-
Measuring the buried interphase between solid electrolytes and lithium metal using neutrons
Authors:
Andrew S. Westover,
Katie L. Browning,
Antonino Cannavo,
Ralph Gilles,
Jiri Vacik,
James F. Browning,
Neelima Paul,
Giovanni Ceccio,
Vasyl Lavrentiev
Abstract:
Interfaces are the key to next generation high energy batteries including solid state Li metal batteries. In solid state batteries, the buried nature of solid solid electrolyte electrode interfaces makes studying them difficult. Neutrons have significant potential to non destructively probe these buried solid solid interfaces. This work presents a comparative study using both neutron depth profili…
▽ More
Interfaces are the key to next generation high energy batteries including solid state Li metal batteries. In solid state batteries, the buried nature of solid solid electrolyte electrode interfaces makes studying them difficult. Neutrons have significant potential to non destructively probe these buried solid solid interfaces. This work presents a comparative study using both neutron depth profiling (NDP) and neutron reflectometry (NR) to study a model lithium metal-lithium phosphorus oxynitride (LiPON) solid electrolyte system. In the NDP data, no distinct interphase is observed at the interface. NR shows a difference between electrodeposited, and vapor deposited LiPON -Li interfaces but finds both are gradient interphases that are less than 30 nm thick. Additional simulations of the LiPON-Li2O-Li system demonstrate that NDP has an excellent resolution in the 50 nm-1 mm regime while NR has an ideal resolution from 0.1 - 200 nm with different sample requirements. Together NDP and NR can provide a complementary understanding of interfaces between Li metal and solid electrolytes across relevant length scales.
△ Less
Submitted 6 December, 2025;
originally announced December 2025.
-
Higher-dimensional Fermiology in bulk moiré metals
Authors:
Kevin P. Nuckolls,
Nisarga Paul,
Alan Chen,
Filippo Gaggioli,
Joshua P. Wakefield,
Avi Auslender,
Jules Gardener,
Austin J. Akey,
David Graf,
Takehito Suzuki,
David C. Bell,
Liang Fu,
Joseph G. Checkelsky
Abstract:
In the past decade, moiré materials have revolutionized how we engineer and control quantum phases of matter. Among incommensurate materials, moiré materials are aperiodic composite crystals whose long-wavelength moiré superlattices enable tunable properties without chemically modifying their layers. To date, nearly all reports of moiré materials have investigated van der Waals heterostructures as…
▽ More
In the past decade, moiré materials have revolutionized how we engineer and control quantum phases of matter. Among incommensurate materials, moiré materials are aperiodic composite crystals whose long-wavelength moiré superlattices enable tunable properties without chemically modifying their layers. To date, nearly all reports of moiré materials have investigated van der Waals heterostructures assembled far from thermodynamic equilibrium. Here we introduce a conceptually new approach to synthesizing high-mobility moiré materials in thermodynamic equilibrium. We report a new family of foliated superlattice materials (Sr$_6$TaS$_8$)$_{1+δ}$(TaS$_2$)$_8$ that are exfoliatable van der Waals crystals with atomically incommensurate lattices. Lattice mismatches between alternating layers generate moiré superlattices, analogous to those of 2D moiré heterobilayers, that are coherent throughout these crystals and are tunable through their synthesis conditions without altering their chemical composition. High-field quantum oscillation measurements map the complex Fermiology of these moiré metals, which can be tuned via the moiré superlattice structure. We find that the Fermi surface of the structurally simplest moiré metal is comprised of over 40 distinct cross-sectional areas, the most observed in any material to our knowledge. This can be naturally understood by postulating that bulk moiré materials can encode electronic properties of higher-dimensional superspace crystals in ways that parallel well-established crystallographic methods used for incommensurate lattices. More broadly, our work demonstrates a scalable synthesis approach potentially capable of producing moiré materials for electronics applications and evidences a novel material design concept for accessing a broad range of physical phenomena proposed in higher dimensions.
△ Less
Submitted 30 October, 2025;
originally announced October 2025.
-
Bound on entanglement in neural quantum states
Authors:
Nisarga Paul
Abstract:
Variational wavefunctions offer a practical route around the exponential complexity of many-body Hilbert spaces, but their expressive power is often sharply constrained. Matrix product states, for instance, are efficient but limited to area law entangled states. Neural quantum states (NQS) are widely believed to overcome such limitations, yet little is known about their fundamental constraints. He…
▽ More
Variational wavefunctions offer a practical route around the exponential complexity of many-body Hilbert spaces, but their expressive power is often sharply constrained. Matrix product states, for instance, are efficient but limited to area law entangled states. Neural quantum states (NQS) are widely believed to overcome such limitations, yet little is known about their fundamental constraints. Here we prove that feed-forward neural quantum states acting on $n$ spins with $k$ scalar nonlinearities, under certain analyticity assumptions, obey a bound on entanglement entropy for any subregion: $S \leq c k\log n$, for a constant $c$. This establishes an NQS analog of the area law constraint for matrix product states and rules out volume law entanglement for NQS with $O(1)$ nonlinearities. We demonstrate analytically and numerically that the scaling with $n$ is tight for a wide variety of NQS. Our work establishes a fundamental constraint on NQS that applies broadly across different network designs, while reinforcing their substantial expressive power.
△ Less
Submitted 13 October, 2025;
originally announced October 2025.
-
Reconstructing the Hamiltonian from the local density of states using neural networks
Authors:
Nisarga Paul,
Andrew Ma,
Kevin P. Nuckolls
Abstract:
Reconstructing a quantum system's Hamiltonian from limited yet experimentally observable information is interesting both as a practical task and from a fundamental standpoint. We pose and investigate the inverse problem of reconstructing a Hamiltonian from a spatial map of the local density of states (LDOS) near a fixed energy. We demonstrate high-quality recovery of Hamiltonians from the LDOS usi…
▽ More
Reconstructing a quantum system's Hamiltonian from limited yet experimentally observable information is interesting both as a practical task and from a fundamental standpoint. We pose and investigate the inverse problem of reconstructing a Hamiltonian from a spatial map of the local density of states (LDOS) near a fixed energy. We demonstrate high-quality recovery of Hamiltonians from the LDOS using supervised learning. In particular, we generate synthetic data from single-particle Hamiltonians in 1D and 2D, train convolutional neural networks, and obtain models that solve the inverse problem with remarkably high accuracy. Moreover, we are able to generalize beyond the training distribution and develop models with strong robustness to noise. Finally, we comment on possible experimental applications to scanning tunneling microscopy, where we propose that maps of the electronic local density of states might be used to reveal a sample's unknown underlying energy landscape.
△ Less
Submitted 11 September, 2025;
originally announced September 2025.
-
Emergent curved space and gravitational lensing in quantum materials
Authors:
Yugo Onishi,
Nisarga Paul,
Liang Fu
Abstract:
We show that an effective gravitational field naturally emerges in quantum materials with long-wavelength spin (or pseudospin) textures. When the itinerant electrons' spin strongly couples to the background spin texture, it effectively behaves as a spinless particle in a curved space, with the curvature arising from quantum corrections to the electron's spin orientation. The emergent curved space…
▽ More
We show that an effective gravitational field naturally emerges in quantum materials with long-wavelength spin (or pseudospin) textures. When the itinerant electrons' spin strongly couples to the background spin texture, it effectively behaves as a spinless particle in a curved space, with the curvature arising from quantum corrections to the electron's spin orientation. The emergent curved space gives rise to the electron lensing effect, an analog of the gravitational lensing. The lensing effect can appear in systems without (emergent) magnetic fields, such as those with coplanar spin textures. Our work shows that novel ``gravitational'' phenomena generically appear in quantum systems due to nonadiabaticity, opening new research directions in quantum physics.
△ Less
Submitted 3 October, 2025; v1 submitted 4 June, 2025;
originally announced June 2025.
-
Gyrotropic magnetic effect in metallic chiral magnets
Authors:
Nisarga Paul,
Takamori Park,
Jung Hoon Han,
Leon Balents
Abstract:
We study the gyrotropic magnetic effect (GME), the low-frequency limit of optical gyrotropy, in metals and semimetals coupled to chiral spin textures. In these systems, the chiral spin texture which lacks inversion symmetry can imprint itself upon the electronic structure through Hund's coupling, leading to novel low-frequency optical activity. Using perturbation theory and numerical diagonalizati…
▽ More
We study the gyrotropic magnetic effect (GME), the low-frequency limit of optical gyrotropy, in metals and semimetals coupled to chiral spin textures. In these systems, the chiral spin texture which lacks inversion symmetry can imprint itself upon the electronic structure through Hund's coupling, leading to novel low-frequency optical activity. Using perturbation theory and numerical diagonalization of both relativistic and non-relativistic models of conduction electrons coupled to spin textures, we analyze how the GME manifests in both single-$q$ and multi-$q$ textures. Analytical expressions for the rotatory power are derived in terms of universal scaling functions. Estimates based on realistic material parameters reveal an experimentally viable range of values for the rotatory power. The GME arises from the orbital and spin magnetic moments of conduction electrons, with the orbital part closely tied to Berry curvature and playing a significant role in relativistic metals but not so in non-relativistic metals where there is no inherent Berry curvature. The spin contribution to the GME can be significant in non-relativistic metals with a large Fermi energy. Our work establishes the GME as a sensitive probe of magnetic chirality and symmetry breaking in metallic chiral magnets.
△ Less
Submitted 12 December, 2025; v1 submitted 28 April, 2025;
originally announced April 2025.
-
Shining light on collective modes in moiré fractional Chern insulators
Authors:
Nisarga Paul,
Ahmed Abouelkomsan,
Aidan Reddy,
Liang Fu
Abstract:
We show that collective excitations and optical responses of moiré fractional Chern insulators (FCIs) drastically differ from those of standard fractional quantum Hall (FQH) states in a Landau level. By constructing a variational wavefunction that incorporates the moiré lattice effect, we capture the collective modes in FCIs across a range of crystal momenta including the roton minimum. Interestin…
▽ More
We show that collective excitations and optical responses of moiré fractional Chern insulators (FCIs) drastically differ from those of standard fractional quantum Hall (FQH) states in a Landau level. By constructing a variational wavefunction that incorporates the moiré lattice effect, we capture the collective modes in FCIs across a range of crystal momenta including the roton minimum. Interestingly, new collective modes -- ``fractional excitons'' -- are found in the long wavelength limit ($\boldsymbol{q} \rightarrow 0$) at low energy below the excitation continuum, distinct from the FQH case. Some of these modes are optically active and manifest as sharp peaks in optical conductivity at THz frequency. We further show that intraband optical absorption and spectral weight in twisted ${\rm MoTe}_2$ are highly tunable by the displacement field. Our work thus establishes optical spectroscopy as a powerful tool to illuminate the unique collective modes of moiré FCIs.
△ Less
Submitted 24 February, 2025;
originally announced February 2025.
-
Stability of sub-dimensional localization to electronic interactions
Authors:
Nisarga Paul,
Philip J. D. Crowley
Abstract:
Sub-dimensional localization, also known as directional localization, arises when 2d electrons are subject to a periodic potential and incommensurate magnetic field which cause them to become exponentially localized along one crystal axis, while remaining extended in the orthogonal direction. We establish that sub-dimensional localization is robust to the presence of electronic interactions, which…
▽ More
Sub-dimensional localization, also known as directional localization, arises when 2d electrons are subject to a periodic potential and incommensurate magnetic field which cause them to become exponentially localized along one crystal axis, while remaining extended in the orthogonal direction. We establish that sub-dimensional localization is robust to the presence of electronic interactions, which are known to generically destabilize single-particle localization. Consequently we predict that sub-dimensional localization may occur in moire materials, where it is manifest as a total absence of conductivity in the localized (transverse) direction, with finite conductivity in the extended (longitudinal) direction. We find the electrons form longitudinal charge density waves which are incommensurate with the moire potential, allowing them to slide and hence conduct in the longitudinal direction. The conductivity remains prohibited in the transverse direction due an emergent charge dipole moment conservation law.
△ Less
Submitted 28 October, 2024;
originally announced October 2024.
-
Designing (higher) Hall crystals
Authors:
Nisarga Paul,
Gal Shavit,
Liang Fu
Abstract:
We introduce a novel platform for realizing interaction-induced Hall crystals with diverse Chern numbers $C$. This platform consists of a two-dimensional semiconductor or graphene subjected to an out-of-plane magnetic field and a one-dimensional modulation, which can be realized by moiré or dielectric engineering. We show that interactions drive the system to spontaneously break the residual trans…
▽ More
We introduce a novel platform for realizing interaction-induced Hall crystals with diverse Chern numbers $C$. This platform consists of a two-dimensional semiconductor or graphene subjected to an out-of-plane magnetic field and a one-dimensional modulation, which can be realized by moiré or dielectric engineering. We show that interactions drive the system to spontaneously break the residual translational symmetry, resulting in Hall crystals with various $C$ (including $|C|>1$). Remarkably, these phases persist across continuous ranges of filling and magnetic field, and the global phase diagram can be understood in a unified manner.
△ Less
Submitted 4 October, 2024;
originally announced October 2024.
-
Band structure engineering using a moiré polar substrate
Authors:
Xirui Wang,
Cheng Xu,
Samuel Aronson,
Daniel Bennett,
Nisarga Paul,
Philip J. D. Crowley,
Clément Collignon,
Kenji Watanabe,
Takashi Taniguchi,
Raymond Ashoori,
Efthimios Kaxiras,
Yang Zhang,
Pablo Jarillo-Herrero,
Kenji Yasuda
Abstract:
Applying long wavelength periodic potentials on quantum materials has recently been demonstrated to be a promising pathway for engineering novel quantum phases of matter. Here, we utilize twisted bilayer boron nitride (BN) as a moiré substrate for band structure engineering. Small-angle-twisted bilayer BN is endowed with periodically arranged up and down polar domains, which imprints a periodic el…
▽ More
Applying long wavelength periodic potentials on quantum materials has recently been demonstrated to be a promising pathway for engineering novel quantum phases of matter. Here, we utilize twisted bilayer boron nitride (BN) as a moiré substrate for band structure engineering. Small-angle-twisted bilayer BN is endowed with periodically arranged up and down polar domains, which imprints a periodic electrostatic potential on a target two-dimensional (2D) material placed on top. As a proof of concept, we use Bernal bilayer graphene as the target material. The resulting modulation of the band structure appears as superlattice resistance peaks, tunable by varying the twist angle, and Hofstadter butterfly physics under a magnetic field. Additionally, we demonstrate the tunability of the moiré potential by altering the dielectric thickness underneath the twisted BN. Finally, we find that near-60°-twisted bilayer BN provides a unique platform for studying the moiré structural effect without the contribution from electrostatic moiré potentials. Tunable moiré polar substrates may serve as versatile platforms to engineer the electronic, optical, and mechanical properties of 2D materials and van der Waals heterostructures.
△ Less
Submitted 6 May, 2024;
originally announced May 2024.
-
Compressible quantum liquid with vanishing Drude weight
Authors:
Ahmed Abouelkomsan,
Nisarga Paul,
Ady Stern,
Liang Fu
Abstract:
We explore the possibility of quantum liquids that are compressible but have vanishing DC conductivity in the absence of disorder. We show that the composite Fermi liquid emerging from strong interaction in a generic Chern band has zero Drude weight, in stark contrast to normal Fermi liquids. Our work establishes the absence of Drude weight as the defining property of the composite Fermi liquid ph…
▽ More
We explore the possibility of quantum liquids that are compressible but have vanishing DC conductivity in the absence of disorder. We show that the composite Fermi liquid emerging from strong interaction in a generic Chern band has zero Drude weight, in stark contrast to normal Fermi liquids. Our work establishes the absence of Drude weight as the defining property of the composite Fermi liquid phase, which distinguishes it from the Fermi liquid or other types of non-Fermi liquids. Our findings point to a possibly wide class of gapless quantum phases with unexpected transport and optical properties.
△ Less
Submitted 1 May, 2025; v1 submitted 21 March, 2024;
originally announced March 2024.
-
Non-Abelian fractionalization in topological minibands
Authors:
Aidan P. Reddy,
Nisarga Paul,
Ahmed Abouelkomsan,
Liang Fu
Abstract:
Motivated by the recent discovery of fractional quantum anomalous Hall states in moiré systems, we consider the possibility of realizing non-Abelian phases in topological minibands. We study a family of moiré systems, skyrmion Chern band models, which can be realized in two-dimensional semiconductor-magnet heterostructures and also capture the essence of twisted transition metal dichalcogenide hom…
▽ More
Motivated by the recent discovery of fractional quantum anomalous Hall states in moiré systems, we consider the possibility of realizing non-Abelian phases in topological minibands. We study a family of moiré systems, skyrmion Chern band models, which can be realized in two-dimensional semiconductor-magnet heterostructures and also capture the essence of twisted transition metal dichalcogenide homobilayers. We show using many-body exact diagonalization that, in spite of strong Berry curvature variations in momentum space, the non-Abelian Moore-Read state can be realized at half filling of the second miniband. These results demonstrate the feasibility of non-Abelian fractionalization in moiré systems without Landau levels and shed light on the desirable conditions for their realization. In particular, we highlight the prospect of realizing the Moore-Read state in twisted semiconductor bilayers.
△ Less
Submitted 8 December, 2024; v1 submitted 29 February, 2024;
originally announced March 2024.
-
Dimensional reduction from magnetic field in moiré superlattices
Authors:
Nisarga Paul,
Philip J. D. Crowley,
Liang Fu
Abstract:
Moiré materials provide a highly tunable platform in which novel electronic phenomena can emerge. We study strained moiré materials in a uniform magnetic field and predict highly anisotropic electrical conductivity which switches easy-axis as magnetic field or strain is varied. The dramatic anisotropy reflects one-dimensional localization (dimensional reduction) of the electron wavefunctions along…
▽ More
Moiré materials provide a highly tunable platform in which novel electronic phenomena can emerge. We study strained moiré materials in a uniform magnetic field and predict highly anisotropic electrical conductivity which switches easy-axis as magnetic field or strain is varied. The dramatic anisotropy reflects one-dimensional localization (dimensional reduction) of the electron wavefunctions along a crystal axis due to quantum interference effects. This can be understood in an effective one-dimensional quasiperiodic Aubry-André-like models, or in a complementary semiclassical picture. This phenomenon should be observable in strained moiré materials at realistic fields and low strain disorder, as well as unstrained systems with anisotropic Fermi surfaces.
△ Less
Submitted 15 November, 2023;
originally announced November 2023.
-
Electronic ratchet effect in a moiré system: signatures of excitonic ferroelectricity
Authors:
Zhiren Zheng,
Xueqiao Wang,
Ziyan Zhu,
Stephen Carr,
Trithep Devakul,
Sergio de la Barrera,
Nisarga Paul,
Zumeng Huang,
Anyuan Gao,
Yang Zhang,
Damien Bérubé,
Kathryn Natasha Evancho,
Kenji Watanabe,
Takashi Taniguchi,
Liang Fu,
Yao Wang,
Su-Yang Xu,
Efthimios Kaxiras,
Pablo Jarillo-Herrero,
Qiong Ma
Abstract:
Electronic ferroelectricity represents a new paradigm where spontaneous symmetry breaking driven by electronic correlations, in contrast to traditional lattice-driven ferroelectricity, leads to the formation of electric dipoles. Despite the potential application advantages arising from its electronic nature, switchable electronic ferroelectricity remains exceedingly rare. Here, we report the disco…
▽ More
Electronic ferroelectricity represents a new paradigm where spontaneous symmetry breaking driven by electronic correlations, in contrast to traditional lattice-driven ferroelectricity, leads to the formation of electric dipoles. Despite the potential application advantages arising from its electronic nature, switchable electronic ferroelectricity remains exceedingly rare. Here, we report the discovery of an electronic ratchet effect that manifests itself as switchable electronic ferroelectricity in a layer-contrasting graphene-boron nitride moiré heterostructure. Our engineered layer-asymmetric moiré potential landscapes result in layer-polarized localized and itinerant electronic subsystems. At particular fillings of the localized subsystem, we find a ratcheting injection of itinerant carriers in a non-volatile manner, leading to a highly unusual ferroelectric response. Strikingly, the remnant polarization can be stabilized at multiple (quasi-continuous) states with behavior markedly distinct from known ferroelectrics. Our experimental observations, simulations, and theoretical analysis suggest that dipolar excitons are the driving force and elementary ferroelectric units in our system. This signifies a new type of electronic ferroelectricity where the formation of dipolar excitons with aligned moments generates a macroscopic polarization and leads to an electronically-driven ferroelectric response, which we term excitonic ferroelectricity. Such new ferroelectrics, driven by quantum objects like dipolar excitons, could pave the way to innovative quantum analog memory and synaptic devices.
△ Less
Submitted 6 June, 2023;
originally announced June 2023.
-
Zero-field composite Fermi liquid in twisted semiconductor bilayers
Authors:
Hart Goldman,
Aidan P. Reddy,
Nisarga Paul,
Liang Fu
Abstract:
Recent experiments have produced evidence for fractional quantum anomalous Hall (FQAH) states at zero magnetic field in the semiconductor moiré superlattice system $t$MoTe$_2$. Here we argue that a composite fermion description, already a unifying framework for the phenomenology of 2d electron gases at high magnetic fields, provides a similarly powerful perspective in this new context. To this end…
▽ More
Recent experiments have produced evidence for fractional quantum anomalous Hall (FQAH) states at zero magnetic field in the semiconductor moiré superlattice system $t$MoTe$_2$. Here we argue that a composite fermion description, already a unifying framework for the phenomenology of 2d electron gases at high magnetic fields, provides a similarly powerful perspective in this new context. To this end, we present exact diagonalization evidence for composite Fermi liquid states at zero magnetic field in $t$MoTe$_2$ at fillings $n=\frac{1}{2}$ and $n=\frac{3}{4}$. We dub these non-Fermi liquid metals anomalous composite Fermi liquids (ACFLs), and we argue that they play a central organizing role in the FQAH phase diagram. We proceed to develop a long wavelength theory for this ACFL state that offers concrete experimental predictions upon doping the composite Fermi sea, including a Jain sequence of FQAH states and a new type of commensurability oscillations originating from the superlattice potential intrinsic to the system.
△ Less
Submitted 23 October, 2023; v1 submitted 4 June, 2023;
originally announced June 2023.
-
Design aspects of dual gate GaAs nanowire FET for room temperature charge qubit operation: A study on diameter and gate engineering
Authors:
Nilayan Paul,
Basudev Nag Chowdhury,
Sanatan Chattopadhyay
Abstract:
The current work explores a geometrically engineered dual gate GaAs nanowire FET with state of the art miniaturized dimensions for high performance charge qubit operation at room temperature. Relevant gate voltages in such device can create two voltage tunable quantum dots (VTQDs) underneath the gates, as well as can manipulate their eigenstate detuning and the inter-dot coupling to generate super…
▽ More
The current work explores a geometrically engineered dual gate GaAs nanowire FET with state of the art miniaturized dimensions for high performance charge qubit operation at room temperature. Relevant gate voltages in such device can create two voltage tunable quantum dots (VTQDs) underneath the gates, as well as can manipulate their eigenstate detuning and the inter-dot coupling to generate superposition, whereas a small drain bias may cause its collapse leading to qubit read out. Such qubit operations, i.e., Initialization, Manipulation, and Measurement, are theoretically modeled in the present work by developing a second quantization filed operator based Schrodinger-Poisson self-consistent framework coupled to non-equilibrium Greens function formalism. The study shows that the Bloch sphere coverage can be discretized along polar and azimuthal directions by reducing the nanowire diameter and increasing the inter-dot separation respectively, that can be utilized for selective information encoding. The theoretically obtained stability diagrams suggest that downscaled nanowire diameter and increased gate separation sharpen the bonding and anti-bonding states with reduced anticrossing leading to a gradual transformation of the hyperbolic current mapping into a pair of straight lines. However, the dephasing time in the proposed GaAs VTQD-based qubit may be significantly improved by scaling down both the nanowire diameter and gate separation. Therefore, the present study suggests an optimization window for geometrical engineering of a dual gate nanowire FET qubit to achieve superior qubit performance. Most importantly, such device is compatible with the mainstream CMOS technology and can be utilized for large scale implementation by little modification of the state of the art fabrication processes.
△ Less
Submitted 20 April, 2023;
originally announced April 2023.
-
Superconductivity and strong interactions in a tunable moiré quasiperiodic crystal
Authors:
Aviram Uri,
Sergio C. de la Barrera,
Mallika T. Randeria,
Daniel Rodan-Legrain,
Trithep Devakul,
Philip J. D. Crowley,
Nisarga Paul,
Kenji Watanabe,
Takashi Taniguchi,
Ron Lifshitz,
Liang Fu,
Raymond C. Ashoori,
Pablo Jarillo-Herrero
Abstract:
Electronic states in quasiperiodic crystals generally preclude a Bloch description, rendering them simultaneously fascinating and enigmatic. Owing to their complexity and relative scarcity, quasiperiodic crystals are underexplored relative to periodic and amorphous structures. Here, we introduce a new type of highly tunable quasiperiodic crystal easily assembled from periodic components. By twisti…
▽ More
Electronic states in quasiperiodic crystals generally preclude a Bloch description, rendering them simultaneously fascinating and enigmatic. Owing to their complexity and relative scarcity, quasiperiodic crystals are underexplored relative to periodic and amorphous structures. Here, we introduce a new type of highly tunable quasiperiodic crystal easily assembled from periodic components. By twisting three layers of graphene with two different twist angles, we form two moiré patterns with incommensurate moiré unit cells. In contrast to many common quasiperiodic structures that are defined on the atomic scale, the quasiperiodicity in our system is defined on moiré length scales of several nanometers. This novel "moiré quasiperiodic crystal" allows us to tune the chemical potential and thus the electronic system between a periodic-like regime at low energies and a strongly quasiperiodic regime at higher energies, the latter hosting a large density of weakly dispersing states. Interestingly, in the quasiperiodic regime we observe superconductivity near a flavor-symmetry-breaking phase transition, the latter indicative of the important role electronic interactions play in that regime. The prevalence of interacting phenomena in future systems with in situ tunability is not only useful for the study of quasiperiodic systems, but it may also provide insights into electronic ordering in related periodic moiré crystals. We anticipate that extending this new platform to engineer quasiperiodic crystals by varying the number of layers and twist angles, and by using different two-dimensional components, will lead to a new family of quantum materials to investigate the properties of strongly interacting quasiperiodic crystals.
△ Less
Submitted 1 February, 2023;
originally announced February 2023.
-
Giant and Broadband THz and IR Emission in Drift-biased Graphene-Based Hyperbolic Nanostructures
Authors:
L. Wang,
N. K. Paul,
J. Hihath,
J. S. Gomez-Diaz
Abstract:
We demonstrate that Cherenkov radiation can be manipulated in terms of operation frequency, bandwidth, and efficiency by simultaneously controlling the properties of drifting electrons and the photonic states supported by their surrounding media. We analytically show that the radiation rate strongly depends on the momentum of the excited photonic state, in terms of magnitude, frequency dispersion,…
▽ More
We demonstrate that Cherenkov radiation can be manipulated in terms of operation frequency, bandwidth, and efficiency by simultaneously controlling the properties of drifting electrons and the photonic states supported by their surrounding media. We analytically show that the radiation rate strongly depends on the momentum of the excited photonic state, in terms of magnitude, frequency dispersion, and its variation versus the properties of the drifting carriers. This approach is applied to design and realize miniaturized, broadband, tunable, and efficient terahertz and far-infrared sources by manipulating and boosting the coupling between drifting electrons and engineered hyperbolic modes in graphene-based nanostructures. The broadband, dispersive, and confined nature of hyperbolic modes relax momentum matching issues, avoid using electron beams, and drastically enhance the radiation rate - allowing that over 90% of drifting electrons emit photons. Our findings open a new paradigm for the development of solid-state terahertz and infrared sources.
△ Less
Submitted 24 November, 2022;
originally announced November 2022.
-
Area-law entanglement from quantum geometry
Authors:
Nisarga Paul
Abstract:
Quantum geometry, which encompasses both Berry curvature and the quantum metric, plays a key role in multi-band interacting electron systems. We study the entanglement entropy of a region of linear size $\ell$ in fermion systems with nontrivial quantum geometry, i.e. whose Bloch states have nontrivial $k$ dependence. We show that the entanglement entropy scales as…
▽ More
Quantum geometry, which encompasses both Berry curvature and the quantum metric, plays a key role in multi-band interacting electron systems. We study the entanglement entropy of a region of linear size $\ell$ in fermion systems with nontrivial quantum geometry, i.e. whose Bloch states have nontrivial $k$ dependence. We show that the entanglement entropy scales as $S = α\ell^{d-1} \ln\ell + β\ell^{d-1} + \cdots$ where the first term is the well-known area-law violating term for fermions and $β$ contains the leading contribution from quantum geometry. We compute this for the case of uniform quantum geometry and cubic domains and provide numerical results for the Su-Schrieffer-Heeger model, 2D massive Dirac cone, and 2D Chern bands. An experimental probe of the quantum geometric entanglement entropy is proposed using particle number fluctuations. We offer an intuitive account of the area-law entanglement related to the spread of maximally localized Wannier functions.
△ Less
Submitted 24 October, 2022;
originally announced October 2022.
-
Lateral recoil optical forces on nanoparticles near nonreciprocal surfaces
Authors:
N. K. Paul,
J. S. Gomez-Diaz
Abstract:
We investigate lateral recoil forces exerted on nanoparticles located near plasmonic platforms with in-plane nonreciprocal response. To this purpose, we first develop a comprehensive theoretical framework based on the Lorentz force within the Rayleigh approximation combined with nonreciprocal Green's functions and then derive approximate analytical expressions to model lateral recoil forces, demon…
▽ More
We investigate lateral recoil forces exerted on nanoparticles located near plasmonic platforms with in-plane nonreciprocal response. To this purpose, we first develop a comprehensive theoretical framework based on the Lorentz force within the Rayleigh approximation combined with nonreciprocal Green's functions and then derive approximate analytical expressions to model lateral recoil forces, demonstrating their explicit dependence on the dispersion relation of the system and unveiling the mechanisms that govern them. In particular, a dominant lateral recoil force component appears due to the momentum imbalance of nonreciprocal surface plasmons supported by the platform. This force can be orders of magnitude larger than other recoil force components, acts only along or against the direction of the external bias, and is quasi-independent of the direction, polarization, and wavelength of the incident plane wave. Lateral recoil forces are explored using drift-biased graphene metasurfaces, a platform that is also proposed to sort nanoparticles as a function of their size. Nonreciprocal plasmonic systems may enable new venues to trap, bind, and manipulate nanoparticles and to alleviate some of the challenges of conventional optical tweezers.
△ Less
Submitted 16 December, 2022; v1 submitted 16 March, 2022;
originally announced March 2022.
-
Moiré Landau fans and magic zeros
Authors:
Nisarga Paul,
Philip J. D. Crowley,
Trithep Devakul,
Liang Fu
Abstract:
We study the energy spectrum of moiré systems under a uniform magnetic field. The superlattice potential generally broadens Landau levels into Chern bands with finite bandwidth. However, we find that these Chern bands become flat at a discrete set of magnetic fields which we dub "magic zeros". The flat band subspace is generally different from the Landau level subspace in the absence of the moiré…
▽ More
We study the energy spectrum of moiré systems under a uniform magnetic field. The superlattice potential generally broadens Landau levels into Chern bands with finite bandwidth. However, we find that these Chern bands become flat at a discrete set of magnetic fields which we dub "magic zeros". The flat band subspace is generally different from the Landau level subspace in the absence of the moiré superlattice. By developing a semiclassical quantization method and taking account of superlattice induced Bragg reflection, we prove that magic zeros arise from the simultaneous quantization of two distinct $k$-space orbits. The flat bands at magic zeros provide a new setting for exploring crystalline fractional quantum Hall physics.
△ Less
Submitted 30 August, 2022; v1 submitted 11 February, 2022;
originally announced February 2022.
-
Giant proximity exchange and flat Chern band in 2D magnet-semiconductor heterostructures
Authors:
Nisarga Paul,
Yang Zhang,
Liang Fu
Abstract:
Van der Waals (vdW) heterostructures formed by two-dimensional magnets and semiconductors have provided a fertile ground for fundamental science and for spintronics. We present first-principles calculations finding a proximity exchange splitting of 14 meV equivalent to an effective Zeeman field of 120 T in the vdW magnet-semiconductor heterostructure MoS$_2$/CrBr$_3$, leading to a 2D spin-polarize…
▽ More
Van der Waals (vdW) heterostructures formed by two-dimensional magnets and semiconductors have provided a fertile ground for fundamental science and for spintronics. We present first-principles calculations finding a proximity exchange splitting of 14 meV equivalent to an effective Zeeman field of 120 T in the vdW magnet-semiconductor heterostructure MoS$_2$/CrBr$_3$, leading to a 2D spin-polarized half-metal with carrier densities ranging up to $10^{13}$ cm$^{-2}$. We consequently explore the effect of large exchange coupling on the electronic bandstructure when the magnetic layer hosts chiral spin textures such as skyrmions. A flat Chern band is found at a "magic" value of magnetization $\overline{m} \sim 0.2$ for Schrödinger electrons, and it generally occurs for Dirac electrons. The magnetic proximity induced anomalous Hall effect enables transport-based detection of chiral spin textures, and flat Chern bands provides an avenue for engineering various strongly correlated states.
△ Less
Submitted 21 January, 2023; v1 submitted 2 November, 2021;
originally announced November 2021.
-
Topological magnetic textures in magnetic topological insulators
Authors:
Nisarga Paul,
Liang Fu
Abstract:
The surfaces of intrinsic magnetic topological insulators (TIs) host magnetic moments exchange-coupled to Dirac electrons. We study the magnetic phases arising from tuning the electron density using variational and exact diagonalization approaches. In the dilute limit, we find that magnetic skrymions are formed which bind to electrons leading to a skyrmion Wigner crystal phase while at higher dens…
▽ More
The surfaces of intrinsic magnetic topological insulators (TIs) host magnetic moments exchange-coupled to Dirac electrons. We study the magnetic phases arising from tuning the electron density using variational and exact diagonalization approaches. In the dilute limit, we find that magnetic skrymions are formed which bind to electrons leading to a skyrmion Wigner crystal phase while at higher densities spin spirals accompanied by chiral 1d channels of electrons are formed. The binding of electrons to textures raises the possibility of manipulating textures with electrostatic gating. We determine the phase diagram capturing the competition of intrinsic spin-spin interactions and carrier density and comment on the possible application to experiments in magnetic TIs and spintronic devices such as skyrmion-based memory.
△ Less
Submitted 9 June, 2021; v1 submitted 3 March, 2021;
originally announced March 2021.
-
Quantum Diffusion in the Strong Tunneling Regime
Authors:
Nisarga Paul,
Ariel Amir
Abstract:
We study the spread of a quantum-mechanical wavepacket in a noisy environment, modeled using a tight-binding Hamiltonian. Despite the coherent dynamics, the fluctuating environment may give rise to diffusive behavior. When correlations between different level-crossing events can be neglected, we use the solution of the Landau-Zener problem to find how the diffusion constant depends on the noise. W…
▽ More
We study the spread of a quantum-mechanical wavepacket in a noisy environment, modeled using a tight-binding Hamiltonian. Despite the coherent dynamics, the fluctuating environment may give rise to diffusive behavior. When correlations between different level-crossing events can be neglected, we use the solution of the Landau-Zener problem to find how the diffusion constant depends on the noise. We also show that when an electric field or external disordered potential is applied to the system, the diffusion constant is suppressed with no drift term arising. The results are relevant to various quantum systems, including exciton diffusion in photosynthesis and electronic transport in solid-state physics.
△ Less
Submitted 10 December, 2018;
originally announced December 2018.
-
Giant Lateral Optical Forces on Rayleigh Particles near Hyperbolic and Extremely Anisotropic Metasurfaces
Authors:
N. K. Paul,
D. Correas-Serrano,
J. S. Gomez-Diaz
Abstract:
We report a dramatic enhancement of the lateral optical forces induced on electrically polarizable Rayleigh particles near hyperbolic and extremely anisotropic metasurfaces under simple plane wave illumination. Such enhancement is enabled by the interplay between the increased density of states provided by these structures and the out-of-plane polarization spin acquired by the particle. The result…
▽ More
We report a dramatic enhancement of the lateral optical forces induced on electrically polarizable Rayleigh particles near hyperbolic and extremely anisotropic metasurfaces under simple plane wave illumination. Such enhancement is enabled by the interplay between the increased density of states provided by these structures and the out-of-plane polarization spin acquired by the particle. The resulting giant lateral forces appear over a broad frequency range and may open unprecedented venues for routing, trapping, and assembling nanoparticles.
△ Less
Submitted 22 October, 2018;
originally announced October 2018.