Deconfined quantum criticality driven by Dirac fermions in SU(2) antiferromagnets

Quantum electrodynamics in 2+1 dimensions is an effective gauge theory for the so called algebraic quantum liquids. A new type of such a liquid, the algebraic charge liquid, has been proposed recently in the context of deconfined quantum critical points [R. K. Kaul {\it et al.}, Nature Physics {\bf 4}, 28 (2008)]. In this context, we show by using the renormalization group in $d=4-\epsilon$ spacetime dimensions, that a deconfined quantum critical point occurs in a SU(2) system provided the number of Dirac fermion species $N_f\geq 4$. The calculations are done in a representation where the Dirac fermions are given by four-component spinors. The critical exponents are calculated for several values of $N_f$. In particular, for $N_f=4$ and $\epsilon=1$ ($d=2+1$) the anomalous dimension of the N\'eel field is given by $\eta_N=1/3$, with a correlation length exponent $\nu=1/2$. These values change considerably for $N_f>4$. For instance, for $N_f=6$ we find $\eta_N\approx 0.75191$ and $\nu\approx 0.66009$. We also investigate the effect of chiral symmetry breaking and analyze the scaling behavior of the chiral holon susceptibility, $G_\chi(x)\equiv$.Comment: 13 pages, 3 figures; published versio

Gauge dependenceof the order parameter anomalous dimension in the Ginzburg-Landau model and the critical fluctuations in superconductors

The critical fluctuations of superconductors are discussed in a fixed dimension scaling suited to describe the type II regime. The gauge dependence of the anomalous dimension of the scalar field is stablished exactly from the Ward-Takahashi identities. Its fixed point value gives the $\eta$ critical exponent and it is shown that $\eta$ is gauge independent, as expected on physical grounds. In the scaling considered, $\eta$ is found to be zero at 1-loop order, while $\nu\approx 0.63$. This result is just the 1-loop values for the XY model obtained in the fixed dimension renormalization group approach. It is shown that this XY behavior holds at all orders. The result $\eta=\eta_{XY}$ should be contrasted with the negative values frequently reported in the literature.Comment: EuroLaTex, 7 pages, 2 figures, reference updated; version to be published in Europhysics Letter

Experimental observation of spatial antibunching of photons

We report an interference experiment that shows transverse spatial antibunching of photons. Using collinear parametric down-conversion in a Young-type fourth-order interference setup we show interference patterns that violate the classical Schwarz inequality and should not exist at all in a classical description.Comment: 4 pages, 7 figure

Deconfinement transition in three-dimensional compact U(1) gauge theories coupled to matter fields

It is shown that permanent confinement in three-dimensional compact U(1) gauge theory can be destroyed by matter fields in a deconfinement transition. This is a consequence of a non-trivial infrared fixed point caused by matter, and an anomalous scaling dimension of the gauge field. This leads to a logarithmic interaction between the defects of the gauge-fields, which form a gas of magnetic monopoles. In the presence of logarithmic interactions, the original electric charges are unconfined. The confined phase which is permanent in the absence of matter fields is reached at a critical electric charge, where the interaction between magnetic charges is screened by a pair unbinding transition in a Kosterlitz-Thouless type of phase-transition.Comment: RevTex4, 4 pages, no figures; version accepted for publication in PR

Wilsonian Renormalization as a Quantum Channel and the Separability of Fixed Points

We show that the Wilsonian formulation of the renormalization group (RG) defines a quantum channel acting on the momentum-space density matrices of a quantum field theory. This information theoretical property of the RG allows us to derive a remarkable consequence for the vacuum of theories at a fixed point: they have no entanglement between momentum scales. Our result can be understood as deriving from the scale symmetry of such theories and leads to constraints on the form of the ground state and on expectation values of momentum space operators.Comment: v2: main sections expanded for clarity, discussion added on the scaling of the time variable and the associated dynamical critical exponent, references added and other minor changes, 7 page

Momentum space entanglement from the Wilsonian effective action

The entanglement between momentum modes of a quantum field theory at different scales is not as well studied as its counterpart in real space, despite the natural connection with the Wilsonian idea of integrating out the high-momentum degrees of freedom. Here, we push such a connection further by developing a novel method to calculate the Rényi and entanglement entropies between slow and fast modes, which is based on the Wilsonian effective action at a given scale. This procedure is applied to the perturbative regime of some scalar theories, comparing the lowest-order results with those from the literature and interpreting them in terms of Feynman diagrams. This method is easily generalized to higher-order or nonperturbative calculations. It has the advantage of avoiding matrix diagonalizations of other techniques

Electromagnetic Pulse Driven Spin-dependent Currents in Semiconductor Quantum Rings

We investigate the non-equilibrium charge and spin-dependent currents in a quantum ring with a Rashba spin orbit interaction (SOI) driven by two asymmetric picosecond electromagnetic pulses. The equilibrium persistent charge and persistent spin-dependent currents are investigated as well. It is shown that the dynamical charge and the dynamical spin-dependent currents vary smoothly with a static external magnetic flux and the SOI provides a SU(2) effective flux that changes the phases of the dynamic charge and the dynamic spin-dependent currents. The period of the oscillation of the total charge current with the delay time between the pulses is larger in a quantum ring with a larger radius. The parameters of the pulse fields control to a certain extent the total charge and the total spin-dependent currents. The calculations are applicable to nano-meter rings fabricated in heterojuctions of III-V and II-VI semiconductors containing several hundreds electrons.Comment: 15pages, 5 figure

Diffusion-weighted imaging: determination of the best pair ofb-values to discriminate breast lesions

In breast diffusion-weighted imaging (DWI), the apparent diffusion coefficient (ADC) is used to discriminate between malignant and benign lesions. As ADC estimates can be affected by the weighting factors, our goal was to determine the optimal pair of b-values for discriminating breast lesions at 3.0 T. METHODS: 152 females with 157 lesions (89 malignant and 68 benign) underwent breast MRI, including a DWI sequence sampling six b-values 50, 200, 400, 600, 800 and 1000 s mm−2. ADC values were computed from different pairs of b-values and compared with ADC obtained by fitting the six b-values using a mono-exponential diffusion model (ADCall). Cut-off ADC values were determined and diagnostic performance evaluated by receiver operating characteristic analysis using Youden statistics. Mean ADCs were determined for normal tissue and lesions. Differences were evaluated by lesion and histological types. RESULTS: Considering the cut-off values 1.46 and 1.49 × 103mm2 s−1, the pairs 50, 1000 and 200, 800 s mm−2 showed the highest accuracy, 77.5% and 75.4% with areas under the curve 84.4% and 84.2%, respectively. The best pair for ADC quantification was 50, 1000 s mm−2 with 38/49 true-negative and 69/89 true-positive cases respectively; mean ADCs were 1.86 ± 0.46, 1.77 ± 0.37 and 1.15 ± 0.46 × 10−3 mm2 s−1 for normal, benign and malignant lesions. There were no significant differences in these ADC values when compared with ADCall (ADC calculated from the full set of b - values) [difference = 0.0075 × 10−3 mm2 s−1; confidence interval 95%: (−0.0036; 0.0186); p = 0.18]. CONCLUSION: The diagnostic performance in differentiating malignant and benign lesions was most accurate for the b-value pair 50, 1000 s mm−2.info:eu-repo/semantics/publishedVersio

Large N study of extreme type II superconductors in a magnetic field

The large N analysis of an extreme type II superconductor is revisited. It is found that the phase transition is of second-order in dimensions 4 < d < 6. For the physical dimension d=3 no sign of phase transition is found, contrary to early claims.Comment: Revtex, 7 pages, no figure

Critical properties of the topological Ginzburg-Landau model

We consider a Ginzburg-Landau model for superconductivity with a Chern-Simons term added. The flow diagram contains two charged fixed points corresponding to the tricritical and infrared stable fixed points. The topological coupling controls the fixed point structure and eventually the region of first order transitions disappears. We compute the critical exponents as a function of the topological coupling. We obtain that the value of the $\nu$ exponent does not vary very much from the XY value, $\nu_{XY}=0.67$. This shows that the Chern-Simons term does not affect considerably the XY scaling of superconductors. We discuss briefly the possible phenomenological applications of this model.Comment: RevTex, 7 pages, 8 figure