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Interface Engineering of Helium Confinement in Argon-Preplated MCM-41 Nanopores
Authors:
Rahul Soni,
Nathan S. Nichols,
Sutirtha Paul,
Garfield Warren,
Paul Sokol,
Adrian Del Maestro
Abstract:
Atomic-scale modification of mesopore interfaces provides a route to tune the confinement experienced by adsorbed fluids, but how a specific interface preparation translates into the resulting microscopic confinement potential remains unclear. Here, we show that preplating MCM-41 with an argon monolayer modifies the effective pore interface by occupying strongly attractive regions of the heterogen…
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Atomic-scale modification of mesopore interfaces provides a route to tune the confinement experienced by adsorbed fluids, but how a specific interface preparation translates into the resulting microscopic confinement potential remains unclear. Here, we show that preplating MCM-41 with an argon monolayer modifies the effective pore interface by occupying strongly attractive regions of the heterogeneous silica surface and screening its atomic-scale corrugation. Grand-canonical Monte Carlo simulations of argon adsorption, low-temperature molecular dynamics, and helium test-particle insertion are combined with adsorption isotherms and neutron-scattering measurements to characterize the preplated pore at the atomic scale. Helium test-particle insertion calculations show that the modified interface shifts the helium adsorption minimum to an annular region inside the pore and produces a confinement landscape dominated by a smooth radial component. The resulting radial confinement potential can be described by a continuum cylindrical model, providing microscopic support for the effective potential used in earlier quantum Monte Carlo studies. Residual corrugation persists over multiple spatial scales and is accurately captured by a Gaussian process surrogate. These results demonstrate how atomic preplating can tailor nanopore confinement and provide an experimentally constrained microscopic potential for predictive studies of confined quantum fluids.
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Submitted 6 August, 2026;
originally announced August 2026.
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Interfacial control of magnetism and electron transport in nonisostructural SrRuO$_3$/SrCuO$_2$ heterostructure
Authors:
Digbijaya Palai,
B. Maharana,
P. Biswal,
Shwetha G. Bhat,
D. Nayak,
D. Sahoo,
R. Soni,
K. Senapati,
Z. Hossain,
D. Samal
Abstract:
Heterointerfaces between structurally dissimilar oxides provide a platform to exploit the interfacial mismatch in the lattice and electronic degrees of freedom to realize emergent functionalities and tunable physical properties with potential applications in oxide electronics. Here, we investigate the magnetic and electronic transport properties of symmetry-mismatched SrRuO$_3$ (SRO)/SrCuO$_2$ (SC…
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Heterointerfaces between structurally dissimilar oxides provide a platform to exploit the interfacial mismatch in the lattice and electronic degrees of freedom to realize emergent functionalities and tunable physical properties with potential applications in oxide electronics. Here, we investigate the magnetic and electronic transport properties of symmetry-mismatched SrRuO$_3$ (SRO)/SrCuO$_2$ (SCO) heterostructure, in which a 4-nm-thick itinerant ferromagnetic metal SRO is interfaced with a planar-type antiferromagnetic insulator SCO, in comparison with a reference SRO (4 nm) film. While both the bare SRO and SRO/SCO bilayer exhibit perpendicular magnetic anisotropy (PMA), the SRO/SCO bilayer shows a pronounced enhancement in the saturation magnetization (M$_S$ $\approx$ 2.7 $μ_B$/Ru) and effective anisotropy constant (K$_{\mathrm{eff}}$ $\approx$ 2.13 $\times$ 10$^6$ erg/cc) relative to the bare SRO film (M$_S$ $\approx$ 1.3 $μ_B$/Ru and K$_{\mathrm{eff}}$ $\approx$ 5.25 $\times$ 10$^5$ erg/cc). Interestingly, the low-temperature resistivity upturn below 5 K arising from disorder-induced quantum corrections in bare SRO is strongly suppressed in SRO/SCO, and it exhibits Fermi-liquid-like transport ($ρ\propto T^2$) down to 2 K. Analysis of the anomalous Hall effect (AHE) reveals dominant intrinsic Berry-curvature driven electron transport in both samples, with enhanced intrinsic and skew-scattering contributions in the SRO/SCO bilayer, indicating the critical role of the interface in modifying the electronic band structure near the Fermi level. Our results demonstrate that the interface acts as an effective control knob, concurrently enhancing the magnetization, PMA, and intrinsic scattering contribution to the AHE in the SRO/SCO heterostructure, while suppressing the disorder-driven quantum corrections to transport behavior observed in the bare SRO film.
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Submitted 15 June, 2026;
originally announced June 2026.
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Anisotropic light-electron-phonon coupling and ultrafast carrier separation in ferroelectric BaTiO$_3$
Authors:
Atal Bihari Swain,
Somnath Kale,
Rohit Soni,
Peter Baum
Abstract:
Ferroelectric materials with built-in electric fields are useful for ultrafast electronics and conversion of light into electrical energy, yet the interaction of carrier motion with ultrafast relaxation processes remains nontrivial. Combining ultrafast electron diffraction with electron microscopy of electromagnetic fields, we capture ultrafast lattice dynamics and nanometer-scale carrier transpor…
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Ferroelectric materials with built-in electric fields are useful for ultrafast electronics and conversion of light into electrical energy, yet the interaction of carrier motion with ultrafast relaxation processes remains nontrivial. Combining ultrafast electron diffraction with electron microscopy of electromagnetic fields, we capture ultrafast lattice dynamics and nanometer-scale carrier transport in ferroelectric BaTiO$_3$. We discover that BaTiO$_3$ reacts to light with an anisotropic electron-phonon coupling that depends on the optical polarization of the excitation light. Excited electrons relax two times faster into phonons when the optical electric field aligns to the ferroelectric symmetry break. Furthermore, ultrafast electron electrometry captures the motion and separation of photo-excited electron-hole pairs in the presence of the ferroelectric field. These combined results provide insight into the tangled and anisotropic interaction of photons with phonons and the ferroelectric field, producing phonons and voltages in a stepwise reaction path.
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Submitted 16 June, 2026; v1 submitted 26 March, 2026;
originally announced March 2026.
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Edge Reconstruction in a Quantum Spin Hall Insulator
Authors:
Rahul Soni,
Matthias Thamm,
Gonzalo Alvarez,
Bernd Rosenow,
Adrian Del Maestro
Abstract:
We study interaction-driven edge reconstruction in a quantum spin Hall insulator described by the Bernevig-Hughes-Zhang model with Kanamori-Hubbard interactions using the real-space density matrix renormalization group method in both the grand-canonical and canonical ensembles. For a two-dimensional cylinder with a smooth edge, we identify discrete particle-number transitions that lead to a spin-p…
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We study interaction-driven edge reconstruction in a quantum spin Hall insulator described by the Bernevig-Hughes-Zhang model with Kanamori-Hubbard interactions using the real-space density matrix renormalization group method in both the grand-canonical and canonical ensembles. For a two-dimensional cylinder with a smooth edge, we identify discrete particle-number transitions that lead to a spin-polarized edge state stabilized by an emergent ferromagnetic exchange interaction. The reconstruction is orbital-selective, occurring predominantly in the $s$-orbital channel. Our results reveal a microscopic mechanism for emergent fluctuating moments at the edge that could compromise the topological protection of helical edge states by time reversal symmetry.
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Submitted 11 July, 2026; v1 submitted 14 August, 2025;
originally announced August 2025.
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Enhanced Phonon-Assisted Tunneling in Metal -- Twisted Bilayer Graphene Junctions
Authors:
Radhika Soni,
Suvronil Datta,
Robin Bajaj,
Saisab Bhowmik,
Shinjan Mandal,
Baladitya Suri,
Kenji Watanabe,
Takashi Taniguchi,
Manish Jain,
U. Chandni
Abstract:
We report planar tunneling spectroscopy measurements on metal-WSe$_2$-twisted bilayer graphene heterostructures across a broad range of gate and bias voltages. The observed experimental features are attributed to phonon-assisted tunneling and the significantly high density of states within the moiré bands. A notable finding is the enhanced phonon-assisted tunneling in twisted bilayer graphene comp…
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We report planar tunneling spectroscopy measurements on metal-WSe$_2$-twisted bilayer graphene heterostructures across a broad range of gate and bias voltages. The observed experimental features are attributed to phonon-assisted tunneling and the significantly high density of states within the moiré bands. A notable finding is the enhanced phonon-assisted tunneling in twisted bilayer graphene compared to Bernal bilayer graphene, which arises from a more relaxed in-plane momentum matching criterion. Theoretical calculations of phonon dispersions enable us to identify low-energy phonon modes in both Bernal and twisted bilayers of graphene, thereby elucidating the underlying mechanism of tunneling. Our results establish planar tunneling as a versatile tool to further understand electron-phonon coupling in twisted van der Waals materials.
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Submitted 17 July, 2025;
originally announced July 2025.
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Interaction-Driven Topological Transitions in Monolayer TaIrTe$_4$
Authors:
Jiangxu Li,
Jian Tang,
Louis Primeau,
Thomas Siyuan Ding,
Rahul Soni,
Tiema Qian,
Kenji Watanabe,
Takashi Taniguchi,
Ni Ni,
Adrian Del Maestro,
Qiong Ma,
Yang Zhang
Abstract:
Discovering materials that combine topological phenomena with correlated electron behavior is a central pursuit in quantum materials research. Monolayer TaIrTe$_4$ has recently emerged as a promising platform in this context, hosting robust quantum spin Hall insulator (QSHI) phases both within a single-particle gap and within a correlation-induced gap arising from van Hove singularities (vHSs), ac…
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Discovering materials that combine topological phenomena with correlated electron behavior is a central pursuit in quantum materials research. Monolayer TaIrTe$_4$ has recently emerged as a promising platform in this context, hosting robust quantum spin Hall insulator (QSHI) phases both within a single-particle gap and within a correlation-induced gap arising from van Hove singularities (vHSs), accessed via electrostatic doping. Its intrinsic monolayer nature offers exceptional tunability and the potential to realize a versatile array of interaction-driven topological phases. In this work, we combine theory and experiment to map the phase landscape of monolayer TaIrTe$_4$. Using Hartree-Fock calculations, we investigate the interaction-driven phase diagram near the vHSs under commensurate filling conditions. By systematically tuning the dielectric screening and strain, we uncover a rich set of ground states--including QSHI, trivial insulator, higher-order topological insulator, and metallic phase--among which are interaction-driven topological phase transitions. Experimentally, we perform both local and nonlocal transport measurements across a broad set of devices, which--due to unavoidable strain variations during fabrication-realize several phases consistent with theoretical predictions. Together, our results lay the groundwork for understanding correlation-driven topological phenomena in TaIrTe$_4$ and open new directions for engineering exotic quantum phases in low-dimensional materials beyond the limitations of moiré superlattices.
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Submitted 23 June, 2025;
originally announced June 2025.
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Multipartite edge modes and tensor networks
Authors:
Chris Akers,
Ronak M. Soni,
Annie Y. Wei
Abstract:
Holographic tensor networks model AdS/CFT, but so far they have been limited by involving only systems that are very different from gravity. Unfortunately, we cannot straightforwardly discretize gravity to incorporate it, because that would break diffeomorphism invariance. In this note, we explore a resolution. In low dimensions gravity can be written as a topological gauge theory, which can be di…
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Holographic tensor networks model AdS/CFT, but so far they have been limited by involving only systems that are very different from gravity. Unfortunately, we cannot straightforwardly discretize gravity to incorporate it, because that would break diffeomorphism invariance. In this note, we explore a resolution. In low dimensions gravity can be written as a topological gauge theory, which can be discretized without breaking gauge-invariance. However, new problems arise. Foremost, we now need a qualitatively new kind of "area operator," which has no relation to the number of links along the cut and is instead topological. Secondly, the inclusion of matter becomes trickier. We successfully construct a tensor network both including matter and with this new type of area. Notably, while this area is still related to the entanglement in "edge mode" degrees of freedom, the edge modes are no longer bipartite entangled pairs. Instead they are highly multipartite. Along the way, we calculate the entropy of novel subalgebras in a particular topological gauge theory. We also show that the multipartite nature of the edge modes gives rise to non-commuting area operators, a property that other tensor networks do not exhibit.
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Submitted 19 June, 2024; v1 submitted 4 April, 2024;
originally announced April 2024.
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Topological and magnetic properties of the interacting Bernevig-Hughes-Zhang model
Authors:
Rahul Soni,
Harini Radhakrishnan,
Bernd Rosenow,
Gonzalo Alvarez,
Adrian Del Maestro
Abstract:
We investigate the effects of electronic correlations on the Bernevig-Hughes-Zhang model using the real-space density matrix renormalization group (DMRG) algorithm. We introduce a method to probe topological phase transitions in systems with strong correlations using DMRG, substantiated by an unsupervised machine learning methodology that analyzes the orbital structure of the real-space edges. Inc…
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We investigate the effects of electronic correlations on the Bernevig-Hughes-Zhang model using the real-space density matrix renormalization group (DMRG) algorithm. We introduce a method to probe topological phase transitions in systems with strong correlations using DMRG, substantiated by an unsupervised machine learning methodology that analyzes the orbital structure of the real-space edges. Including the full multi-orbital Hubbard interaction term, we construct a phase diagram as a function of a gap parameter ($m$) and the Hubbard interaction strength ($U$) via exact DMRG simulations on $N\times 4$ cylinders. Our analysis confirms that the topological phase persists in the presence of interactions, consistent with previous studies, but it also reveals an intriguing phase transition from a paramagnetic to a stripey antiferromagnetic topological insulator. The combination of the magnetic structure factor, strength of magnetic moments, and the orbitally resolved density, provides real-space information on both topology and magnetism in a strongly correlated system.
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Submitted 14 September, 2024; v1 submitted 26 October, 2023;
originally announced October 2023.
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Spin-orbit coupling-enhanced valley ordering of malleable bands in twisted bilayer graphene on WSe2
Authors:
Saisab Bhowmik,
Bhaskar Ghawri,
Youngju Park,
Dongkyu Lee,
Suvronil Datta,
Radhika Soni,
K. Watanabe,
T. Taniguchi,
Arindam Ghosh,
Jeil Jung,
U. Chandni
Abstract:
New phases of matter can be stabilized by a combination of diverging electronic density of states, strong interactions, and spin-orbit coupling. Recent experiments in magic-angle twisted bilayer graphene (TBG) have uncovered a wealth of novel phases as a result of interaction-driven spin-valley flavour polarization. In this work, we explore correlated phases appearing due to the combined effect of…
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New phases of matter can be stabilized by a combination of diverging electronic density of states, strong interactions, and spin-orbit coupling. Recent experiments in magic-angle twisted bilayer graphene (TBG) have uncovered a wealth of novel phases as a result of interaction-driven spin-valley flavour polarization. In this work, we explore correlated phases appearing due to the combined effect of spin-orbit coupling-enhanced valley polarization and large density of states below half filling ($ν\lesssim 2$) of the moiré band in a TBG coupled to tungsten diselenide. We observe anomalous Hall effect, accompanied by a series of Lifshitz transitions, that are highly tunable with carrier density and magnetic field. Strikingly, the magnetization shows an abrupt sign change in the vicinity of half-filling, confirming its orbital nature. The coercive fields reported are about an order of magnitude higher than previous studies in graphene-based moiré systems, presumably aided by a Stoner instability favoured by the van Hove singularities in the malleable bands. While the Hall resistance is not quantized at zero magnetic fields, indicative of a ground state with partial valley polarization, perfect quantization and complete valley polarization are observed at finite fields. Our findings illustrate that singularities in the flat bands in the presence of spin-orbit coupling can stabilize ordered phases even at non-integer moiré band fillings.
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Submitted 2 November, 2022;
originally announced November 2022.
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Majorana corner states on the dice lattice
Authors:
Narayan Mohanta,
Rahul Soni,
Satoshi Okamoto,
Elbio Dagotto
Abstract:
Lattice geometry continues providing exotic topological phases in condensed matter physics. Exciting recent examples are the higher-order topological phases, manifesting via localized lower-dimensional boundary states. Moreover, flat electronic bands with a non-trivial topology arise in various lattices and can hold a finite superfluid density, bounded by the Chern number $C$. Here we consider att…
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Lattice geometry continues providing exotic topological phases in condensed matter physics. Exciting recent examples are the higher-order topological phases, manifesting via localized lower-dimensional boundary states. Moreover, flat electronic bands with a non-trivial topology arise in various lattices and can hold a finite superfluid density, bounded by the Chern number $C$. Here we consider attractive interaction in the dice lattice that hosts flat bands with $C=\pm2$ and show that the induced superconducting state exhibits a second-order topological phase with mixed singlet-triplet pairing. The second-order nature of the topological superconducting phase is revealed by the zero-energy Majorana bound states at the lattice corners. Hence, the topology of the normal state dictates the nature of the Majorana localization. These findings suggest that flat bands with a higher Chern number provide feasible platforms for inducing higher-order topological superconductivity.
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Submitted 7 September, 2023; v1 submitted 18 October, 2022;
originally announced October 2022.
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Electronic structure, magnetic properties and pairing tendencies of the copper-based honeycomb lattice Na$_2$Cu$_2$TeO$_6$
Authors:
Ling-Fang Lin,
Rahul Soni,
Yang Zhang,
Shang Gao,
Adriana Moreo,
Gonzalo Alvarez,
Andrew D. Christianson,
Matthew B. Stone,
Elbio Dagotto
Abstract:
Spin-$1/2$ chains with alternating antiferromagnetic and ferromagnetic couplings have attracted considerable interest due to the topological character of their spin excitations. Here, using density functional theory and density matrix renormalization group methods, we have systematically studied the dimerized chain system Na$_2$Cu$_2$TeO$_6$. Near the Fermi level, the dominant states are mainly co…
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Spin-$1/2$ chains with alternating antiferromagnetic and ferromagnetic couplings have attracted considerable interest due to the topological character of their spin excitations. Here, using density functional theory and density matrix renormalization group methods, we have systematically studied the dimerized chain system Na$_2$Cu$_2$TeO$_6$. Near the Fermi level, the dominant states are mainly contributed by the Cu $3d_{x^2-y^2}$ orbitals highly hybridized with the O $2p$ orbitals in the nonmagnetic phase, leading to an "effective" single-orbital low-energy model. Furthermore, the bandwidth of the Cu $3d_{x^2-y^2}$ states is small ($\sim 0.8$ eV), suggesting that electronic correlations will strongly affect this system. By introducing such electronic correlations, we found this system is a Mott insulator. Moreover, by calculating the magnetic exchange interactions ($J_1$, $J_2$ and $J_3$), we explained the size and sign of the exchange interactions in Na$_2$Cu$_2$TeO$_6$, in agreement with neutron experiments. In addition, we constructed a single-orbital Hubbard model for this dimerized chain system, where the quantum fluctuations are taken into account. Both AFM and FM coupling ($\uparrow$-$\downarrow$-$\downarrow$-$\uparrow$) along the chain were found in our DMRG and Lanczos calculations, in agreement with DFT and neutron results. We also calculated the hole pairing binding energy $ΔE$ which becomes negative at Hubbard $U \sim 11$ eV, indicating incipient pairing tendencies. Finally, we also looked at various cases of hole doping that always exhibit tight pairs. Thus, we believe our results for Na$_2$Cu$_2$TeO$_6$ could provide guidance to experimentalists and theorists working on this dimerized chain system, such as short-range magnetic coupling, doping effects, and possible pairing tendencies.
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Submitted 4 April, 2022;
originally announced April 2022.
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Range of biquadratic and triquadratic Heisenberg effective couplings deduced from multiorbital Hubbard models
Authors:
Rahul Soni,
Nitin Kaushal,
Cengiz Şen,
Fernando A. Reboredo,
Adriana Moreo,
Elbio Dagotto
Abstract:
We studied a multi-orbital Hubbard model at half-filling for two and three orbitals per site on a two-site cluster via full exact diagonalization, in a wide range for the onsite repulsion $U$, from weak to strong coupling, and multiple ratios of the Hund coupling $J_H$ to $U$. The hopping matrix elements among the orbitals were also varied extensively. At intermediate and large $U$, we mapped the…
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We studied a multi-orbital Hubbard model at half-filling for two and three orbitals per site on a two-site cluster via full exact diagonalization, in a wide range for the onsite repulsion $U$, from weak to strong coupling, and multiple ratios of the Hund coupling $J_H$ to $U$. The hopping matrix elements among the orbitals were also varied extensively. At intermediate and large $U$, we mapped the results into a Heisenberg model. For two orbitals per site, the mapping is into a $S=1$ Heisenberg model where by symmetry both nearest-neighbor $(\mathbf{S}_{i}\cdot\mathbf{S}_{j})$ and $(\mathbf{S}_{i}\cdot\mathbf{S}_{j})^{2}$ are allowed, with respective couplings $J_1$ and $J_2$. For the case of three orbitals per site, the mappping is into a $S=3/2$ Heisenberg model with $(\mathbf{S}_{i}\cdot\mathbf{S}_{j})$, $(\mathbf{S}_{i}\cdot\mathbf{S}_{j})^{2}$, and $(\mathbf{S}_{i}\cdot\mathbf{S}_{j})^{3}$ terms, and respective couplings $J_1$, $J_2$, and $J_3$. The strength of these coupling constants in the Heisenberg models depend on the $U$, $J_H$, and hopping amplitudes of the underlying Hubbard model. Our study allows to establish bounds on how large the ratios $J_2/J_1$ and $J_3/J_1$ can be. We show that those ratios are severely limited and, as a crude guidance, we conclude that $J_2/J_1$ is less than 0.4 and $J_3/J_1$ is less than 0.2, establishing bounds on effective models for strongly correlated Hubbard systems.
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Submitted 21 December, 2021;
originally announced December 2021.
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Multitude of Topological Phase Transitions in Bipartite Dice and Lieb Lattices with Interacting Electrons and Rashba Coupling
Authors:
Rahul Soni,
Amit Bikram Sanyal,
Nitin Kaushal,
Satoshi Okamoto,
Adriana Moreo,
Elbio Dagotto
Abstract:
We report the results of a Hartree-Fock study applied to interacting electrons moving in two different bipartite lattices: the dice and the Lieb lattices, at half-filling. Both lattices develop ferrimagnetic order in the phase diagram $U$-$λ$, where $U$ is the Hubbard onsite repulsion and $λ$ the Rashba spin-orbit coupling strength. Our main result is the observation of an unexpected multitude of…
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We report the results of a Hartree-Fock study applied to interacting electrons moving in two different bipartite lattices: the dice and the Lieb lattices, at half-filling. Both lattices develop ferrimagnetic order in the phase diagram $U$-$λ$, where $U$ is the Hubbard onsite repulsion and $λ$ the Rashba spin-orbit coupling strength. Our main result is the observation of an unexpected multitude of topological phases for both lattices. All these phases are ferrimagnetic, but they differ among themselves in their set of six Chern numbers (six numbers because the unit cells have three atoms). The Chern numbers $|C|$ observed in our study range from 0 to 3, showing that large Chern numbers can be obtained by the effect of electronic correlations, adding to the recently discussed methodologies to increase $|C|$ based on extending the hopping range in tight-binding models, using sudden quenches, or photonic crystals, all without including electronic interactions.
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Submitted 21 December, 2021; v1 submitted 7 October, 2021;
originally announced October 2021.
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Intertwined charge, spin, and pairing orders in doped iron ladders
Authors:
Bradraj Pandey,
Rahul Soni,
Ling-Fang Lin,
Gonzalo Alvarez,
Elbio Dagotto
Abstract:
Motivated by recent experimental progress on iron-based ladder compounds, we study the doped two-orbital Hubbard model for the two-leg ladder BaFe$_2$S$_3$. The model is constructed by using {\it ab initio} hopping parameters and the ground state properties are investigated using the density matrix renormalization group method. We show that the $(π,0)$ magnetic ordering at half-filling, with ferro…
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Motivated by recent experimental progress on iron-based ladder compounds, we study the doped two-orbital Hubbard model for the two-leg ladder BaFe$_2$S$_3$. The model is constructed by using {\it ab initio} hopping parameters and the ground state properties are investigated using the density matrix renormalization group method. We show that the $(π,0)$ magnetic ordering at half-filling, with ferromagnetic rungs and antiferromagnetic legs, becomes incommensurate upon hole doping. Moreover, depending on the strength of the Hubbard $U$ coupling, other magnetic patterns, such as $(0,π)$, are also stabilized. We found that the binding energy for two holes becomes negative for intermediate Hubbard interaction strength, indicating hole pairing. Due to the crystal-field split among orbitals, the holes primarily reside in one orbital, with the other one remaining half-filled. This resembles orbital selective Mott states. The formation of tight hole pairs continues with increasing hole density, as long as the magnetic order remains antiferromagnetic in one direction. The study of pair-pair correlations indicates the dominance of the intra-orbital spin-singlet channel, as opposed to other pairing channels. Although in a range of hole doping pairing correlations decay slowly, our results can also be interpreted as corresponding to a charge-density-wave made of pairs, a precursor of eventual superconductivity after interladder couplings are included. Such scenario of intertwined orders has been extensively discussed before in the cuprates, and our results suggest a similar physics could exist in ladder iron-based superconductors. Finally, we also show that a robust Hund's coupling is needed for pairing to occur.
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Submitted 23 March, 2021; v1 submitted 10 March, 2021;
originally announced March 2021.
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Origin of the Magnetic and Orbital ordering in $α$-Sr$_2$CrO$_4$
Authors:
Bradraj Pandey,
Yang Zhang,
Nitin Kaushal,
Rahul Soni,
Ling-Fang Lin,
Wen-Jun Hu,
Gonzalo Alvarez,
Elbio Dagotto
Abstract:
Motivated by recent experimental progress in transition metal oxides with the K$_2$NiF$_4$ structure, we investigate the magnetic and orbital ordering in $α$-Sr$_2$CrO$_4$. Using first principles calculations, first we derive a three-orbital Hubbard model, which reproduces the {\it ab initio} band structure near the Fermi level. The unique reverse splitting of $t_{2g}$ orbitals in $α$-Sr$_2$CrO…
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Motivated by recent experimental progress in transition metal oxides with the K$_2$NiF$_4$ structure, we investigate the magnetic and orbital ordering in $α$-Sr$_2$CrO$_4$. Using first principles calculations, first we derive a three-orbital Hubbard model, which reproduces the {\it ab initio} band structure near the Fermi level. The unique reverse splitting of $t_{2g}$ orbitals in $α$-Sr$_2$CrO$_4$, with the $3d^2$ electronic configuration for the Cr$^{4+}$ oxidation state, opens up the possibility of orbital ordering in this material. Using real-space Hartree-Fock for multi-orbital systems, we constructed the ground-state phase diagram for the two-dimensional compound $α$-Sr$_2$CrO$_4$. We found stable ferromagnetic, antiferromagnetic, antiferro-orbital, and staggered orbital stripe ordering in robust regions of the phase diagram. Furthermore, using the density matrix renormalization group method for two-leg ladders with the realistic hopping parameters of $α$-Sr$_2$CrO$_4$, we explore magnetic and orbital ordering for experimentally relevant interaction parameters. Again, we find a clear signature of antiferromagnetic spin ordering along with antiferro-orbital ordering at moderate to large Hubbard interaction strength. We also explore the orbital-resolved density of states with Lanczos, predicting insulating behavior for the compound $α$-Sr$_2$CrO$_4$, in agreement with experiments. Finally, an intuitive understanding of the results is provided based on a hierarchy between orbitals, with $d_{xy}$ driving the spin order, while electronic repulsion and the effective one dimensionality of the movement within the $d_{xz}$ and $d_{yz}$ orbitals driving the orbital order.
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Submitted 20 October, 2020;
originally announced October 2020.
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Prediction of exotic magnetic states in the alkali metal quasi-one-dimensional iron selenide compound Na$_2$FeSe$_2$
Authors:
Bradraj Pandey,
Ling-Fang Lin,
Rahul Soni,
Nitin Kaushal,
Jacek Herbrych,
Gonzalo Alvarez,
Elbio Dagotto
Abstract:
The magnetic and electronic phase diagram of a model for the quasi-one-dimensional alkali metal iron selenide compound Na$_2$FeSe$_2$ is presented. The novelty of this material is that the valence of iron is Fe$^{2+}$ contrary to most other iron-chain compounds with valence Fe$^{3+}$. Using first-principles techniques, we developed a three-orbital tight-binding model that reproduces the {\it ab in…
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The magnetic and electronic phase diagram of a model for the quasi-one-dimensional alkali metal iron selenide compound Na$_2$FeSe$_2$ is presented. The novelty of this material is that the valence of iron is Fe$^{2+}$ contrary to most other iron-chain compounds with valence Fe$^{3+}$. Using first-principles techniques, we developed a three-orbital tight-binding model that reproduces the {\it ab initio} band structure near the Fermi level. Including Hubbard and Hund couplings and studying the model via the density matrix renormalization group and Lanczos methods, we constructed the ground state phase diagram. A robust region where the block state $\uparrow \uparrow \downarrow \downarrow \uparrow \uparrow \downarrow \downarrow$ is stabilized was unveiled. The analog state in iron ladders, employing 2$\times$2 ferromagnetic blocks, is by now well-established, but in chains a block magnetic order has not been observed yet in real materials. The phase diagram also contains a large region of canonical staggered spin order $\uparrow \downarrow \uparrow \downarrow \uparrow \downarrow \uparrow$ at very large Hubbard repulsion. At the block to staggered transition region, a novel phase is stabilized with a mixture of both states: an inhomogeneous orbital-selective charge density wave with the exotic spin configuration $\uparrow \uparrow \downarrow \uparrow \downarrow \downarrow \uparrow \downarrow$. Our predictions for Na$_2$FeSe$_2$ may guide crystal growers and neutron scattering experimentalists towards the realization of block states in one-dimensional iron-selenide chain materials.
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Submitted 26 May, 2020;
originally announced May 2020.
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BCS-BEC crossover in a $(t_{2g})^4$ Excitonic Magnet
Authors:
Nitin Kaushal,
Rahul Soni,
Alberto Nocera,
Gonzalo Alvarez,
Elbio Dagotto
Abstract:
The condensation of spin-orbit-induced excitons in $(t_{2g})^4$ electronic systems is attracting considerable attention. In the large Hubbard U limit, antiferromagnetism was proposed to emerge from the Bose-Einstein Condensation (BEC) of triplons ($J_{\textrm{eff}} = 1$). In this publication, we show that even for the weak and intermediate U regimes, the spin-orbit exciton condensation is possible…
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The condensation of spin-orbit-induced excitons in $(t_{2g})^4$ electronic systems is attracting considerable attention. In the large Hubbard U limit, antiferromagnetism was proposed to emerge from the Bose-Einstein Condensation (BEC) of triplons ($J_{\textrm{eff}} = 1$). In this publication, we show that even for the weak and intermediate U regimes, the spin-orbit exciton condensation is possible leading also to staggered magnetic order. The canonical electron-hole excitations (excitons) transform into local triplon excitations at large U , and this BEC strong coupling regime is smoothly connected to the intermediate U excitonic insulator region. We solved the degenerate three-orbital Hubbard model with spin-orbit coupling ($λ$) in one-dimensional geometry using the Density Matrix Renormalization Group, while in two-dimensional square clusters we use the Hartree-Fock approximation (HFA). Employing these techniques, we provide the full $λ$ vs U phase diagrams for both one- and two- dimensional lattices. Our main result is that at the intermediate Hubbard U region of our focus, increasing $λ$ at fixed U the system transitions from an incommensurate spin-density-wave metal to a Bardeen-Cooper-Schrieffer (BCS) excitonic insulator, with coherence length r coh of O(a) and O(10a) in 1d and 2d, respectively, with a the lattice spacing. Further increasing $λ$, the system eventually crosses over to the BEC limit (with r coh << a).
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Submitted 17 February, 2020;
originally announced February 2020.
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Flat Bands and Ferrimagnetic Order in Electronically Correlated Dice-Lattice Ribbons
Authors:
Rahul Soni,
Nitin Kaushal,
Satoshi Okamoto,
Elbio Dagotto
Abstract:
We study ribbons of the dice two-dimensional lattice (that we call ``dice ladders'') known to have nontrivial topological properties, such as Chern numbers 2 [Wang and Y. Ran, Phys. Rev. B {\bf 84}, 241103 (2011)]. Our main results are two folded: (1) Analyzing the tight-binding model in the presence of Rashba spin-orbit coupling and an external magnetic field, we observed that dice ladders qualit…
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We study ribbons of the dice two-dimensional lattice (that we call ``dice ladders'') known to have nontrivial topological properties, such as Chern numbers 2 [Wang and Y. Ran, Phys. Rev. B {\bf 84}, 241103 (2011)]. Our main results are two folded: (1) Analyzing the tight-binding model in the presence of Rashba spin-orbit coupling and an external magnetic field, we observed that dice ladders qualitatively display properties similar to their two-dimensional counterpart all the way to the limit of only two legs in the short direction. This includes flat bands near the Fermi level, edge currents and edge charge localization near zero energy when open boundary conditions are used, two chiral edge modes, and a nonzero Hall conductance. (2) We studied the effect of Hubbard correlation $U$ in the two-leg dice ladder using Lanczos and density matrix renormalization group techniques. We show that increasing $U$ the flat bands split without the need of introducing external fields. Moreover, robust ferrimagnetic order develops. Overall, our work establishes dice ladders as a promising playground to study the combined effect of topology and correlation effects, one of the frontiers in Quantum Materials.
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Submitted 7 June, 2020; v1 submitted 25 November, 2019;
originally announced November 2019.
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A non-invasive sub-surface electrical probe to encapsulated layers in van der Waals heterostructures
Authors:
Mrityunjay Pandey,
Radhika Soni,
Avi Mathur,
Srinivasan Raghavan,
U. Chandni
Abstract:
Van der Waals heterostructures formed by stacking different atomically thin layered materials have emerged as the sought-after device platform for electronic and optoelectronic applications. Determining the spatial extent of all the encapsulated components in such vertical stacks is key to optimal fabrication methods and improved device performance. Here we employ electrostatic force microscopy as…
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Van der Waals heterostructures formed by stacking different atomically thin layered materials have emerged as the sought-after device platform for electronic and optoelectronic applications. Determining the spatial extent of all the encapsulated components in such vertical stacks is key to optimal fabrication methods and improved device performance. Here we employ electrostatic force microscopy as a fast and non-invasive microscopic probe that provides compelling images of two dimensional layers buried over 30 nm below the sample surface. We demonstrate the versatility of the technique by studying heterojunctions comprising graphene, hexagonal boron nitride and transition metal dichalcogenides. Work function of each constituent layer acts as a unique fingerprint during imaging, thereby providing important insights into the charge environment, disorder, structural imperfections and doping profile. The technique holds great potential for gaining a comprehensive understanding of the quality, flatness as well as local electrical properties of buried layers in a large class of nanoscale materials and vertical heterostructures.
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Submitted 6 June, 2019;
originally announced June 2019.
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First-principles high pressure studies on group-14 element pernitrides
Authors:
Sharad Babu Pillai,
Himadri R. Soni,
Prafulla K. Jha
Abstract:
The search for ultra hard materials is inevitable in high pressure device applications. Nitrides of group-14 elements have been foreseen as potential candidates in replacing existing hard materials. In a recent experiment, pyrite structures of SiN2, GeN2 and SnN2 have been synthesized at high pressure and were shown to have high bulk modulus. Though their existence and bulk modulus are known, limi…
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The search for ultra hard materials is inevitable in high pressure device applications. Nitrides of group-14 elements have been foreseen as potential candidates in replacing existing hard materials. In a recent experiment, pyrite structures of SiN2, GeN2 and SnN2 have been synthesized at high pressure and were shown to have high bulk modulus. Though their existence and bulk modulus are known, limited studies have been devoted to SiN2, GeN2 and SnN2. In the present work, we perform first-principles calculations to investigate structural, electronic, mechanical and vibrational properties at ambient as well as high pressure condition. The physical properties of SnN2 and high pressure lattice dynamical properties of SiN2 and GeN2 are explored for the first time. SiN2 has higher bulk modulus among all MN2 (where M = Si, Sn and Ge), and increases further with increase in pressure. The increase in elastic moduli of MN2 have been related to the shortening of M-N bond length at high pressures. Electronic properties of these pyrite structures suggest that the bandgap increases with applying pressure. We further characterize these MN2 compounds at high pressures using theoretically calculated Raman spectroscopy. Frequency of N-N stretching Ag and Tg modes confirms the single bond character of nitrogen dimer in all MN2.
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Submitted 23 August, 2019; v1 submitted 13 May, 2019;
originally announced May 2019.
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Entanglement Entropy, Relative Entropy and Duality
Authors:
Upamanyu Moitra,
Ronak M Soni,
Sandip P. Trivedi
Abstract:
A definition for the entanglement entropy in both Abelian and non-Abelian gauge theories has been given in the literature, based on an extended Hilbert space construction. The result can be expressed as a sum of two terms, a classical term and a quantum term. It has been argued that only the quantum term is extractable through the processes of quantum distillation and dilution. Here we consider ga…
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A definition for the entanglement entropy in both Abelian and non-Abelian gauge theories has been given in the literature, based on an extended Hilbert space construction. The result can be expressed as a sum of two terms, a classical term and a quantum term. It has been argued that only the quantum term is extractable through the processes of quantum distillation and dilution. Here we consider gauge theories in the continuum limit and argue that quite generically, the classical piece is dominated by modes with very high momentum, of order the cut-off, in the direction normal to the entangling surface. As a result, we find that the classical term does not contribute to the relative entropy or the mutual information, in the continuum limit, for states which only carry a finite amount of energy above the ground state. We extend these considerations for $p$-form theories, and also discuss some aspects pertaining to electric-magnetic duality.
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Submitted 14 August, 2019; v1 submitted 16 November, 2018;
originally announced November 2018.
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The First Precise Determination of Graphene Functionalisation by in situ Raman Spectroscopy
Authors:
Philipp Vecera,
Julio C. Chacón-Torres,
Thomas Pichler,
Stephanie Reich,
Himadri R. Soni,
Andreas Görling,
Konstantin Edelthalhammer,
Herwig Peterlik,
Frank Hauke,
Andreas Hirsch
Abstract:
We report, for the first time, a comprehensive study involving in situ Raman spectroscopy supported by quantum mechanical calculations to exactly monitor the covalent binding to graphene with unprecedented precision. As a model reaction we have chosen the hydrogenation of reduced graphite ($KC_8$) with $H_2O$ and compared it with the corresponding exposure to $H_2$ and $O_2$. The early stages of g…
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We report, for the first time, a comprehensive study involving in situ Raman spectroscopy supported by quantum mechanical calculations to exactly monitor the covalent binding to graphene with unprecedented precision. As a model reaction we have chosen the hydrogenation of reduced graphite ($KC_8$) with $H_2O$ and compared it with the corresponding exposure to $H_2$ and $O_2$. The early stages of graphene hydrogenation are accompanied by the evolution of a series of so far undiscovered D-bands ($D_1$-$D_5$). Using quantum mechanical calculations, we were able to unambiguously assign these bands to distinct lattice vibrations in the neighborhood of the covalently bound addend. Interestingly, the exposure of $KC_8$ to $H_2$ and $O_2$ didn't cause covalent binding, but intercalation of molecular $H_2$ or partial oxidation, respectively. A combination of $H_2O$ and $O_2$ treatment led to the formation of additional hydroxyl (-OH) functionalities. The latter reaction represents a very suitable model for the decomposition of graphenides under ambient conditions (hydrogenation and hydroxylation). We have applied this Raman analysis to simulate and satisfactorily characterize a series of additional covalently functionalised graphene derivatives prepared as bulk materials with different composition (e.g. degree of functionalisation and the nature of covalent addend) demonstrating the generality of the concept and the fundamental value for graphene chemistry.
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Submitted 7 March, 2017;
originally announced March 2017.
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Entanglement Entropy in (3+1)-d Free $U(1)$ Gauge Theory
Authors:
Ronak M Soni,
Sandip P. Trivedi
Abstract:
We consider the entanglement entropy for a free $U(1)$ theory in $3 + 1$ dimensions in the extended Hilbert space definition. By taking the continuum limit carefully we obtain a replica trick path integral which calculates this entanglement entropy. The path integral is gauge invariant, with a gauge fixing delta function accompanied by a Faddeev-Popov determinant. For a spherical region it follows…
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We consider the entanglement entropy for a free $U(1)$ theory in $3 + 1$ dimensions in the extended Hilbert space definition. By taking the continuum limit carefully we obtain a replica trick path integral which calculates this entanglement entropy. The path integral is gauge invariant, with a gauge fixing delta function accompanied by a Faddeev-Popov determinant. For a spherical region it follows that the result for the logarithmic term in the entanglement, which is universal, is given by the $a$ anomaly coefficient. We also consider the extractable part of the entanglement, which corresponds to the number of Bell pairs which can be obtained from entanglement distillation or dilution. For a spherical region we show that the coefficient of the logarithmic term for the extractable part is different from the extended Hilbert space result. We argue that the two results will differ in general, and this difference is accounted for by a massless scalar living on the boundary of the region of interest.
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Submitted 1 August, 2016;
originally announced August 2016.
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Aspects of Entanglement Entropy for Gauge Theories
Authors:
Ronak M Soni,
Sandip P. Trivedi
Abstract:
A definition for the entanglement entropy in a gauge theory was given recently in arXiv:1501.02593. Working on a spatial lattice, it involves embedding the physical state in an extended Hilbert space obtained by taking the tensor product of the Hilbert space of states on each link of the lattice. This extended Hilbert space admits a tensor product decomposition by definition and allows a density m…
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A definition for the entanglement entropy in a gauge theory was given recently in arXiv:1501.02593. Working on a spatial lattice, it involves embedding the physical state in an extended Hilbert space obtained by taking the tensor product of the Hilbert space of states on each link of the lattice. This extended Hilbert space admits a tensor product decomposition by definition and allows a density matrix and entanglement entropy for the set of links of interest to be defined. Here, we continue the study of this extended Hilbert space definition with particular emphasis on the case of Non-Abelian gauge theories.
We extend the electric centre definition of Casini, Huerta and Rosabal to the Non-Abelian case and find that it differs in an important term. We also find that the entanglement entropy does not agree with the maximum number of Bell pairs that can be extracted by the processes of entanglement distillation or dilution, and give protocols which achieve the maximum bound. Finally, we compute the topological entanglement entropy which follows from the extended Hilbert space definition and show that it correctly reproduces the total quantum dimension in a class of Toric code models based on Non-Abelian discrete groups.
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Submitted 29 November, 2015; v1 submitted 26 October, 2015;
originally announced October 2015.
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On The Entanglement Entropy For Gauge Theories
Authors:
Sudip Ghosh,
Ronak M. Soni,
Sandip P. Trivedi
Abstract:
We propose a definition for the entanglement entropy of a gauge theory on a spatial lattice. Our definition applies to any subset of links in the lattice, and is valid for both Abelian and Non-Abelian gauge theories. For $\mathbb{Z}_N$ and $U(1)$ theories, without matter, our definition agrees with a particular case of the definition given by Casini, Huerta and Rosabal. We also argue that in gener…
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We propose a definition for the entanglement entropy of a gauge theory on a spatial lattice. Our definition applies to any subset of links in the lattice, and is valid for both Abelian and Non-Abelian gauge theories. For $\mathbb{Z}_N$ and $U(1)$ theories, without matter, our definition agrees with a particular case of the definition given by Casini, Huerta and Rosabal. We also argue that in general, both for Abelian and Non-Abelian theories, our definition agrees with the entanglement entropy calculated using a definition of the replica trick. Our definition, however, does not agree with some standard ways to measure entanglement, like the number of Bell pairs which can be produced by entanglement distillation.
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Submitted 21 July, 2015; v1 submitted 12 January, 2015;
originally announced January 2015.
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Vortex Dynamics of Rotating Bose-Einstein Condensate of Microcavity Polaritons
Authors:
Bikash Padhi,
Romain Duboscq,
Ankita Niranjan,
Ravi K. Soni
Abstract:
In this work we perform a numerical study of a rotating, harmonically trapped, Bose-Einstein condensate of microcavity polaritons. An efficient numerical method (toolbox) to solve the complex Gross-Pitaevskii equation is developed. Using this method, we investigate how the behavior of the number of vortices formed inside the condensate changes as the various system parameters are varied. In contra…
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In this work we perform a numerical study of a rotating, harmonically trapped, Bose-Einstein condensate of microcavity polaritons. An efficient numerical method (toolbox) to solve the complex Gross-Pitaevskii equation is developed. Using this method, we investigate how the behavior of the number of vortices formed inside the condensate changes as the various system parameters are varied. In contrast to the atomic condensates, we show, there exists an (experimentally realizable) range of parameter values in which all the vortices can be made to vanish even when there is a high rotation. We further explore how this region can be tuned through other free parameters and also discuss how this study can help to realize the synthetic magnetic field for polaritons and hence paving the way for the realization of the quantum Hall physics and many other exotic phenomena.
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Submitted 4 April, 2015; v1 submitted 20 October, 2014;
originally announced October 2014.
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Fundamental Issues and Problems in the Realization of Memristors
Authors:
Paul Meuffels,
Rohit Soni
Abstract:
In 2008, researchers at the Hewlett-Packard (HP) laboratories claimed to have found an analytical physical model for a genuine memristor device [1]. The model is considered for a thin TiO_2 film containing a region which is highly self-doped with oxygen vacancies and a region which is less doped, i.e., a single-phase material with a built-in chemical inhomogeneity sandwiched between two platinum e…
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In 2008, researchers at the Hewlett-Packard (HP) laboratories claimed to have found an analytical physical model for a genuine memristor device [1]. The model is considered for a thin TiO_2 film containing a region which is highly self-doped with oxygen vacancies and a region which is less doped, i.e., a single-phase material with a built-in chemical inhomogeneity sandwiched between two platinum electrodes. On base of the proposed model, Strukov et al. [1] were able to obtain the characteristic dynamical state equation and current-voltage relation for a genuine memristor. However, some fundamental facts of electrochemistry have been overlooked by the authors while putting forward their model, namely the coupling of diffusion currents at the boundary between both regions. The device will operate for a certain time like a "chemical capacitor" until the chemical inhomogeneity is balanced out, thus violating the essential requirement on a genuine memristor, the so-called "no energy discharge property". Moreover, the dynamical state equation for the HP-memristor device must fail as this relation violates by itself Landauer's principle of the minimum energy costs for information processing. Maybe, such an approach might be upheld if one introduces an additional prerequisite by specifying the minimum amount of electric power input to the device which is required to continuously change internal, physical states of the considered system. However, we have reasonable doubts with regard to this.
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Submitted 31 July, 2012;
originally announced July 2012.