Scaling dimensions of monopole operators in the CPNb−1\mathbb{CP}^{N_b - 1} theory in 2+12+1 dimensions

We study monopole operators at the conformal critical point of the $\mathbb{CP}^{N_b - 1}$ theory in $2+1$ spacetime dimensions. Using the state-operator correspondence and a saddle point approximation, we compute the scaling dimensions of these operators to next-to-leading order in $1/N_b$. We find remarkable agreement between our results and numerical studies of quantum antiferromagnets on two-dimensional lattices with SU($N_b$) global symmetry, using the mapping of the monopole operators to valence bond solid order parameters of the lattice antiferromagnet.Comment: 29 pages + Appendices, 3 figures; v2 corrected an important minus sign error and made various improvement

Monopole operators from the 4−ϵ4-\epsilon expansion

Three-dimensional quantum electrodynamics with $N$ charged fermions contains monopole operators that have been studied perturbatively at large $N$. Here, we initiate the study of these monopole operators in the $4-\epsilon$ expansion by generalizing them to codimension-3 defect operators in $d = 4-\epsilon$ spacetime dimensions. Assuming the infrared dynamics is described by an interacting CFT, we define the "conformal weight" of these operators in terms of the free energy density on $S^2 \times \mathbb{H}^{2-\epsilon}$ in the presence of magnetic flux through the $S^2$, and calculate this quantity to next-to-leading order in $\epsilon$. Extrapolating the conformal weight to $\epsilon = 1$ gives an estimate of the scaling dimension of the monopole operators in $d=3$ that does not rely on the $1/N$ expansion. We also perform the computation of the conformal weight in the large $N$ expansion for any $d$ and find agreement between the large $N$ and the small $\epsilon$ expansions in their overlapping regime of validity.Comment: 45 pages, 3 figures, version accepted by journa

Momentum-resolved electron-phonon interaction in lead determined by neutron resonance spin-echo spectroscopy

Neutron resonance spin-echo spectroscopy was used to monitor the temperature evolution of the linewidths of transverse acoustic phonons in lead across the superconducting transition temperature, $T_c$, over an extended range of the Brillouin zone. For phonons with energies below the superconducting energy gap, a linewidth reduction of maximum amplitude $\sim 6 \mu$eV was observed below $T_c$. The electron-phonon contribution to the phonon lifetime extracted from these data is in satisfactory overall agreement with {\it ab-initio} lattice-dynamical calculations, but significant deviations are found

Shape functions of dipolar ferromagnets at the Curie point

We present a complete mode coupling theory for the critical dynamics of ferromagnets above the Curie point with both short range exchange and long range dipolar interaction. This theory allows us to determine the full Kubo relaxation functions at the critical point. In particular, we are able to explain recent spin echo measurements

Monopole Operators in U(1)U(1) Chern-Simons-Matter Theories

We study monopole operators at the infrared fixed points of $U(1)$ Chern-Simons-matter theories (QED$_3$, scalar QED$_3$, ${\cal N} =1$ SQED$_3$, and ${\cal N} = 2$ SQED$_3$) with $N$ matter flavors and Chern-Simons level $k$. We work in the limit where both $N$ and $k$ are taken to be large with $\kappa = k/N$ fixed. In this limit, we extract information about the low-lying spectrum of monopole operators from evaluating the $S^2 \times S^1$ partition function in the sector where the $S^2$ is threaded by magnetic flux $4 \pi q$. At leading order in $N$, we find a large number of monopole operators with equal scaling dimensions and a wide range of spins and flavor symmetry irreducible representations. In two simple cases, we deduce how the degeneracy in the scaling dimensions is broken by the $1/N$ corrections. For QED$_3$ at $\kappa=0$, we provide conformal bootstrap evidence that this near-degeneracy is in fact maintained to small values of $N$. For ${\cal N} = 2$ SQED$_3$, we find that the lowest dimension monopole operator is generically non-BPS.Comment: 52 pages plus appendices, 9 figures, v2: minor correction

Large Scales - Long Times: Adding High Energy Resolution to SANS

The Neutron Spin Echo (NSE) variant MIEZE (Modulation of IntEnsity by Zero Effort), where all beam manipulations are performed before the sample position, offers the possibility to perform low background SANS measurements in strong magnetic fields and depolarising samples. However, MIEZE is sensitive to differences \DeltaL in the length of neutron flight paths through the instrument and the sample. In this article, we discuss the major influence of \DeltaL on contrast reduction of MIEZE measurements and its minimisation. Finally we present a design case for enhancing a small-angle neutron scattering (SANS) instrument at the planned European Spallation Source (ESS) in Lund, Sweden, using a combination of MIEZE and other TOF options, such as TISANE offering time windows from ns to minutes. The proposed instrument allows studying fluctuations in depolarizing samples, samples exposed to strong magnetic fields, and spin-incoherently scattering samples in a straightforward way up to time scales of \mus at momentum transfers up to 0.01 {\AA}-1, while keeping the instrumental effort and costs low.Comment: 5 pages, 8 figure

Quantum phase transitions in holographic models of magnetism and superconductors

We study a holographic model realizing an "antiferromagnetic" phase in which a global SU(2) symmetry representing spin is broken down to a U(1) by the presence of a finite electric charge density. This involves the condensation of a neutral scalar field in a charged AdS black hole. We observe that the phase transition for both neutral and charged (as in the standard holographic superconductor) order parameters can be driven to zero temperature by a tuning of the UV conformal dimension of the order parameter, resulting in a quantum phase transition of the Berezinskii-Kosterlitz-Thouless type. We also characterize the antiferromagnetic phase and an externally forced ferromagnetic phase by showing that they contain the expected spin waves with linear and quadratic dispersions respectively.Comment: 24 pages; v2: references added, typos correcte