Sampled data analysis of a computer-controlled manipulator

A comprehensive sampled data analysis of a computer-controlled manipulator is presented in terms of root loci for gain selection and transient responses to step input functions. All parameter values and their derivations where applicable were tabulated. The analysis, while quite specific, uses normalized gain parameters, which allows the results to be applied to any similar system regardless of individual hardware parameter values

Remnant Fermi Surfaces in Photoemission

Recent experiments have introduced a new concept for analyzing the photoemission spectra of correlated electrons -- the remnant Fermi surface (rFs), which can be measured even in systems which lack a conventional Fermi surface. Here, we analyze the rFs in a number of interacting electron models, and find that the results fall into two classes. For systems with pairing instabilities, the rFs is an accurate replica of the true Fermi surface. In the presence of nesting instabilities, the rFs is a map of the resulting superlattice Brillouin zone. The results suggest that the gap in Ca_2CuO_2Cl_2 is of nesting origin.Comment: 4 pages LaTex, 3 ps figure

Noisy metrology beyond the standard quantum limit

Parameter estimation is of fundamental importance in areas from atomic spectroscopy and atomic clocks to gravitational wave detection. Entangled probes provide a significant precision gain over classical strategies in the absence of noise. However, recent results seem to indicate that any small amount of realistic noise restricts the advantage of quantum strategies to an improvement by at most a multiplicative constant. Here, we identify a relevant scenario in which one can overcome this restriction and attain superclassical precision scaling even in the presence of uncorrelated noise. We show that precision can be significantly enhanced when the noise is concentrated along some spatial direction, while the Hamiltonian governing the evolution which depends on the parameter to be estimated can be engineered to point along a different direction. In the case of perpendicular orientation, we find superclassical scaling and identify a state which achieves the optimum.Comment: Erroneous expressions with inconsistent units have been corrected. 5 pages, 3 figures + Appendi

Renormalization of f-levels away from the Fermi energy in electron excitation spectroscopies: Density functional results of Nd2−x_{2-x}Cex_xCuO4_4

Relaxation energies for photoemission, when an occupied electronic state is excited, and for inverse photoemission, when an empty state is filled, are calculated within the density functional theory with application to Nd$_{2-x}$Ce$_x$CuO$_4$. The associated relaxation energies are obtained by computing differences in total energies between the ground state and an excited state in which one hole or electron is added into the system. The relaxation energies of f-electrons are found to be of the order of several eV's, indicating that f-bands will appear substantially away from the Fermi energy ($E_F$) in their spectroscopic images, even if these bands lie near $E_F$. Our analysis explains why it would be difficult to observe f electrons at the $E_F$ even in the absence of strong electronic correlations.Comment: 6 pages, 1 figure, 1 tabl

Proposal to determine the Fermi-surface topology of a doped iron-based superconductor using bulk-sensitive Fourier-transform Compton scattering

We have carried out first-principles calculations of the Compton scattering spectra to demonstrate that the filling of the hole Fermi surface in LaO$_{1-x}$F$_{x}$FeAs produces a distinct signature in the Fourier transformed Compton spectrum when the momentum transfer vector lies along the [100] direction. We thus show how the critical concentration $x_c$, where hole Fermi surface pieces are filled up and the superconductivity mediated by antiferromagnetic spin fluctuations is expected to be suppressed, can be obtained in a bulk-sensitive manner.Comment: 4 pages, 6 figures, accepted in Physical Review

Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates

The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping from a pseudogap state in the underdoped cuprates to a superconducting state at optimal and overdoping, has been interpreted as evidence that the pseudogap must be due to precursor pairing. We suggest an alternative explanation, that the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the pseudogap could actually be due to any of a number of nesting instabilities, including charge or spin density waves or more exotic phases. We present a detailed analysis of this competition for one particular model: the pinned Balseiro-Falicov model of competing charge density wave and (s-wave) superconductivity. We show that most of the anomalous features of both tunneling and photoemission follow naturally from the model, including the smooth crossover, the general shape of the pseudogap phase diagram, the shrinking Fermi surface of the pseudogap phase, and the asymmetry of the tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1 and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be described in detail by this model, but we suggest a simple generalization to account for inhomogeneity, which does provide an adequate description. We show that it should be possible, with a combination of photoemission and tunneling, to demonstrate the extent of pinning of the Fermi level to the Van Hove singularity. A preliminary analysis of the data suggests pinning in the underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure

Effects of excess or deficiency of oxygen content on the electronic structure of high-TCT_C cuprates

Band structure calculations are presented for large supercells of Ba$_2$CuO$_4$ (BCO) with O-vacancies in planar or apic al positions, and of superoxygenated La$_2$CuO$_4$ (LCO) with oxygen interstitials in the La$_2$O$_2$ layers. It is foun d that apical oxygen vacancies in BCO act as electron dopants and makes the electronic structure similar to that of hole doped LCO. Excess oxygen interstitials forming wires in the La$_2$O$_2$ layers of LCO are shown to yield a much larger density-of-states at the Fermi energy than for the stoichiometric compound related with a segmentation of the Fermi surface. Anti-ferromagnetic (AFM) spin fluctuations are strengthened by O-vacancies in BCO as well as by oxygen interstitials in LCO, but are strongly suppressed in O-deficient LCO. Our results indicate the complexity of doping by O-vacancies, and by ordered defects that are a significant factor contr olling the electronic properties of cuprates.Comment: 5 pages, 3 figures, Proceedings Superstripe 2012 Conference, Erice July 201

Phase Separation Models for Cuprate Stripe Arrays

An electronic phase separation model provides a natural explanation for a large variety of experimental results in the cuprates, including evidence for both stripes and larger domains, and a termination of the phase separation in the slightly overdoped regime, when the average hole density equals that on the charged stripes. Several models are presented for charged stripes, showing how density waves, superconductivity, and strong correlations compete with quantum size effects (QSEs) in narrow stripes. The energy bands associated with the charged stripes develop in the middle of the Mott gap, and the splitting of these bands can be understood by considering the QSE on a single ladder.Comment: significant revisions: includes island phase, 16 eps figures, revte