643 research outputs found
Spin-lattice instability to a fractional magnetization state in the spinel HgCr2O4
Magnetic systems are fertile ground for the emergence of exotic states when
the magnetic interactions cannot be satisfied simultaneously due to the
topology of the lattice - a situation known as geometrical frustration.
Spinels, AB2O4, can realize the most highly frustrated network of
corner-sharing tetrahedra. Several novel states have been discovered in
spinels, such as composite spin clusters and novel charge-ordered states. Here
we use neutron and synchrotron X-ray scattering to characterize the fractional
magnetization state of HgCr2O4 under an external magnetic field, H. When the
field is applied in its Neel ground state, a phase transition occurs at H ~ 10
Tesla at which each tetrahedron changes from a canted Neel state to a
fractional spin state with the total spin, Stet, of S/2 and the lattice
undergoes orthorhombic to cubic symmetry change. Our results provide the
microscopic one-to-one correspondence between the spin state and the lattice
distortion
Experimental modulation of capsule size in Cryptococcus neoformans
Experimental modulation of capsule size is an important technique for the study of the virulence of the encapsulated pathogen Cryptococcus neoformans. In this paper, we summarize the techniques available for experimental modulation of capsule size in this yeast and describe improved methods to induce capsule size changes. The response of the yeast to the various stimuli is highly dependent on the cryptococcal strain. A high CO(2) atmosphere and a low iron concentration have been used classically to increase capsule size. Unfortunately, these stimuli are not reliable for inducing capsular enlargement in all strains. Recently we have identified new and simpler conditions for inducing capsule enlargement that consistently elicited this effect. Specifically, we noted that mammalian serum or diluted Sabouraud broth in MOPS buffer pH 7.3 efficiently induced capsule growth. Media that slowed the growth rate of the yeast correlated with an increase in capsule size. Finally, we summarize the most commonly used media that induce capsule growth in C. neoformans
BPS States, Refined Indices, and Quiver Invariants
For D=4 BPS state construction, counting, and wall-crossing thereof, quiver
quantum mechanics offers two alternative approaches, the Coulomb phase and the
Higgs phase, which sometimes produce inequivalent counting. The authors have
proposed, in arXiv:1205.6511, two conjectures on the precise relationship
between the two, with some supporting evidences. Higgs phase ground states are
naturally divided into the Intrinsic Higgs sector, which is insensitive to
wall-crossings and thus an invariant of quiver, plus a pulled-back ambient
cohomology, conjectured to be an one-to-one image of Coulomb phase ground
states. In this note, we show that these conjectures hold for all cyclic
quivers with Abelian nodes, and further explore angular momentum and R-charge
content of individual states. Along the way, we clarify how the protected spin
character of BPS states should be computed in the Higgs phase, and further
determine the entire Hodge structure of the Higgs phase cohomology. This shows
that, while the Coulomb phase states are classified by angular momentum, the
Intrinsic Higgs states are classified by R-symmetry.Comment: 51 pages, 5 figure
A Study of Wall-Crossing: Flavored Kinks in D=2 QED
We study spectrum of D=2 N=(2,2) QED with N+1 massive charged chiral
multiplets, with care given to precise supermultiplet countings. In the
infrared the theory flows to CP^N model with twisted masses, where we construct
generic flavored kink solitons for the large mass regime, and study their
quantum degeneracies. These kinks are qualitatively different and far more
numerous than those of small mass regime, with features reminiscent of
multi-pronged (p,q) string web, complete with the wall-crossing behavior. It
has been also conjectured that spectrum of this theory is equivalent to the
hypermultiplet spectrum of a certain D=4 Seiberg-Witten theory. We find that
the correspondence actually extends beyond hypermultiplets in D=4, and that
many of the relevant indices match. However, a D=2 BPS state is typically
mapped to several different kind of dyons whose individual supermultiplets are
rather complicated; the match of index comes about only after summing over
indices of these different dyons. We note general wall-crossing behavior of
flavored BPS kink states, and compare it to those of D=4 dyons.Comment: 47 pages, 5 figures; typos fixed; references adde
Non-Supersymmetric String Theory
A class of non-supersymmetric string backgrounds can be constructed using
twists that involve space-time fermion parity. We propose a non-perturbative
definition of string theory in these backgrounds via gauge theories with
supersymmetry softly broken by twisted boundary conditions. The perturbative
string spectrum is reproduced, and qualitative effects of the interactions are
discussed. Along the way, we find an interesting mechanism for inflation. The
end state of closed string tachyon condensation is a highly excited state in
the gauge theory which, in all likelihood, does not have a geometric
interpretation.Comment: 35 pages, 2 figures; revision adds a computation of the relevant
orbifold state
BPS Spectrum, Indices and Wall Crossing in N=4 Supersymmetric Yang-Mills Theories
BPS states in N=4 supersymmetric SU(N) gauge theories in four dimensions can
be represented as planar string networks with ends lying on D3-branes. We
introduce several protected indices which capture information on the spectrum
and various quantum numbers of these states, give their wall crossing formula
and describe how using the wall crossing formula we can compute all the indices
at all points in the moduli space.Comment: LaTeX file, 33 pages, 15 figure
Negative discriminant states in N=4 supersymmetric string theories
Single centered BPS black hole solutions exist only when the charge carried
by the black hole has positive discriminant. On the other hand the exact dyon
spectrum in heterotic string theory compactified on T^6 is known to contain
states with negative discriminant. We show that all of these negative
discriminant states can be accounted for as two centered black holes. Thus
after the contribution to the index from the two centered black holes is
subtracted from the total microscopic index, the index for states with negative
discriminant vanishes even for finite values of charges, in agreement with the
results from the black hole side. Bound state metamorphosis -- which requires
us to identify certain apparently different two centered configurations
according to a specific set of rules -- plays a crucial role in this analysis.
We also generalize these results to a class of CHL string theories.Comment: LaTeX file, 32 pages; v2: reference added; v3: added new section 3.
BPS dyons and Hesse flow
We revisit BPS solutions to classical N=2 low energy effective gauge
theories. It is shown that the BPS equations can be solved in full generality
by the introduction of a Hesse potential, a symplectic analog of the
holomorphic prepotential. We explain how for non-spherically symmetric,
non-mutually local solutions, the notion of attractor flow generalizes to
gradient flow with respect to the Hesse potential. Furthermore we show that in
general there is a non-trivial magnetic complement to this flow equation that
is sourced by the momentum current in the solution.Comment: 25 pages, references adde
A holographic model for the fractional quantum Hall effect
Experimental data for fractional quantum Hall systems can to a large extent
be explained by assuming the existence of a modular symmetry group commuting
with the renormalization group flow and hence mapping different phases of
two-dimensional electron gases into each other. Based on this insight, we
construct a phenomenological holographic model which captures many features of
the fractional quantum Hall effect. Using an SL(2,Z)-invariant
Einstein-Maxwell-axio-dilaton theory capturing the important modular
transformation properties of quantum Hall physics, we find dyonic diatonic
black hole solutions which are gapped and have a Hall conductivity equal to the
filling fraction, as expected for quantum Hall states. We also provide several
technical results on the general behavior of the gauge field fluctuations
around these dyonic dilatonic black hole solutions: We specify a sufficient
criterion for IR normalizability of the fluctuations, demonstrate the
preservation of the gap under the SL(2,Z) action, and prove that the
singularity of the fluctuation problem in the presence of a magnetic field is
an accessory singularity. We finish with a preliminary investigation of the
possible IR scaling solutions of our model and some speculations on how they
could be important for the observed universality of quantum Hall transitions.Comment: 86 pages, 16 figures; v.2 references added, typos fixed, improved
discussion of ref. [39]; v.3 more references added and typos fixed, several
statements clarified, v.4 version accepted for publication in JHE
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