1,129 research outputs found
Shear Viscosity in a Non-Fermi Liquid Phase of a Quadratic Semimetal
We study finite temperature transport in the Luttinger-Abrikosov-Beneslavskii
phase -- an interacting, scale invariant, non-Fermi liquid phase found in
quadratic semimetals. We develop a kinetic equation formalism to describe the
d.c. transport properties, which are dominated by collisions, and compute the
shear viscosity . The ratio of shear viscosity to entropy density
is a measure of the strength of interaction between the excitations of
a quantum fluid. As a consequence of the quantum critical nature of the system,
is a universal number and we find it to be consistent with a bound
proposed from gauge-gravity duality.Comment: 5+5 pages, 2 figures; Published Versio
Exploring Curved Superspace (II)
We extend our previous analysis of Riemannian four-manifolds M admitting
rigid supersymmetry to N=1 theories that do not possess a U(1)_R symmetry. With
one exception, we find that M must be a Hermitian manifold. However, the
presence of supersymmetry imposes additional restrictions. For instance, a
supercharge that squares to zero exists, if the canonical bundle of the
Hermitian manifold M admits a nowhere vanishing, holomorphic section. This
requirement can be slightly relaxed if M is a torus bundle over a Riemann
surface, in which case we obtain a supercharge that squares to a complex
Killing vector. We also analyze the conditions for the presence of more than
one supercharge. The exceptional case occurs when M is a warped product S^3 x
R, where the radius of the round S^3 is allowed to vary along R. Such manifolds
admit two supercharges that generate the superalgebra OSp(1|2). If the S^3
smoothly shrinks to zero at two points, we obtain a squashed four-sphere, which
is not a Hermitian manifold.Comment: 34 pages; reference adde
Supercurrents and Brane Currents in Diverse Dimensions
We systematically analyze all possible supersymmetry multiplets that include
the supersymmetry current and the energy-momentum tensor in various dimensions,
focusing on N=1 in four dimensions. The most general such multiplet is the
S-multiplet, which includes 16 bosonic and 16 fermionic operators. In special
situations it can be decomposed, leading to smaller multiplets with 12+12 or
even 8+8 operators. Physically, these multiplets give rise to different brane
charges in the supersymmetry algebra. The S-multiplet is needed when the
algebra contains both string and domain wall charges. In lower dimensions (or
in four-dimensional N=2 theories) the algebra can include space-filling brane
charges, which are associated with partial supersymmetry breaking. This
phenomenon is physically distinct from ordinary spontaneous supersymmetry
breaking. Our analysis leads to new results about the dynamics of
supersymmetric field theories. These include constraints on the existence of
certain charged branes and the absence of magnetic charges in U(1) gauge
theories with a Fayet-Iliopoulos term.Comment: 47 pages; some minor typos corrected thanks to input from reader
Candidate Phases for SU(2) Adjoint QCD with Two Flavors from Supersymmetric Yang-Mills Theory
We study four-dimensional adjoint QCD with gauge group SU(2) and two Weyl
fermion flavors, which has an chiral symmetry. The infrared behavior
of this theory is not firmly established. We explore candidate infrared phases
by embedding adjoint QCD into supersymmetric Yang-Mills theory
deformed by a supersymmetry-breaking scalar mass M that preserves all global
symmetries and 't Hooft anomalies. This includes 't Hooft anomalies that are
only visible when the theory is placed on manifolds that do not admit a spin
structure. The consistency of this procedure is guaranteed by a nonabelian
spin-charge relation involving the symmetry that is familiar from
topologically twisted theories. Since every vacuum on the
Coulomb branch of the theory necessarily matches all 't Hooft
anomalies, we can generate candidate phases for adjoint QCD by deforming the
theories in these vacua while preserving all symmetries and 't Hooft anomalies.
One such deformation is the supersymmetry-breaking scalar mass M itself, which
can be reliably analyzed when M is small. In this regime it gives rise to an
exotic Coulomb phase without chiral symmetry breaking. By contrast, the theory
near the monopole and dyon points can be deformed to realize a candidate phase
with monopole-induced confinement and chiral symmetry breaking. The low-energy
theory consists of two copies of a sigma model, which we
analyze in detail. Certain topological couplings that are likely to be present
in this model turn the confining solitonic string of the model
into a topological insulator. We also examine the behavior of various candidate
phases under fermion mass deformations. We speculate on the possible large-M
behavior of the deformed theory and conjecture that the
phase eventually becomes dominant.Comment: 94 pages, 1 figur
Superconductivity and Nematic Fluctuations in a model of FeSe monolayers: A Determinant Quantum Monte Carlo Study
In contrast to bulk FeSe, which exhibits nematic order and low temperature
superconductivity, atomic layers of FeSe reverse the situation, having high
temperature superconductivity appearing alongside a suppression of nematic
order. To investigate this phenomenon, we study a minimal electronic model of
FeSe, with interactions that enhance nematic fluctuations. This model is sign
problem free, and is simulated using determinant quantum Monte Carlo (DQMC). We
developed a DQMC algorithm with parallel tempering, which proves to be an
efficient source of global updates and allows us to access the region of strong
interactions. Over a wide range of intermediate couplings, we observe
superconductivity with an extended s-wave order parameter, along with enhanced,
but short ranged, ferro-orbital (nematic) order. These results are
consistent with approximate weak coupling treatments that predict that nematic
fluctuations lead to superconducting pairing. Surprisingly, in the parameter
range under study, we do not observe nematic long range order. Instead, at
stronger coupling an unusual insulating phase with
antiferro-orbital order appears, which is missed by weak coupling
approximations.Comment: 9 pages, 9 figures; v3: adds two short appendices, fixes minor typos;
published versio
Hidden Symmetry Decoupling of Majorana Fermions
Multiple zero-energy Majorana fermions (MFs) with spatially overlapping wave
functions can survive only if their splitting is prevented by an underlying
symmetry. Here we show that, in quasi-one-dimensional (Q1D) time reversal
invariant topological superconductors (class DIII), a realistic model for
superconducting lithium molybdenum purple bronze and certain families of
organic superconductors, multiple Majorana-Kramers pairs with strongly
overlapping wave functions persist at zero energy even in the absence of an
easily identifiable symmetry. We find that similar results hold in the case of
Q1D semiconductor-superconductor heterostructures (class D) with transverse
hopping t_{perp} much smaller than longitudinal hopping t_x. Our results,
explained in terms of special properties of the Hamiltonian and wave functions,
underscore the importance of hidden accidental symmetries in topological
superconductors.Comment: 4+ pages, 3 figure
Deformations of Superconformal Theories
We classify possible supersymmetry-preserving relevant, marginal, and
irrelevant deformations of unitary superconformal theories in
dimensions. Our method only relies on symmetries and unitarity. Hence, the
results are model independent and do not require a Lagrangian description. Two
unifying themes emerge: first, many theories admit deformations that reside in
multiplets together with conserved currents. Such deformations can lead to
modifications of the supersymmetry algebra by central and non-central charges.
Second, many theories with a sufficient amount of supersymmetry do not admit
relevant or marginal deformations, and some admit neither. The classification
is complicated by the fact that short superconformal multiplets display a rich
variety of sporadic phenomena, including supersymmetric deformations that
reside in the middle of a multiplet. We illustrate our results with examples in
diverse dimensions. In particular, we explain how the classification of
irrelevant supersymmetric deformations can be used to derive known and new
constraints on moduli-space effective actions.Comment: 73 pages, 34 table
Anomalies, Renormalization Group Flows, and the a-Theorem in Six-Dimensional (1,0) Theories
We establish a linear relation between the -type Weyl anomaly and the 't
Hooft anomaly coefficients for the -symmetry and gravitational anomalies in
six-dimensional superconformal field theories. For RG flows onto the
tensor branch, where conformal symmetry is spontaneously broken, supersymmetry
relates the anomaly mismatch to the square of a four-derivative
interaction for the dilaton. This establishes the -theorem for all such
flows. The four-derivative dilaton interaction is in turn related to the
Green-Schwarz-like terms that are needed to match the 't Hooft anomalies on the
tensor branch, thus fixing their relation to . We use our formula to
obtain exact expressions for the -anomaly of small instantons, as
well as M5-branes probing an orbifold singularity, and verify the
-theorem for RG flows onto their Higgs branches. We also discuss aspects of
supersymmetric RG flows that terminate in scale but not conformally invariant
theories with massless gauge fields.Comment: 38 pages, 3 figures; added references and an appendi
Higher Derivative Terms, Toroidal Compactification, and Weyl Anomalies in Six-Dimensional (2,0) Theories
We systematically analyze the effective action on the moduli space of (2,0)
superconformal field theories in six dimensions, as well as their toroidal
compactification to maximally supersymmetric Yang-Mills theories in five and
four dimensions. We present a streamlined approach to non-renormalization
theorems that constrain this effective action. The first several orders in its
derivative expansion are determined by a one-loop calculation in
five-dimensional Yang-Mills theory. This fixes the leading higher-derivative
operators that describe the renormalization group flow into theories residing
at singular points on the moduli space of the compactified (2,0) theories. This
understanding allows us to compute the a-type Weyl anomaly for all (2,0)
superconformal theories. We show that it decreases along every renormalization
group flow that preserves (2,0) supersymmetry, thereby establishing the
a-theorem for this class of theories. Along the way, we encounter various
field-theoretic arguments for the ADE classification of (2,0) theories.Comment: 48 pages + appendix, 3 figure
String order parameters for 1d Floquet Symmetry Protected Topological Phases
Floquet symmetry protected topological (FSPT) phases are non-equilibrium
topological phases enabled by time-periodic driving. FSPT phases of 1d chains
of bosons, spins, or qubits host dynamically protected edge states that can
store quantum information without decoherence, making them promising for use as
quantum memories. While FSPT order cannot be detected by any local measurement,
here we construct non-local string order parameters that directly measure
general 1d FSPT order. We propose a superconducting-qubit array based
realization of the simplest Ising-FSPT, which can be implemented with existing
quantum computing hardware. We devise an interferometric scheme to directly
measure the non-local string order using only simple one- and two- qubit
operations and single-qubit measurements.Comment: 5+4 pages; 4+1 figures; v2. updates Fig. 3, adds additional
reference
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