Quantum-Mechanical Position Operator and Localization in Extended Systems

We introduce a fundamental complex quantity, $z_{L}$, which allows us to discriminate between a conducting and non-conducting thermodynamic phase in extended quantum systems. Its phase can be related to the expectation value of the position operator, while its modulus provides an appropriate definition of a localization length. The expressions are valid for {\it any} fractional particle filling. As an illustration we use $z_{L}$ to characterize insulator to ``superconducting'' and Mott transitions in one-dimensional lattice models with infinite on-site Coulomb repulsion at quarter filling.Comment: 4 pages, REVTEX, 1 ps figure

Strong-correlation effects in Born effective charges

Large values of Born effective charges are generally considered as reliable indicators of the genuine tendency of an insulator towards ferroelectric instability. However, these quantities can be very much influenced by strong electron correlation and metallic behavior, which are not exclusive properties of ferroelectric materials. In this paper we compare the Born effective charges of some prototypical ferroelectrics with those of magnetic, non-ferroelectric compounds using a novel, self-interaction free methodology that improves on the local-density approximation description of the electronic properties. We show that the inclusion of strong-correlation effects systermatically reduces the size of the Born effective charges and the electron localization lengths. Furthermore we give an interpretation of the Born effective charges in terms of band energy structure and orbital occupations which can be used as a guideline to rationalize their values in the general case.Comment: 10 pages, 4 postscript figure

Dielectric catastrophe at the Mott transition

We study the Mott transition as a function of interaction strength in the half-filled Hubbard chain with next-nearest-neighbor hopping t' by calculating the response to an external electric field using the Density Matrix Renormalization Group. The electric susceptibility chi diverges when approaching the critical point from the insulating side. We show that the correlation length xi characterizing this transition is directly proportional to fluctuations of the polarization and that chi ~ xi^2. The critical behavior shows that the transition is infinite-order for all t', whether or not a spin gap is present, and that hyperscaling holds.Comment: 4 pages, 4 eps figures, REVTe

From band insulator to Mott insulator in one dimension

We derive the phase diagram for the one-dimensional model of a ferroelectric perovskite recently introduced by Egami, Ishihara and Tachiki [Science, {\bf 261}, 1307 (1993)]. We show that the interplay between covalency, ionicity and strong correlations results in a spontaneously dimerized phase which separates the weak-coupling band insulator from the strong-coupling Mott insulator. The transition from the band insulator to the dimerized phase is identified as an Ising critical point. The charge gap vanishes at this single point with the optical conductivity diverging as $\sigma(\omega)\sim \omega^{-3/4}$. The spin excitations are gapless above the second transition to the Mott insulator phase.Comment: 4 pages LaTex (RevTex) and 1 postscript figure included by eps

Quantum Spin Pump in S=1/2 antiferromagnetic chains -Holonomy of phase operators in sine-Gordon theory-

In this paper, we propose the quantum spin pumping in quantum spin systems where an applied electric field ($E$) and magnetic field ($H$) cause a finite spin gap to its critical ground state. When these systems are subject to alternating electromangetic fields; $(E,H)=(\sin\frac{2\pi t}{T},\cos\frac{2\pi t}{T})$ and travel along the {\it{loop}} $\Gamma_{\rm{loop}}$ which encloses their critical ground state in this $E$-$H$ phase diagram, the locking potential in the sine-Gordon model slides and changes its minimum. As a result, the phase operator acquires $2\pi$ holonomy during one cycle along $\Gamma_{\rm{loop}}$, which means that the quantized spin current has been transported through the bulk systems during this adiabatic process. The relevance to real systems such as Cu-benzoate and ${\rm{Yb}}_4{\rm{As}}_3$ is also discussed.Comment: 10 pages, 5 figures, to be published in J. Phys. Soc. Jpn. 74 (2005) no. 4. Typos corrected in the revised versio

DEVELOPMENT OF A FAST MICRON-RESOLUTION BEAM POSITION MONITOR SIGNAL PROCESSOR FOR LINEAR COLLIDER BEAMBASED FEEDBACK SYSTEMS

We present the design of a prototype fast beam position monitor (BPM) signal processor for use in inter-bunch beam-based feedbacks for linear colliders and electron linacs. We describe the FONT4 intra-train beam-based digital position feedback system prototype deployed at the Accelerator test facility (ATF) extraction line at KEK, Japan. The system incorporates a fast analogue beam position monitor front-end signal processor, a digital feedback board, and a fast kicker-driver amplifier. The total feedback system latency is less than 150ns, of which less than 10ns is used for the BPM processor. We report preliminary results of beam tests using electron bunches separated by c. 150ns. Position resolution of order 1 micron was obtained

BEAM TEST RESULTS WITH THE FONT4 ILC PROTOTYPE INTRA-TRAIN BEAM FEEDBACK SYSTEM

We present the design and beam test results of a prototype beam-based digital feedback system for the Interaction Point of the International Linear Collider. A custom analogue front-end processor, FPGA-based digital signal processing board, and kicker drive amplifier have been designed, built, and tested on the extraction line of the KEK Accelerator Test Facility (ATF). The system was measured to have a latency of approximately 140 ns

A quantum Monte Carlo study of the one-dimensional ionic Hubbard model

Quantum Monte Carlo methods are used to study a quantum phase transition in a 1D Hubbard model with a staggered ionic potential (D). Using recently formulated methods, the electronic polarization and localization are determined directly from the correlated ground state wavefunction and compared to results of previous work using exact diagonalization and Hartree-Fock. We find that the model undergoes a thermodynamic transition from a band insulator (BI) to a broken-symmetry bond ordered (BO) phase as the ratio of U/D is increased. Since it is known that at D = 0 the usual Hubbard model is a Mott insulator (MI) with no long-range order, we have searched for a second transition to this state by (i) increasing U at fixed ionic potential (D) and (ii) decreasing D at fixed U. We find no transition from the BO to MI state, and we propose that the MI state in 1D is unstable to bond ordering under the addition of any finite ionic potential. In real 1D systems the symmetric MI phase is never stable and the transition is from a symmetric BI phase to a dimerized BO phase, with a metallic point at the transition

Latest Beam Test Results of the FONT4 ILC Intra-train Feedback System Prototype

We present the design and preliminary results of a prototype beam-based digital feedback system for the Interaction Point of the International Linear Collider. A custom analogue front-end processor, FPGA-based digital signal processing board, and kicker drive amplifier have been designed, built, and tested on the extraction line of the KEK Accelerator Test Facility (ATF). The system was measured to have a latency of approximately 140 ns.Comment: 4 pages, 6 figures, Proceedings of LCWS/ILC0