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Alberto F. Morpurgo

Publications and source records attributed to Alberto F. Morpurgo.

At least 19 recordsLinked to original sources
arXiv2026

Switchable giant room-temperature nonlinear Hall effect in Bilayer Graphene

Utilizing quantum second-order nonlinear transport for practical junction-free devices require materials with large and tunable nonlinearites at room temperature -- a current materials platform challenge. Here, we report the nonlinear Hall effect (NLHE) in double-ionic gated bilayer graphene devices that enable unusually strong inversion breaking. We observe NLHE that are readily switchable (on, off, and sign reversed) with second order nonlinear susceptibilities $χ^{(2)}_{yxx}$ that reaches giant room-temperature values of $3\,10^{-3}\,μ\mathrm{m}\,\mathrm{S/V}$, comparable to values commonly observed at low temperature in WTe$_2$ or in graphene-based moiré superlattices, and three-to-four orders of magnitude larger than values reported in material systems recently employed in search of a room-temperature NLHE. Our devices produce corresponding THz voltage responsivities $\simeq 4\,10^{4}\,\mathrm{V/W}$, comparable to commercially available Schottky diodes. These are orders of magnitude better than for previously reported room-temperature NLHE devices rendering double-ionic gated bilayer graphene a choice platform for junction-free nonlinear technology.

cond-mat.mes-hall↗
arXiv2026

Quasi-one-dimensional topological band structure and van Hove singularities in monolayer TaIrTe$_4$ from laser $μ$-ARPES

Recent transport experiments reported a quantum spin Hall insulator phase in monolayer 1T-TaIrTe$_4$ gated away from charge neutrality. This phase is not predicted by band structure calculations and has been attributed to an electronic instability induced by strong correlations at a putative van Hove singularity. Here, we investigate the electronic structure of exfoliated monolayer 1T-TaIrTe$_4$ using micro-focus laser angle resolved photoemission. We find a strongly anisotropic band structure susceptible to density wave instabilities. Our data further reveal a saddle point singularity in the density of states. However, we find that the saddle point lies at a density for which transport experiments found no anomalies. Moreover, the quasiparticle line widths suggest weak electron correlations only. This points to a secondary role of van Hove singularities and electron correlations in the transport phase diagram of monolayer 1T-TaIrTe$_4$.

cond-mat.mtrl-sci↗
arXiv2026

Fluctuation electrodynamics of quantum capacitance in electron bilayers

Capacitance is a thermodynamic probe of interacting electrons: it measures the energy cost of moving charge between conductors, and in low-dimensional systems this cost is shaped by exchange and correlation as much as by electrostatics. We develop a theory of the quantum capacitance of electron double layers, semiconductor quantum wells as well as monolayer and bilayer graphene devices, focusing on the contribution generated by interlayer correlations. Within a functional-integral formulation we show that the separation-dependent part of the ground-state energy is, at the level of ring diagrams, exactly the nonretarded Lifshitz expression for the van der Waals energy of two conducting sheets, with reflection amplitudes built from the layer polarizabilities. The interlayer correction to the inverse capacitance is the second density derivative of this energy: a Casimir compressibility. The zero-point fluctuations that generate Casimir forces between mirrors are here the coupled plasmons of the bilayer, and their contribution to the capacitance is obtained in closed form, with a universal coefficient; its sign shows that interlayer correlations oppose charging at high density. A Gell-Mann-Brueckner analysis gives the exact high-density limit in terms of universal functions. For graphene we find that monolayers follow the electron-gas template, while bilayer graphene is anomalous: interband screening suppresses the correction by orders of magnitude and reverses its sign in the experimentally relevant range of separations. We delineate the limits of the theory, identifying the dilute Wigner-crystal regime and electron-hole double layers near exciton condensation as regimes where the capacitance becomes a probe of interlayer pairing.

cond-mat.mes-hall↗
arXiv2026

Coupling between magnetism and band structure in a 2D semiconductor

Van der Waals semiconducting magnets exhibit a cornucopia of physical phenomena originating from the interplay of their semiconducting and magnetic properties. However, a comprehensive understanding of how semiconducting processes and magnetism are coupled is lacking. We address this question by performing scanning tunneling spectroscopy (STS) measurements on the magnetic semiconductor CrPS$_4$, and by comparing the results to photoluminescence experiments and density functional theory (DFT) calculations. Below the magnetic transition, STS exhibit multiple features absent in the paramagnetic state, caused by the proliferation of electronic bands due to spin splitting with a large ($\simeq 0.5$ eV) exchange energy. The energetic differences between the band edges determined by STS match all observed photoluminescence transitions, which also proliferate in the magnetic state. DFT calculations quantitatively predict the relative positions of all detected bands, explain which pairs of bands lead to radiative transitions, and also reproduce the measured spatial dependence of electronic wavefunctions. Our results reveal how all basic optoelectronic processes observed in CrPS$_4$ can be understood in terms of the evolution of the electronic band structure when entering the magnetic state, and allow us to conclude that individual bands are fully spin-polarized over a broad energy interval.

cond-mat.mtrl-sci↗
arXiv2026

Persistence of large and gate-tunable anisotropic magnetoresistance in an atomically thin antiferromagnet

Anisotropic magnetoresistance (AMR) offers a robust electrical readout of antiferromagnetic (AFM) states, playing a central role in the rapidly advancing field of AFM spintronics. Despite its great versatility, electrical probing of the Néel vector via AMR remains challenging in the ultrathin limit due to interface disorder and reduced dimensionality. Here, we demonstrate electrical readout of the Néel vector down to 1.3 nm (two layers) in the two-dimensional van der Waals (vdW) AFM semiconductor NiPS3. Leveraging spin-flop-mediated rotation of the Néel vector and using both transistor and tunnel-junction device geometries, we identify two distinct AMR contributions in NiPS3, that dominate at low and high charge densities, respectively. We achieve full gate control over these contributions, enabling tunability of both the magnitude and sign of magnetoresistance. Our results establish semiconducting vdW antiferromagnets as a rich platform for studying AMR in the ultrathin limit, opening new avenues for multifunctional AFM spintronic devices.

cond-mat.mes-hall↗
arXiv2026

Interband State Transfer in Double-Gated Bilayer Graphene at High Electric Field

The band structure of Bernal-stacked bilayer graphene can be tuned using double-gated transistors to apply a perpendicular electric field that generates an interlayer potential energy difference $Δ$. Dielectric breakdown limits the operation of conventional devices to the $Δ\ll t_\perp \simeq 360$ meV regime. We employ double ionic gating to reach fields past $ 1$ V/nm, for which $Δ> t_\perp$. We find that for $Δ\simeq t_\perp$, the evolution of the longitudinal resistance ($R_{xx}$) peak as a function of applied gate voltages undergoes a sharp change in slope, exhibiting a pronounced "knee". Increasing $Δ$ past the "knee" results in an unusual evolution transport properties: the peak in $R_{xx}$ decreases in magnitude, it exhibits a splitting concomitant with multiple sign reversals of the Hall resistance, and hysteresis in the peak position emerges. We explain the observed phenomenology in terms of in-gap bound states, whose energy strongly depends on the perpendicular electric field, and crosses the mid-gap level for sufficiently large $Δ> t_\perp$. The phenomenon causes large changes in the electronic density of in-gap states that profoundly affect the evolution of the chemical potential. Our experimental results and their interpretation reveal unique aspects of the physics of in-gap states in Bernal bilayer graphene and demonstrate that double ionic gating enables investigating the large-$Δ$ regime, which has remained experimentally inaccessible so far.

cond-mat.mes-hall↗
arXiv2025

Nature of 2D XY antiferromagnetism in a van der Waals monolayer

Two-dimensional antiferromagnetism has long attracted significant interest in many areas of condensed matter physics, but only recently has experimental exploration become feasible due to the isolation of van der Waals antiferromagnetic monolayers. Probing the magnetic phase diagram of these monolayers remains however challenging because established experimental techniques often lack the required sensitivity. Here, we investigate antiferromagnetism in atomically thin van der Waals magnet NiPS3 using magnetotransport measurements in field-effect transistor devices. Temperature-dependent conductance and magnetoresistance data reveal a distinct magnetic behavior in monolayers as compared to thicker samples. While bilayer and multilayer NiPS3 exhibit a single magnetic phase transition into a zig-zag antiferromagnetic state driven by uniaxial anisotropy, monolayer NiPS3 undergoes two magnetic transitions, with a low-temperature phase governed by in-plane hexagonal magnetic anisotropy. The experimentally constructed phase diagram for monolayer NiPS3 matches theoretical predictions from the six-state clock and 2D-XY models incorporating hexagonal anisotropy.

cond-mat.mes-hall↗
arXiv2025

Switching on and off the spin polarization of the conduction band in antiferromagnetic bilayer transistors

Antiferromagnetic conductors with suitably broken spatial symmetries host spin-polarized bands, which lead to transport phenomena commonly observed in metallic ferromagnets. In bulk materials, it is the given crystalline structure that determines whether symmetries are broken and spin-polarized bands are present. Here we demonstrate experimentally that double-gate transistors realized on bilayers of van der Waals antiferromagnetic semiconductor CrPS4 allow the relevant symmetry to be controlled by a perpendicular electric displacement field. Such a level of control enables the spin-polarization of the conduction band to be switched on and off. Because conduction band states with opposite spin-polarizations are hosted in the different layers and are spatially separated, these devices also give control over the magnetization of the electrons that are accumulated electrostatically. Our experiments show that double-gated CrPS4 transistors provide a viable platform to create gate-induced conductors with near unity spin polarization at the Fermi level, as well as devices with a full electrostatic control of the total magnetization of the system.

cond-mat.mes-hall↗
arXiv2025

Spin valve effect in junctions with a single ferromagnet

Spin valves are essential components in spintronic memory devices, whose conductance is modulated by controlling spin-polarized electron tunneling through the alignment of the magnetization in ferromagnetic elements. Whereas conventional spin valves unavoidably require at least two ferromagnetic elements, here we demonstrate a van der Waals spin valve based on a tunnel junction that comprises only one such ferromagnetic layer. Our devices combine a Fe3GeTe2 electrode acting as spin injector together with a paramagnetic tunnel barrier, formed by a CrBr3 multilayer operated above its Curie temperature. We show that these devices exhibit a conductance modulation with values comparable to that of conventional spin valves. A quantitative analysis of the magnetoconductance that accounts for the field-induced magnetization of CrBr3, and that includes the effect of exchange interaction, confirms that the spin valve effect originates from the paramagnetic response of the barrier, in the absence of spontaneous magnetization in CrBr3.

cond-mat.mtrl-sci↗
arXiv2024

Moire magnetism in CrBr3 multilayers emerging from differential strain

Interfaces between twisted 2D materials host a wealth of physical phenomena originating from the long-scale periodicity associated with the resulting moire structure. Besides twisting, an alternative route to create structures with comparably long or even longer periodicities is inducing a differential strain between adjacent layers in a van der Waals (vdW) material. Despite recent theoretical efforts analyzing its benefits, this route has not yet been implemented experimentally. Here we report evidence for the simultaneous presence of ferromagnetic and antiferromagnetic regions in CrBr3 _a hallmark of moire magnetism_ from the observation of an unexpected magnetoconductance in CrBr3 tunnel barriers with ferromagnetic Fe3GeTe2 and graphene electrodes. The observed magnetoconductance evolves with temperature and magnetic field as the magnetoconductance measured in small angle CrBr3 twisted junctions, in which moire magnetism occurs. Consistent with Raman measurements and theoretical modeling, we attribute the phenomenon to the presence of a differential strain in the CrBr3 multilayer, which locally modifies the stacking and the interlayer exchange between adjacent CrBr3 layers, resulting in spatially modulated spin textures. Our conclusions indicate that inducing differential strain in vdW multilayers is a viable strategy to create moire-like superlattices, which in the future may offer in-situ continuous tunability even at low temperatures.

cond-mat.mtrl-sci↗
arXiv2024

Positive oscillating magnetoresistance in a van der Waals antiferromagnetic semiconductor

In all van der Waals layered antiferromagnetic semiconductors investigated so far a negative magnetoresistance has been observed in vertical transport measurements, with characteristic trends that do not depend on applied bias. Here we report vertical transport measurements on layered antiferromagnetic semiconductor CrPS$_4$ that exhibit a drastically different behavior, namely a strongly bias dependent, positive magnetoresistance that is accompanied by pronounced oscillations for devices whose thickness is smaller than 10 nm. We establish that this unexpected behavior originates from transport being space-charge limited, and not injection limited as for layered antiferromagetic semiconductors studid earlier. Our analysis indicates that the positive magnetoresistance and the oscillations only occur when electrons are injected into in-gap defect states, whereas when electrons are injected into the conduction band the magnetoresistance vanishes. We propose a microscopic explanation for the observed phenomena that combines concepts typical of transport through disordered semiconductors with known properties of the CrPS$_4$ magnetic state, which captures all basic experimental observations. Our results illustrate the need to understand in detail the nature of transport through vdW magnets, to extract information about the nature of the order magnetic states and its microscopic properties.

cond-mat.mes-hall↗
arXiv2024

Unconventional gate-induced superconductivity in transition-metal dichalcogenides

Superconductivity in few-layer semiconducting transition metal dichalcogenides (TMDs) can be induced by field-effect doping through ionic-liquid gating. While several experimental observations have been collected over the years, a fully-consistent theoretical picture is still missing. Here we develop a realistic framework that combines the predictive power of first-principles simulations with the versatility and insight of Bardeen-Cooper-Schrieffer gap equations to rationalize such experiments. The multi-valley nature of semiconducting TMDs is taken into account, together with the doping- and momentum-dependent electron-phonon and Coulomb interactions. Consistently with experiments, we find that superconductivity occurs when the electron density is large enough that the Q valleys get occupied, as a result of a large enhancement of electron-phonon interactions. Despite being phonon-driven, the superconducting state is predicted to be sensitive to Coulomb interactions, which can lead to the appearance of a relative sign difference between valleys and thus to a $s_{+-}$ character. We discuss qualitatively how such scenario may account for many of the observed physical phenomena for which no microscopic explanation has been found so far, including in particular the presence of a large subgap density of states, and the sample-dependent dome-shaped dependence of $T_c$ on accumulated electron density. Our results provide a comprehensive analysis of gate-induced superconductivity in semiconducting TMDs, and introduce an approach that will likely be valuable for other multivalley electronic systems, in which superconductivity occurs at relatively low electron density.

cond-mat.supr-con↗
arXiv2024

Spectroscopic evidence for a first-order transition to the orbital Fulde-Ferrell-Larkin-Ovchinnikov state

A conventional superconducting state may be replaced by another dissipationless state hosting Cooper pairs with a finite momentum, leaving thermodynamic footprints for such a phase transition. Recently, a novel type of finite momentum pairing, so-called orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, has been proposed to occur in spin-orbit coupled superconductors such as bilayer $2\mathrm{H-NbSe_{2}}$. So far, a thermodynamic demonstration, which is key for establishing this exotic phase, has been lacking. Here, we reveal a first-order quantum phase transition to the orbital FFLO state in tunneling spectroscopic measurements on multilayer $2\mathrm{H-NbSe_{2}}$. The phase transition manifests itself as a sudden enhancement of the superconducting gap at an in-plane magnetic field $B_{//}$ well below the upper critical field. Furthermore, this transition shows prominent hysteresis by sweeping $B_{//}$ back and forth and quickly disappears once the magnetic field is tilted away from the sample plane by less than one degree. We obtain a comprehensive phase diagram for the orbital FFLO state and compare it with the theoretical calculation that takes into account the rearrangement of Josephson vortices. Our work elucidates the microscopic mechanism for the emergence of the orbital FFLO state.

cond-mat.supr-con↗
arXiv2024

Influence of magnetism on vertical hopping transport in CrSBr

We investigate the c-direction conduction in CrSBr in the linear regime, not accessible in other van der Waals (vdW) magnetic semiconductors, because of the unmeasurably low current. The resistivity -- $10^8$ to $10^{11}$ times larger than in the a- and b-directions -- exhibits magnetic state dependent thermally activated and variable range hopping transport. In the spin-flip phase at 2 T, the activation energy is 20 meV lower than in the antiferromagnetic state, due to a downshift of the conduction band edge, in agreement with ab-initio calculations. In the variable range hopping regime, the average hopping length decreases from twice the interlayer distance to the interlayer distance at 2 T, because in the antiferromagnetic state the large exchange energy impedes electrons hopping between adjacent layers. Our work demonstrates that the linear transport regime provides new information about electronic processes in vdW magnetic semiconductors, and shows how magnetism influences these processes both in real and reciprocal space.

cond-mat.mtrl-sci↗
arXiv2023

Magnetism-induced band-edge shift as mechanism for magnetoconductance in CrPS$_4$ transistors

Transistors realized on 2D antiferromagnetic semiconductor CrPS$_4$ exhibit large magnetoconductance, due to magnetic-field-induced changes in magnetic state. The microscopic mechanism coupling conductance and magnetic state is not understood. We identify it by analyzing the evolution of the parameters determining the transistor behavior -- carrier mobility and threshold voltage -- with temperature and magnetic field. For temperatures T near the Néel temperature $T_N$, the magnetoconductance originates from a mobility increase due to the applied magnetic field that reduces spin fluctuation induced disorder. For $T << T_N$, instead, what changes is the threshold voltage, so that increasing the field at fixed gate voltage increases the density of accumulated electrons. The phenomenon is explained by a conduction band-edge shift correctly predicted by \emph{ab-initio} calculations. Our results demonstrate that the bandstructure of CrPS$_4$ depends on its magnetic state and reveal a mechanism for magnetoconductance that had not been identified earlier.

cond-mat.mes-hall↗
arXiv2023

Multiple antiferromagnetic phases and magnetic anisotropy in exfoliated CrBr$_3$ multilayers

In twisted two-dimensional (2D) magnets, the stacking dependence of the magnetic exchange interaction can lead to regions of ferromagnetic and antiferromagnetic interlayer order, separated by non-collinear, skyrmion-like spin textures. Recent experimental searches for these textures have focused on CrI$_3$, known to exhibit either ferromagnetic or antiferromagnetic interlayer order, depending on layer stacking. However, the very strong uniaxial anisotropy of CrI$_3$ disfavors smooth non-collinear phases in twisted bilayers. Here, we report the experimental observation of three distinct magnetic phases -- one ferromagnetic and two antiferromagnetic -- in exfoliated CrBr$_3$ multilayers, and reveal that the uniaxial anisotropy is significantly smaller than in CrI$_3$. These results are obtained by magnetoconductance measurements on CrBr$_3$ tunnel barriers and Raman spectroscopy, in conjunction with density functional theory calculations, which enable us to identify the stackings responsible for the different interlayer magnetic couplings. The detection of all locally stable magnetic states predicted to exist in CrBr$_3$ and the excellent agreement found between theory and experiments, provide complete information on the stacking-dependent interlayer exchange energy and establish twisted bilayer CrBr$_3$ as an ideal system to deterministically create non-collinear magnetic phases.

cond-mat.mtrl-sci↗
arXiv2023

Electronic structure of few-layer black phosphorus from $μ$-ARPES

Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based micro-focus angle resolved photoemission ($μ$-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment. Our measurements unveil ladders of anisotropic, quantized subbands at energies that deviate from the scaling observed in conventional semiconductor quantum wells. We quantify the anisotropy of the effective masses and determine universal tight-binding parameters which provide an accurate description of the electronic structure for all thicknesses.

cond-mat.mes-hall↗
arXiv2023

Gate-controlled Magnetotransport and Electrostatic Modulation of Magnetism in 2D magnetic semiconductor CrPS$_4$

Using field-effect transistors (FETs) to explore atomically thin magnetic semiconductors with transport measurements is difficult, because the very narrow bands of most 2D magnetic semiconductors cause carrier localization, preventing transistor operation. Here, we show that exfoliated layers of CrPS$_4$ -- a 2D layered antiferromagnetic semiconductor whose bandwidth approaches 1 eV -- allow the realization of FETs that operate properly down to cryogenic temperature. Using these devices, we perform conductance measurements as a function of temperature and magnetic field, to determine the full magnetic phase diagram, which includes a spin-flop and a spin-flip phase. We find that the magnetoconductance depends strongly on gate voltage, reaching values as high as 5000 % near the threshold for electron conduction. The gate voltage also allows the magnetic states to be tuned, despite the relatively large thickness of the CrPS$_4$ multilayers employed in our study. Our results show the need to employ 2D magnetic semiconductors with sufficiently large bandwidth to realize properly functioning transistors, and identify a candidate material to realize a fully gate-tunable half-metallic conductor.

cond-mat.mes-hall↗