arXiv subjects

R. Gupta

Publications and source records attributed to R. Gupta.

At least 73 records · Page 4Linked to original sources

arXiv2020

Spin texture in weak topological insulators: the role of bulk states and band bending

We derive the spin texture of a weak topological insulator via a supersymmetric approach that includes the roles of the bulk gap edge states and surface band bending. We find the spin texture can take one of four forms: (i) helical, (ii) hyperbolic, (iii) hedgehog, with spins normal to the Dirac-Weyl cone of the surface state, and (iv) hyperbolic hedgehog. Band bending determines the winding number in the case of a helical texture, and for all textures can be used to tune the spin texture polarization to zero. For the weak topological insulator SnTe, we show that inclusion of band bending is crucial to obtain the correct texture winding number for the (111) surface facet $Γ$-point Dirac-Weyl cone. We argue that hedgehogs will be found only in low symmetry situations.

cond-mat.mtrl-sci↗

arXiv2020

A general relation between stacking order and Chern index: a topological map of minimally twisted bilayer graphene

We derive a general relation between the stacking vector ${\bf u}$ describing the relative shift of two layers of bilayer graphene and the Chern index. We find $C = ν- \text{sign}\left(|V_{AB}|-|V_{BA}|\right)$, where $ν$ is a valley index and $|V_{αβ}|$ the absolute value of stacking potentials that depend on ${\bf u}$ and that uniquely determine the interlayer interaction; AA stacking plays no role in the topological character. With this expression we show that while ideal and relaxed minimally twisted bilayer graphene appear so distinct as to be almost different materials, their Chern index maps are, remarkably, identical. The topological physics of this material is thus strongly robust to lattice relaxations.

cond-mat.mes-hall↗

arXiv2020

Higgs boson potential at colliders: status and perspectives

This document summarises the current theoretical and experimental status of the di-Higgs boson production searches, and of the direct and indirect constraints on the Higgs boson self-coupling, with the wish to serve as a useful guide for the next years. The document discusses the theoretical status, including state-of-the-art predictions for di-Higgs cross sections, developments on the effective field theory approach, and studies on specific new physics scenarios that can show up in the di-Higgs final state. The status of di-Higgs searches and the direct and indirect constraints on the Higgs self-coupling at the LHC are presented, with an overview of the relevant experimental techniques, and covering all the variety of relevant signatures. Finally, the capabilities of future colliders in determining the Higgs self-coupling are addressed, comparing the projected precision that can be obtained in such facilities. The work has started as the proceedings of the Di-Higgs workshop at Colliders, held at Fermilab from the 4th to the 9th of September 2018, but it went beyond the topics discussed at that workshop and included further developments.

hep-ph↗

arXiv2020

FLAG Review 2019

We review lattice results related to pion, kaon, $D$-meson, $B$-meson, and nucleon physics with the aim of making them easily accessible to the nuclear and particle physics communities. More specifically, we report on the determination of the light-quark masses, the form factor $f_+(0)$ arising in the semileptonic $K \to π$ transition at zero momentum transfer, as well as the decay constant ratio $f_K/f_π$ and its consequences for the CKM matrix elements $V_{us}$ and $V_{ud}$. Furthermore, we describe the results obtained on the lattice for some of the low-energy constants of $SU(2)_L\times SU(2)_R$ and $SU(3)_L\times SU(3)_R$ Chiral Perturbation Theory. We review the determination of the $B_K$ parameter of neutral kaon mixing as well as the additional four $B$ parameters that arise in theories of physics beyond the Standard Model. For the heavy-quark sector, we provide results for $m_c$ and $m_b$ as well as those for $D$- and $B$-meson decay constants, form factors, and mixing parameters. These are the heavy-quark quantities most relevant for the determination of CKM matrix elements and the global CKM unitarity-triangle fit. We review the status of lattice determinations of the strong coupling constant $α_s$. Finally, in this review we have added a new section reviewing results for nucleon matrix elements of the axial, scalar and tensor bilinears, both isovector and flavor diagonal.

hep-lat↗

arXiv2020

Ultrafast magnetization dynamics in half-metallic Co$_2$FeAl Heusler alloy

We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy $\mathrm{Co_{2}FeAl}$, probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to results from electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we find that the demagnetization time ($τ_{M}$) in films of $\mathrm{Co_{2}FeAl}$ is almost independent of varying structural order, and that it is similar to that in elemental 3d ferromagnets. In contrast, the slower process of magnetization recovery, specified by $τ_{R}$, is found to occur on picosecond time scales, and is demonstrated to correlate strongly with the Gilbert damping parameter ($α$). Our results show that $\mathrm{Co_{2}FeAl}$ is unique, in that it is the first material that clearly demonstrates the importance of the damping parameter in the remagnetization process. Based on these results we argue that for $\mathrm{Co_{2}FeAl}$ the remagnetization process is dominated by magnon dynamics, something which might have general applicability.

cond-mat.mtrl-sci↗

arXiv2019

Origin of Ferroelectricity in Orthorhombic LuFeO$_3$

We demonstrate that small but finite ferroelectric polarization ($\sim$0.01 $μ$C/cm$^2$) emerges in orthorhombic LuFeO$_3$ ($Pnma$) at $T_N$ ($\sim$600 K) because of commensurate (k = 0) and collinear magnetic structure. The synchrotron x-ray and neutron diffraction data suggest that the polarization could originate from enhanced bond covalency together with subtle contribution from lattice. The theoretical calculations indicate enhancement of bond covalency as well as the possibility of structural transition to the polar $Pna2_1$ phase below $T_N$. The $Pna2_1$ phase, in fact, is found to be energetically favorable below $T_N$ in orthorhombic LuFeO$_3$ ($albeit$ with very small energy difference) than in isostructural and nonferroelectric LaFeO$_3$ or NdFeO$_3$. Application of electric field induces finite piezostriction in LuFeO$_3$ via electrostriction resulting in clear domain contrast images in piezoresponse force microscopy.

cond-mat.str-el↗

arXiv2019

Deformation induced pseudo-magnetic fields in complex carbon architectures

We show that the physics of deformation in $α$-, $β$-, and $6,6,12$-graphyne is, despite their significantly more complex lattice structures, remarkably close to that of graphene, with inhomogeneously strained graphyne described at low energies by an emergent Dirac-Weyl equation augmented by strain induced electric and pseudo-magnetic fields. To show this we develop two continuum theories of deformation in these materials: one that describes the low energy degrees of freedom of the conical intersection, and is spinor valued as in graphene, and one describing the full sub-lattice space. The spinor valued continuum theory agrees very well with the full continuum theory at low energies, showing that the remarkable physics of deformation in graphene generalizes to these more complex carbon architectures. In particular, we find that deformation induced pseudospin polarization and valley current loops, key phenomena in the deformation physics of graphene, both have their counterpart in these more complex carbon materials.

cond-mat.mtrl-sci↗

arXiv2019

Moiré quantum well states in tiny angle two dimensional semi-conductors

The valence band edge in tiny angle twist bilayers of MoS$_2$ and phosphorene is shown to consist of highly localized energy levels created by a `moiré quantum well', i.e. trapped by the interlayer moiré potential. These approximately uniformly spaced energy levels exhibit a richly modulated charge density, becoming ultra-localized at the valence band maximum. The number and spacing of such levels is controllable by the twist angle and interlayer interaction strength, suggesting the possibility of `moiré engineering' ordered arrays of quantum dots in 2d twist semi-conductors.

cond-mat.mes-hall↗

arXiv2019

Moiré ordered current loops in the graphene twist bilayer

While a typical material exhibits field induced currents only at the boundary, a uniform out-of-plane magnetic field applied to two mutually rotated layers of graphene is shown to result in an ordered array of permanent current loops throughout the material. Each current loop consists of an interlayer current flowing through the open AA stacked regions of the moiré created by rotation, which then flows back through the neighboring AB regions to form a circuit, with significant current strength even at small fields. Similar moiré ordered arrays of current loops are also shown to exist in non-equilibrium transport states, where they manifest as current back flowing against the applied bias in the device. Such current loops thus represent an intrinsic feature of the twist bilayer in conditions of broken time reversal symmetry, and exist both as a low field imprint of the moiré lattice on Landau physics, and as measurable moiré scale current configurations in transport states.

cond-mat.mes-hall↗

arXiv2019

A non-perturbative theory of effective Hamiltonians: example of moiré materials

We demonstrate that there exists a continuum Hamiltonian $H(\bf{r},\bf{p})$ that is formally the operator equivalent of the general tight-binding method, inheriting the associativity and Hermiticity of the latter operator. This provides a powerful and controlled method of obtaining effective Hamiltonians via Taylor expansion with respect to momentum and, optionally, deformation fields. In particular, for fundamentally non-perturbative defects, such as twist faults and partial dislocations, the method allows the deformation field to be retained to all orders, providing an efficient scheme for the generation of transparent and compact Hamiltonians for such defects. We apply the method to a survey of incommensurate physics in twist bilayers of graphene, graphdiyne, MoS$_2$, and phosphorene. For graphene we are able to reproduce the `reflected Dirac cones' of the $30^\circ$ quasi-crystalline bilayer found in a recent ARPES experiment, and show it is an example of a more general phenomena of coupling by the moiré momentum. We show that incommensurate physics is governed by the decay of the interlayer interaction on the scale of the single layer reciprocal lattices, and demonstrate that if this is slow incommensurate scattering effects lead to very rapid broadening of band manifolds as the twist angle is tuned through commensurate values.

cond-mat.mtrl-sci↗

arXiv2018

Dislocation and node states in bilayer graphene systems

We investigate the electronic structure of realistic partial dislocation networks in bilayer graphene that feature annihilating, wandering, and intersecting partial lines. We find charge accumulation states at partials that are sensitive to Fermi energy and partial Burgers vector but not to the screw versus edge character of the partial. These states are shown to be current carrying, with the current density executing a spiral motion along the dislocation line with a strong interlayer component to the current. Close to the Dirac point localization on partials switches to localization on intersections of partials, with a corresponding complex current flow around the nodes.

cond-mat.mes-hall↗

arXiv2018

Straintronics beyond homogeneous deformation

We present a continuum theory of graphene treating on an equal footing both homogeneous Cauchy-Born (CB) deformation, as well as the microscopic degrees of freedom associated with the two sublattices. While our theory recovers all extant results from homogeneous continuum theory, the Dirac-Weyl equation is found to be augmented by new pseudo-gauge and chiral fields fundamentally different from those that result from homogeneous deformation. We elucidate three striking electronic consequences: (i) non-CB deformations allow for the transport of valley polarized charge over arbitrarily long distances e.g. along a designed ridge; (ii) the triaxial deformations required to generate an approximately uniform magnetic field are unnecessary with non-CB deformation; and finally (iii) the vanishing of the effects of a one dimensional corrugation seen in \emph{ab-initio} calculation upon lattice relaxation are explained as a compensation of CB and non-CB deformation.

cond-mat.mes-hall↗

arXiv2018

High Rate RPC detector for LHC

The High Luminosity LHC (HL-LHC) phase is designed to increase by an order of magnitude the amount of data to be collected by the LHC experiments. The foreseen gradual increase of the instantaneous luminosity of up to more than twice its nominal value of $10\times10^{34}\ {\rm cm}^{-1}{\rm s}^{-2}$ during Phase I and Phase II of the LHC running, presents special challenges for the experiments. The region with high pseudo rapidity ($η$) region of the forward muon spectrometer ($2.4 > |η| > 1.9$) is not equipped with RPC stations. The increase of the expected particles rate up to 2 kHz cm$^{-1}$ ( including a safety factor 3 ) motivates the installation of RPC chambers to guarantee redundancy with the CSC chambers already present. The current CMS RPC technology cannot sustain the expected background level. A new generation of Glass-RPC (GRPC) using low-resistivity glass was proposed to equip the two most far away of the four high $η$ muon stations of CMS. In their single-gap version they can stand rates of few kHz cm$^{-1}$. Their time precision of about 1 ns can allow to reduce the noise contribution leading to an improvement of the trigger rate. The proposed design for large size chambers is examined and some preliminary results obtained during beam tests at Gamma Irradiation Facility (GIF++) and Super Proton Synchrotron (SPS) at CERN are shown. They were performed to validate the capability of such detectors to support high irradiation environment with limited consequence on their efficiency.

physics.ins-det↗

arXiv2018

Moiré edge states in twisted graphene nanoribbons

The edge physics of graphene based systems is well known to be highly sensitive to the atomic structure at the boundary, with localized zero mode edge states found only on the zigzag type termination of the lattice. Here we demonstrate that the graphene twist bilayer supports an additional class of edge states, that (i) are found for all edge geometries and thus are robust against edge roughness, (ii) occur at energies coinciding with twist induced van Hove singularities in the bulk and (iii) possess an electron density strongly modulated by the moiré lattice. Interestingly, these "moiré edge states" exist only for certain lattice commensurations and thus the edge physics of the twist bilayer is, in dramatic contrast to that of the bulk, not uniquely determined by the twist angle.

cond-mat.mes-hall↗

arXiv2016

Dimension-5 CP-odd operators: QCD mixing and renormalization

We study the off-shell mixing and renormalization of flavor-diagonal dimension-5 T- and P-odd operators involving quarks, gluons, and photons, including quark electric dipole and chromo-electric dipole operators. We present the renormalization matrix to one-loop in the $\bar{\rm MS}$ scheme. We also provide a definition of the quark chromo-electric dipole operator in a regularization-independent momentum-subtraction scheme suitable for non-perturbative lattice calculations and present the matching coefficients with the $\bar{\rm MS}$ scheme to one-loop in perturbation theory, using both the naive dimensional regularization and 't Hooft-Veltman prescriptions for $γ_5$.

hep-ph↗

arXiv2016

Going beyond k.p theory: a general method for obtaining effective Hamiltonians in both high and low symmetry situations

We provide a method for the generation of effective continuum Hamiltonians that goes beyond the well known k.p method in being equally effective in both high, and low (or no) symmetry situations. Our approach is based on a surprising exact map of the two-centre tight-binding method onto a compact continuum Hamiltonian, with a precise condition given for the hermiticity of the latter object. We apply this method to a broad range of low dimensional systems of both high and low symmetry: graphene, graphdiyne, γ-graphyne, 6,6,12-graphyne, twist bilayer graphene, and partial dislocation networks in Bernal stacked bilayer graphene. For the single layer systems the method yields Hamiltonians for the ideal lattices, as well as a systematic theory for corrections due to deformation. In the case of bilayer graphene we provide a compact expression for an effective field capable of describing any stacking deformation of the bilayer; twist bilayer graphene, as well as the partial dislocation network in AB stacked graphene, emerge as special cases of this field. For the latter system we find (i) charge pooling on the mosaic of AB and AC segments near the Dirac point and (ii) localized current carrying states on the partials with the current density characterized by both intralayer and interlayer components.

cond-mat.mes-hall↗

arXiv2016

Lattice QCD calculations of nucleon transverse momentum-dependent parton distributions using clover and domain wall fermions

We present a lattice QCD calculation of transverse momentum dependent parton distribution functions (TMDs) of protons using staple-shaped Wilson lines. For time-reversal odd observables, we calculate the generalized Sivers and Boer-Mulders transverse momentum shifts in SIDIS and DY cases, and for T-even observables we calculate the transversity related to the tensor charge and the generalized worm-gear shift. The calculation is done on two different n_f=2+1 ensembles: domain-wall fermion (DWF) with lattice spacing 0.084 fm and pion mass of 297 MeV, and clover fermion with lattice spacing 0.114 fm and pion mass of 317 MeV. The results from those two different discretizations are consistent with each other.

hep-lat↗

arXiv2014

Imaging polarimetry of the rotating Bok globule CB67

Polarimetric observations of about 50 stars located in a close vicinity of the Bok globule CB67 having significantly nonspherical shape and rapid rotation are performed. The data obtained are compared with the available observations of this globule at radio and submillimeter wavelengths as well as some theoretical calculations. It is found that the elongation and the rotation moment of CB67 are oriented rather perpendicular to the magnetic fields, which is unusual for Bok globules and is difficult to be explained from the theoretical point of view.

astro-ph.GA↗