Physics Department Faculty Publication SeriesCopyright (c) 2018 University of Massachusetts Amherst All rights reserved.
https://scholarworks.umass.edu/physics_faculty_pubs
Recent documents in Physics Department Faculty Publication Seriesen-usSun, 14 Jan 2018 15:09:05 PST3600Gapless Symmetry-Protected Topological Order
https://scholarworks.umass.edu/physics_faculty_pubs/1246
https://scholarworks.umass.edu/physics_faculty_pubs/1246Wed, 20 Dec 2017 07:37:57 PST
We introduce exactly solvable gapless quantum systems in d dimensions that support symmetry-protected topological (SPT) edge modes. Our construction leads to long-range entangled, critical points or phases that can be interpreted as critical condensates of domain walls “decorated” with dimension (d−1) SPT systems. Using a combination of field theory and exact lattice results, we argue that such gapless SPT systems have symmetry-protected topological edge modes that can be either gapless or symmetry broken, leading to unusual surface critical properties. Despite the absence of a bulk gap, these edge modes are robust against arbitrary symmetry-preserving local perturbations near the edges. In two dimensions, we construct wave functions that can also be interpreted as unusual quantum critical points with diffusive scaling in the bulk but ballistic edge dynamics.
]]>
Scaffidi, Thomas et al.Raman Scattering and Anomalous Stokes-anti-Stokes Ratio in MoTe2 Atomic Layers
https://scholarworks.umass.edu/physics_faculty_pubs/1245
https://scholarworks.umass.edu/physics_faculty_pubs/1245Tue, 14 Feb 2017 12:27:16 PST
Stokes and anti-Stokes Raman scattering are performed on atomic layers of hexagonal molybdenum ditelluride (MoTe2), a prototypical transition metal dichalcogenide (TMDC) semiconductor. The data reveal all six types of zone center optical phonons, along with their corresponding Davydov splittings, which have been challenging to see in other TMDCs. We discover that the anti-Stokes Raman intensity of the low energy layer-breathing mode becomes more intense than the Stokes peak under certain experimental conditions, and find the effect to be tunable by excitation frequency and number of atomic layers. These observations are interpreted as a result of resonance effects arising from the C excitons in the vicinity of the Brillouin zone center in the photon-electron-phonon interaction process.
]]>
Goldstein, Thomas et al.Polarization Dependent Switching of Asymmetric Nanorings with a Circular Field
https://scholarworks.umass.edu/physics_faculty_pubs/1244
https://scholarworks.umass.edu/physics_faculty_pubs/1244Thu, 19 Jan 2017 13:58:53 PST
We experimentally investigated the switching from onion to vortex states in asymmetric cobalt nanorings by an applied circular field. An in-plane field is applied along the symmetric or asymmetric axis of the ring to establish domain walls (DWs) with symmetric or asymmetric polarization. A circular field is then applied to switch from the onion state to the vortex state, moving the DWs in the process. The asymmetry of the ring leads to different switching fields depending on the location of the DWs and direction of applied field. For polarization along the asymmetric axis, the field required to move the DWs to the narrow side of the ring is smaller than the field required to move the DWs to the larger side of the ring. For polarization along the symmetric axis, establishing one DW in the narrow side and one on the wide side, the field required to switch to the vortex state is an intermediate value.
]]>
Pradhan, Nihar R. et al.Very effective string model?
https://scholarworks.umass.edu/physics_faculty_pubs/1243
https://scholarworks.umass.edu/physics_faculty_pubs/1243Tue, 22 Feb 2011 12:17:57 PST
Additional evidence is presented for a recently proposed effective string model, conjectured to hold throughout the parameter space of the basic 5 dimensional, triply charged black holes, which includes the effects of brane excitations, as well as momentum modes. We compute the low energy spacetime absorption coefficient σ for the scattering of a triply charged scalar field in the near extremal case, and conjecture an exact form for σ. It is shown that this form of σ arises simply from the effective string model. This agreement encompasses both statistical factors coming from the Bose distributions of string excitations and a prefactor which depends on the effective string radius. An interesting feature of the effective string model is that the change in mass of the effective string system in an emission process is not equal to the change in the energies of the effective string excitations. If the model is valid, this may hold clues towards understanding back reaction due to Hawking radiation. A number of weak spots and open questions regarding the model are also noted.
]]>
Kastor, David et al.An extended Kerr–Schild ansatz
https://scholarworks.umass.edu/physics_faculty_pubs/1242
https://scholarworks.umass.edu/physics_faculty_pubs/1242Tue, 22 Feb 2011 12:17:54 PST
We present an analysis of the vacuum Einstein equations for a recently proposed extension of the Kerr–Schild ansatz that includes a spacelike vector field as well as the usual Kerr–Schild null vector. We show that many, although not all, of the simplifications that occur in the Kerr–Schild case continue to hold for the extended ansatz. In particular, we find a simple set of sufficient conditions on the vectors such that the vacuum field equations truncate beyond quadratic order in an expansion around a general vacuum background solution. We extend our analysis to the electrovac case with a related ansatz for the gauge field.
]]>
Ett, Benjamin et al.BPS force balances via spin-spin interactions
https://scholarworks.umass.edu/physics_faculty_pubs/1241
https://scholarworks.umass.edu/physics_faculty_pubs/1241Tue, 22 Feb 2011 12:17:50 PST
We study two systems of BPS solitons in which spin-spin interactions are important in establishing the force balances which allow static, multi-soliton solutions to exist. Solitons in the Israel-Wilson-Perjes (IWP) spacetimes each carry arbitrary, classical angular momenta. Solitons in the Aichelburg-Embacher `superpartner' spacetimes carry quantum mechanical spin, which originates in the zero modes of the gravitino field of N = 2 supergravity in an extreme Reissner-Nordström background. In each case we find a cancellation between gravitational spin-spin and magnetic dipole-dipole forces, in addition to the usual one between Newtonian gravitational attraction and Coulombic electrostatic repulsion. In both cases, we analyse the forces between two solitons by treating one of the solitons as a probe or test particle, with the appropriate properties, moving in the background of the other. In the IWP case, the equation of motion for a spinning test particle, originally due to Papapetrou, includes a coupling between the background curvature and the spin of the test particle. In the superpartner case, the relevant equation of motion follows from a -symmetric superparticle action.
]]>
Kastor, David et al.Breaking Cosmic Strings without Monopoles
https://scholarworks.umass.edu/physics_faculty_pubs/1240
https://scholarworks.umass.edu/physics_faculty_pubs/1240Tue, 22 Feb 2011 12:17:47 PST
It is shown that topologically stable cosmic strings can, in fact, appear to end or to break, even in theories without monopoles. This can occur whenever the spatial topology of the universe is nontrivial. For the case of Abelian-Higgs strings, we describe the gauge and scalar field configurations necessary for a string to end on a black hole. We give a lower bound for the rate at which a cosmic string will break via black hole pair production, using an instanton calculation based on the Euclidean C-metric.
]]>
Eardley, D et al.Calibrations and Fayyazuddin-Smith spacetimes
https://scholarworks.umass.edu/physics_faculty_pubs/1239
https://scholarworks.umass.edu/physics_faculty_pubs/1239Tue, 22 Feb 2011 12:17:44 PST
We show that a class of spacetimes introduced by Fayyazuddin and Smith to describe intersecting M5-branes admits a generalized Kähler calibration. Equipped with this understanding, we are able to construct spacetimes corresponding to further classes of calibrated p-brane world-volume solitons. We note that these classes of spacetimes also describe the fields of p-branes wrapping certain supersymmetric cycles of Calabi-Yau manifolds.
]]>
Cho, Hyunji et al.C-functions in Lovelock gravity
https://scholarworks.umass.edu/physics_faculty_pubs/1238
https://scholarworks.umass.edu/physics_faculty_pubs/1238Tue, 22 Feb 2011 12:17:41 PST
We present C-functions for static and spherically symmetric spacetimes in Lovelock gravity theories. These functions are monotonically increasing functions of the outward radial coordinate and acquire their minima when evaluated on the horizon. Unlike the case of Einstein gravity, where there is a single C-function, we find that this function is non-unique in the case of Lovelock gravity. We define two C-functions, which agree at the horizon giving the black hole entropy, and state the different energy conditions that must hold in order for these functions to satisfy the monotonicity condition.
]]>
Anber, Mohamed et al.Conserved gravitational charges from Yano tensors
https://scholarworks.umass.edu/physics_faculty_pubs/1237
https://scholarworks.umass.edu/physics_faculty_pubs/1237Tue, 22 Feb 2011 12:17:39 PST
The defining properties of Yano tensors naturally generalize those of Killing vectors to anti-symmetric tensor fields of arbitrary rank. We show how the Yano tensors of flat spacetime can be used to construct new, conserved gravitational charges for transverse asymptotically flat spacetimes. The relationship of these new charges to Yano tensors parallels that of ordinary ADM conserved charges to Killing vectors. Hence, we call them Y-ADM charges. A rank n Y-ADM charge is given by an integral over a co-dimension n slice of spatial infinity. In particular, a rank (p+1) Y-ADM charge in a p-brane spacetime is given by an integral over only the sphere SD−(p+2) surrounding the brane and may be regarded as an intensive property of the brane.
]]>
Kastor, David et al.