological Insulators in the Honeycomb Lattice

We investigate realizations of topological insulators with spin-1 bosons loaded in a honeycomb optical lattice and subjected to a $SU(3)$ spin-orbit coupling - a situation which can be realized experimentally using cold atomic gases. In this paper, we focus on the topological properties of the single-particle band structure, namely Chern numbers (lattice with periodic boundary conditions) and edge states (lattice with strip geometry). While $SU(2)$ spin-orbit couplings always lead to time-reversal symmetric Hubbard models, and thereby to topologically trivial band structures, suitable $SU(3)$ spin-orbit couplings can break time reversal symmetry and lead to topologically non-trivial bulk band structures and to edge states in the strip geometry. In addition, we show that one can trigger a series of topological transitions (i.e. integer changes of the Chern numbers) that are specific to the geometry of the honeycomb lattice by varying a single parameter in the Hamiltonian.

cond-mat.quant-gas↗

arXiv2018

Pairing and superconductivity in the flat band: Creutz lattice

We use unbiased numerical methods to study the onset of pair superfluidity in a system that displays flat bands in the noninteracting regime. This is achieved by using a known example of flat band systems, namely the Creutz lattice, where we investigate the role of local attractive interactions in the $U < 0$ Hubbard model. Going beyond the standard approach used in these systems where weak interactions are considered, we map the superfluid behavior for a wide range of interaction strengths and exhibit a crossover between BCS and tightly bound bosonic fermion pairs. We further contrast these results with a standard two-leg fermionic ladder, showing that the pair correlations, although displaying algebraic decay in both cases, are longer ranged in the Creutz lattice, signifying the robustness of pairing in this system.

cond-mat.str-el↗

arXiv2018

Non-Abelian adiabatic geometric transformations in a cold Strontium gas

Topology, geometry, and gauge fields play key roles in quantum physics as exemplified by fundamental phenomena such as the Aharonov-Bohm effect, the integer quantum Hall effect, the spin Hall, and topological insulators. The concept of topological protection has also become a salient ingredient in many schemes for quantum information processing and fault-tolerant quantum computation. The physical properties of such systems crucially depend on the symmetry group of the underlying holonomy. We study here a laser-cooled gas of strontium atoms coupled to laser fields through a 4-level resonant tripod scheme. By cycling the relative phases of the tripod beams, we realize non-Abelian SU(2) geometrical transformations acting on the dark-states of the system and demonstrate their non-Abelian character. We also reveal how the gauge field imprinted on the atoms impact their internal state dynamics. It leads to a new thermometry method based on the interferometric displacement of atoms in the tripod beams.

quant-ph↗

arXiv2018

Two-dimensional network of atomtronic qubits

Through a combination of laser beams, we engineer a two-dimensional optical lattice of Mexican hat potentials able to host atoms in its ring-shaped wells. When tunneling can be ignored (at high laser intensities), we show that a well-defined qubit can be associated with the states of the atoms trapped in each of the rings. Each of these two-level systems can be manipulated by a suitable configuration of Raman laser beams imprinting a synthetic flux onto each Mexican hat cell of the lattice. Overall, we believe that the system has the potential to form a scalable architecture for atomtronic flux qubits.

cond-mat.quant-gas↗