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We investigate the quantum transport of the heat and the charge through a quantum dot coupled to fermionic contacts under the influence of time modulation of temperatures. We derive, within the nonequilibrium Keldysh Green's function formalism, generic formulas for the charge and heat currents by extending the concept of gravitational field introduced by Luttinger to the dynamically driven system and by identifying the correct form of dynamical contact energy. In linear response regime our formalism is validated from satisfying the Onsager reciprocity relations and demonstrates its utility to reveal nontrivial dynamical effects of the Coulomb interaction on charge and energy relaxations., Comment: 36 pages, 6 figures

BERT has shown a lot of sucess in a wide variety of NLP tasks. But it has a limitation dealing with long inputs due to its attention mechanism. Longformer, ETC and BigBird addressed this issue and effectively solved the quadratic dependency problem. However we find that these models are not sufficient, and propose LittleBird, a novel model based on BigBird with improved speed and memory footprint while maintaining accuracy. In particular, we devise a more flexible and efficient position representation method based on Attention with Linear Biases (ALiBi). We also show that replacing the method of global information represented in the BigBird with pack and unpack attention is more effective. The proposed model can work on long inputs even after being pre-trained on short inputs, and can be trained efficiently reusing existing pre-trained language model for short inputs. This is a significant benefit for low-resource languages where large amounts of long text data are difficult to obtain. As a result, our experiments show that LittleBird works very well in a variety of languages, achieving high performance in question answering tasks, particularly in KorQuAD2.0, Korean Question Answering Dataset for long paragraphs., Comment: Accepted to EMNLP 2022. see https://aclanthology.org/2022.emnlp-main.352/

We investigate the effects of Majorana bound states on the ac response of a quantum resistor-capacitor circuit which is composed of a topological superconducting wire whose two ends are tunnel-coupled to a lead and a spinless quantum dot, respectively. The Majorana states formed at the two ends of the wire are found to suppress completely or enhance greatly the dissipation, depending on the strength of the overlap between two Majorana modes and/or the dot level. We compare the relaxation resistance and the quantum capacitance of the system with those of non-Majorana counterparts to find that the effects of the Majorana state on the ac response are genuine and cannot be reproduced in ordinary fermonic systems., Comment: 12 pages, 4 figures

We study combined interference effects due to the Aharonov-Bohm (AB) and Aharonov-Casher (AC) phases in a Josephson supercurrent of local and nonlocal (split) Cooper pairs. We analyze a junction between two superconductors interconnected through a normal-state nanostructure with either (i) a ring, where single-electron interference is possible, or (ii) two parallel nanowires, where the single-electron interference can be absent, but the cross Andreeev reflection can occur. In the low-transmission regime in both geometries the AB and AC effects can be related to only local or nonlocal Cooper pair transport, respectively.

We analyze a Josephson junction between two superconductors interconnected through a normal-state nanostructure made of two parallel nanowires with embedded quantum dots. We study the influence of interference effects due to the Aharonov-Bohm (AB) and Aharonov-Casher (AC) phases for local and nonlocal (split) Cooper pairs. In the AB effect the phase of electron is affected by magnetic flux, while in the AC effect the phase of the electron in solid state can be modified due to the Rashba spin-orbit coupling. In the low-transmission regime the AB and AC effects can be related to only local or nonlocal Cooper pair transport, respectively. We demonstrate that by the addition of the quantum dots the Cooper pair splitting can be made perfectly efficient, and that the AC phase is different for non-spin-flip and spin-flip transport processes.

We consider a quantum dot coupled to both superconducting and spin-polarized electrodes, and study the triad interplay of the Kondo effect, superconductivity, and ferromagnetism, any pair of which compete with and suppress each other. We find that the interplay leads to a mixed-valence quantum phase transition, which for other typical sysmstems is merely a crossover rather than a true transition. At the transition, the system changes from the spin doublet to singlet state. The singlet phase is adiabatically connected (through crossovers) to the so-called 'charge Kondo state' and to the superconducting state. We analyze in detail the physical characteristics of different states and propose that the measurement of the cross-current correlation and the charge relaxation resistance can clearly distinguish between them., Comment: 8 figures; to appear in Phys. Rev. B

We consider a topological superconducting wire and use the string order parameter to investigate the spatiotemporal evolution of the topological order upon a quantum quench across the critical point. We also analyze the propagation of the initially localized Majorana bound states after the quench, in order to examine the connection between the topological order and the unpaired Majorana states, which has been well established at equilibrium but remains illusive in dynamical situations. It is found that after the quench the string order parameters decay over a finite time and that the decaying behavior is universal, independent of the wire length and the final value of the chemical potential (the quenching parameter). It is also found that the topological order is revived repeatedly although the amplitude gradually decreases. Further, the topological order can propagate into the region which was initially in the non-topological state. It is observed that all these behaviors are in parallel and consistent with the propagation and dispersion of the Majorana wave functions. Finally, we propose a local probing method which can measure the non-local topological order., Comment: 21 pages; 11 figures

The Kibble-Zurek mechanism (KZM) is generalized to a class of multi-level systems and applied to study the quenching dynamics of one-dimensional (1D) topological superconductors (TS) with open ends. Unlike the periodic boundary condition, the open boundary condition, that is crucial for the zero-mode Majorana states localized at the boundaries, requires to consider many coupled levels. which is ultimately related to the zero-mode Majorana modes. Our generalized KZM predictions agree well with the numerically exact results for the 1D TS., Comment: 5 pages, 2 figures; The last section completely rewritten with more relevant plots in Fig. 2 (Fig 3 removed)

We investigate the linear thermoelectric response of an interacting quantum dot side-coupled by one of two Majorana fermions (MFs) formed at the ends of a topological superconducting wire. We employ the numerical renormalization group technique to obtain the thermoelectrical conductance $L$ as well as the electrical conductance $G$ when the background temperature $T$ and the dot gate are tuned. We distinguish two transport regimes in which $L$ displays different features: the weak- $(\Gamma_{m} < T_{K})$ and strong-coupling $(\Gamma_{m} > T_{K})$ regimes, where $\Gamma_{m}$ and $T_{K}$ are the Majorana-dot coupling and the Kondo temperature, respectively. For an ideal (infinitely long) nanowire where the Majorana end states do not overlap $(\epsilon_{m} = 0)$, the thermoelectrical conductance $L$ in the weak-coupling regime exhibits a peak at $T \sim \Gamma_{m}$. This peak is ascribed to the anti-Fano resonance between the asymmetric Kondo resonance and the zero-energy MF mode. Interestingly, in the strong-coupling regime, the Kondo-induced peak in $L$ is shifted due to the MF-induced Zeeman splitting in the dot. For finite but small $\epsilon_{m} > 0$, the interference between two MFs restores the Kondo effect in the dot in a smaller energy scale $\Gamma^{\prime}_{m}$ and gives rise to an additional peak in $L$ at $T \sim \Gamma^{\prime}_{m}$, whose sign is opposite to that at $T\sim\Gamma_{m}$. In the strong-coupling regime this additional peak can cause a non-monotonic behavior of $L$ with respect to the dot gate. Finally, we examine the case in which an ordinary spin-polarized fermion is coupled to the dot and identify the fingerprint of MFs by comparing two cases., Comment: 10 pages, 8 figures

We consider spin-polarized transport in a quantum spin Hall antidot system coupled to normal leads. Due to the helical nature of the conducting edge states, the screening potential at the dot region becomes spin dependent without external magnetic fields nor ferromagnetic contacts. Therefore, the electric current due to voltage or temperature differences becomes spin polarized, its degree of polarization being tuned with the dot level position or the base temperature. This spin-filter effect arises in the nonlinear transport regime only and has a purely interaction origin. Likewise, we find a spin polarization of the heat current which is asymmetric with respect to the bias direction. Interestingly, our results show that a pure spin current can be generated by thermoelectric means: when a temperature gradient is applied, the created thermovoltage (Seebeck effect) induces a spin-polarized current for vanishingly small charge current. An analogous effect can be observed for the heat transport: a pure spin heat flows in response to a voltage shift even if the thermal current is zero., Comment: 10 pages, 7 figures

We investigate the mesoscopic resistor-capacitor circuit consisting of a quantum dot coupled to spatially separated Majorana fermion modes in a chiral topological superconductor. We find substantially enhanced relaxation resistance due to the nature of Majorana fermions, which are their own anti-particles and composed of particle and hole excitations in the same abundance. Further, if only a single Majorana mode is involved, the zero-frequency relaxation resistance is completely suppressed due to a destructive interference. As a result, the Majorana mode opens an exotic dissipative channel on a superconductor which is typically regarded as dissipationless due to its finite superconducting gap., Comment: To appear in Phys. Rev. Lett.; Supplemental Material available at http://iff.korea.ac.kr/Opus/ChoiMS14c-Suppl.pdf; References and Suppl. Mat. updated

We discuss the thermoelectrical properties of nanowires hosting Majorana edge states. For a Majorana nanowire directly coupled to two normal reservoirs the thermopower always vanishes regardeless of the value of the Majorana hybridization. This situation changes drastically if we insert a quantum dot. Then, the dot Majorana side coupled system exhibits a different behavior for the thermopower depending on the Majorana hybridization parameter. Thermopower reverses its sign when the half fermionic state is fully developed. As long as the hybridization becomes finite the Seebeck coefficient behaves similarly to a resonant level system. The sign change of the thermopower when Majorana physics takes place and the fact that both, the electrical and thermal conductances reach, their half fermionic value could serve as a proof of the existence of Majorana edge states in nanowires. Finally, we perform some predictions about the gate dependence of the Seebeck coefficient when Kondo correlations are present in the dot., Comment: 7 pages, 6 figures

We propose a two-terminal spin-orbit interferometer with a hot molecule inserted in one of its arms to generate pure spin currents. Local heating is achieved by coupling the vibrational modes of the molecule to a third (phononic) reservoir. We show that this spin calorimetric effect is due to the combined influence of spin-dependent wave interference and inelastic scattering. Remarkably, the device converts heat flow into spin-polarized current even without applying any voltage or temperature difference to the electronic terminals., Comment: 4 pages, 3 figures

We investigate the Majorana physics and its effect on the electron transport in the non-topological superconductor(NS)-topological superconductor(TS) double junctions of a ring geometry. We find that, depending on the ratio between the lengths of two topologically different regions and the localization lengths of the Majorana fermions formed between them, two completely different transport mechanisms are working: perfect crossed Andreev reflection (CAR) for the short NS region and perfect normal Andreev reflection for the short TS region. The difference is explained in terms of the topologically distinct properties of subgap states in the NS-TS double junction system, which have not been revealed so far. The exotic dependence of the CAR process on the magnetic flux threading the ring is uncovered and can be used to detect the Majorana fermions., Comment: 15 pages, 21 figures

Nonlinear transport coefficients do not obey, in general, reciprocity relations. We here discuss the magnetic-field asymmetries that arise in thermoelectric and heat transport of mesoscopic systems. Based on a scattering theory of weakly nonlinear transport, we analyze the leading-order symmetry parameters in terms of the screening potential response to either voltage or temperature shifts. We apply our general results to a quantum Hall antidot system. Interestingly, we find that certain symmetry parameters show a dependence on the measurement configuration., Comment: 17 pages, 3 figures

We investigate the dynamical and transport features of a Kondo dot side-coupled to a topological superconductor (TS). The Majorana fermion states (MFS) formed at the ends of the TS are found to be able to alter the Kondo physics profoundly: For the ideal setup where the MFS do not overlap ($\epsilon_m=0$) a finite dot-MFS coupling $(\Gamma_m)$ reduces the unitary-limit value of the linear conductance by exactly a factor 3/4 in the Kondo-dominant regime $(\Gamma_mT_K)$, on the other hand, the spin-split Kondo resonance takes place due to the MFS-induced Zeeman splitting, which is a genuine many-body effect of the strong Coulomb interaction and the topological superconductivity. We find that the original Kondo resonance is fully restored once the MFSs are strongly hybridized ($\epsilon_m > \Gamma_m$). This unusual interaction between the Kondo effect and the MFS can thus serve to detect the Majorana fermions unambiguously and quantify the degree of overlap between the MFSs in the TS., Comment: 5 pages, 4 figures

We propose a general scattering matrix formalism that guarantees the charge conservation at junctions between conducting arms with arbitrary spin interactions. By using our formalism, we find that the spin-flip scattering can happen even at nonmagnetic junctions if the spin eigenstates in arms are not orthogonal. We apply our formalism to the Aharonov-Bohm interferometer consisting of $n$-type semiconductor ring with both the Rashba spin-orbit coupling and the Zeeman splitting. We discuss the characteristics of the interferometer as conditional/unconditional spin switch in the weak/strong-coupling limit, respectively., Comment: 14 pages, 13 figures

We investigate in a nonperturbative way the dynamics of a correlated quantum capacitor. We find that the many-body correlations do not disturb the universal low-frequency relaxation resistance per channel, $R_q(\omega=0) = h/4e^2$ ensured by the Korringa-Shiba rule whereas the interpretation of the quantum capacitance $C_q$ in terms of the density of states fails when strong correlations are present. The AC resistance $R_q(\omega)$ shows huge peaks (with values larger than $h/4e^2$) at $\hbar\omega \approx \pm \Gamma^*$, where $\Gamma^*$ is the renormalized level broadening. These peaks are merged to a single one at $\omega=0$ when a finite Zeeman field is applied comparable to $\Gamma^*$. The observed features of $R_q$, being most evident in the Kondo regime, are attributed to the generation of particle-hole excitations in the contacts accomplished by spin-flip processes in the dot., Comment: 5 pages, 4 figures

We study the transport properties of a serial double quantum dot (DQD) coupled to two superconducting leads, focusing on the Josephson current through the DQD and the associated 0-$\pi$ transitions which result from the subtle interplay between the superconductivity, the Kondo physics, and the inter-dot superexchange interaction. We examine the competition between the superconductivity and the Kondo physics by tuning the relative strength $\Delta/T_K$ of the superconducting gap $\Delta$ and the Kondo temperature $T_K$, for different strengths of the superexchange coupling determined by the interdot tunneling $t$ relative to the dot level broadening $\Gamma$. We find strong renormalization of $t$, a significant role of the superexchange coupling $J$, and a rich phase diagram of the 0 and $\pi$-junction regimes. In particular, when both the superconductivity and the exchange interaction are in close competion with the Kondo physics ($\Delta\sim J\sim T_K$), there appears an island of $\pi'$-phase at large values of the superconducting phase difference., Comment: 4 pages, 4 figures

We investigate the Josephson effect through a two-level quantum dot with an exchange coupling between two dot electrons. We compute the superconducting phase relationship and construct the phase diagram in the superconducting gap--exchange coupling plane in the regime of the singlet-triplet transition driven by the exchange coupling. In our study two configurations for the dot-lead coupling are considered: one where effectively only one channel couples to the dot, and the other where the two dot orbitals have opposite parities. Perturbative analysis in the weak-coupling limit reveals that the system experiences transitions from 0 to $\pi$ (negative critical current) behavior, depending on the parity of the orbitals and the spin correlation between dot electrons. The strong coupling regime is tackled with the numerical renormalization group method, which first characterizes the Kondo correlations due to the dot-lead coupling and the exchange coupling in the absence of superconductivity. In the presence of superconductivity, many-body correlations such as two-stage Kondo effect compete with the superconductivity and the comparison between the gap and the relevant Kondo temperature scales allows to predict a rich variety of phase diagrams for the ground state of the system and for the Josephson current. Numerical calculations predicts that our system can exhibit Kondo-driven 0-$\pi$-0 or $\pi$-0-$\pi$ double transitions and, more interestingly, that if proper conditions are met a Kondo-assisted $\pi$-junction can arise, which is contrary to a common belief that the Kondo effect opens a resonant level and makes the 0-junction. Our predictions could be probed experimentally for a buckminster fullerene sandwiched between two superconductors., Comment: 19 pages, 15 figures

We study a series-coupled double quantum dot in the Kondo regime modeled by the two-impurity Anderson model and find a new conduction-band mediated superexchange interaction that competes with Kondo physics in the strong Coulomb interaction limit. Our numerical renormalization group results, complemented with the higher-order Rayleigh-Schr\"odinger perturbation theory, show that the novel exchange mechanism leads to clear experimental consequences that can be checked in transport measurements through double quantum dots.

We investigate the intrinsic spin Hall effect in two-dimensional electron gases in quantum wells with two subbands, where a new intersubband-induced spin-orbit coupling is operative. The bulk spin Hall conductivity $\sigma^z_{xy}$ is calculated in the ballistic limit within the standard Kubo formalism in the presence of a magnetic field $B$ and is found to remain finite in the B=0 limit, as long as only the lowest subband is occupied. Our calculated $\sigma^z_{xy}$ exhibits a nonmonotonic behavior and can change its sign as the Fermi energy (the carrier areal density $n_{2D}$) is varied between the subband edges. We determine the magnitude of $\sigma^z_{xy}$ for realistic InSb quantum wells by performing a self-consistent calculation of the intersubband-induced spin-orbit coupling., Comment: 7 pages, 3 figures

We study the coherent transport of heavy holes through a one-dimensional ring in the presence of spin-orbit coupling. Spin-orbit interaction of holes, cubic in the in-plane components of momentum, gives rise to an angular momentum dependent spin texture of the eigenstates and influences transport. We analyze the dependence of the resulting differential conductance of the ring on hole polarization of the leads and the signature of the textures in the Aharonov-Bohm oscillations when the ring is in a perpendicular magnetic field. We find that the polarization-resolved conductance reveals whether the dominant spin-orbit coupling is of Dresselhaus or Rashba type, and that the cubic spin-orbit coupling can be distinguished from the conventional linear coupling by observing the four-peak structure in the Aharonov-Bohm oscillations., Comment: 12 pages, 11 figures

We investigate the Josephson effect through a molecular quantum dot magnet connected to superconducting leads. The molecule contains a magnetic atom, whose spin is assumed to be isotropic. It is coupled to the electron spin on the dot via exchange coupling. Using the numerical renormalization group method we calculate the Andreev levels and the supercurrent and examine intertwined effect of the exchange coupling, Kondo correlation, and superconductivity on the current. Exchange coupling typically suppresses the Kondo correlation so that the system undergoes a phase transition from 0 to $\pi$ state as the modulus of exchange coupling increases. Antiferromagnetic coupling is found to drive exotic transitions: the reentrance to the $\pi$ state for a small superconducting gap and the restoration of 0 state for large antiferromagnetic exchange coupling. We suggest that the asymmetric dependence of supercurrent on the exchange coupling could be used as to detect its sign in experiments.

We investigate the spin-orbit (s-o) interaction in two-dimensional electron gases (2DEGs) in quantum wells with two subbands. From the $8\times 8$ Kane model, we derive a new inter-subband-induced s-o term which resembles the functional form of the Rashba s-o -- but is non-zero even in \emph{symmetric} structures. This follows from the distinct parity of the confined states (even/odd) which obliterates the need for asymmetric potentials. We self-consistently calculate the new s-o coupling strength for realistic wells and find it comparable to the usual Rashba constant. Our new s-o term gives rise to a non-zero ballistic spin-Hall conductivity, which changes sign as a function of the Fermi energy ($\epsilon_F$), and can induce an unusual \emph{zitterbewegung} with cycloidal trajectories \textit{without} magnetic fields., Comment: v2: 4 two-column pages, 3 figures (added spin Hall conductivity and self-consistent calculation)

We introduce a new spin filter based on spin-resolved Fano resonances due to spin-split levels in a quantum ring (QR) side-coupled to a quantum wire (QW). Spin-orbit coupling inside the QR, together with external magnetic fields, induces spin splitting, and the Fano resonances due to the spin-split levels result in perfect or considerable suppression of the transport of either spin direction. Using the numerical renormalization group method, we find that the Coulomb interaction in the QR enhances the spin filter operation by widening the separation between dips in conductances for different spins and by allowing perfect blocking for one spin direction and perfect transmission for the other. The spin-filter effect persists as long as the temperature is less than the broadening of QR levels due to the QW-QR coupling. We discuss realistic conditions for the QR-based spin filter and its advantages to other similar devices., Comment: 5 pages, 4 figures

We investigate static charge and spin density modulation patterns along a ferromagnet/semiconductor single junction quantum wire in the presence of spin-orbit coupling. Coherent scattering theory is used to calculate the charge and spin densities in the ballistic regime. The observed oscillatory behavior is explained in terms of the symmetry of the charge and spin distributions of eigenstates in the semiconductor quantum wire. Also, we discuss the condition that these charge and spin density oscillations can be observed experimentally., Comment: 7 pages, 8 figures (low-resolution)

In a recent work [Phys. Rev. Lett. 93, 047002 (2004)], Siano and Egger (SE) studied Josephson current through a quantum dot in the Kondo regime using the quantum Monte Carlo (QMC) method. Several of their results were inconsistent with those from the numerical renormalization group (NRG) calculations [Phys. Rev. B 70, R020502 (2004); J. Phys. Soc. Jpn. 69, 1812 (2000)] among other previous studies. The results of SE are not reliable for the following two reasons: (i) The definition of the Kondo temperature was not correct; (ii) There were substantial fintie-temperature effects.

We investigate the ballistic spin transport through a two-dimensional mesoscopic metal/semiconductor/metal double junctions in the presence of spin-orbit interactions. It is shown that \emph{real} longitudinal and/or transverse spin currents can flow in the presence of the Rashba and Dresselhaus terms., Comment: 5 pages, 6 figures

We investigate theoretically the transport properties of a closed Aharonov-Bohm interferometer containing two quantum dots in the strong coupling regime. We find two distinct physical scenarios depending on the strength of the interdot Coulomb interaction. When the interdot Coulomb interaction is negligible only spin fluctuations are important and each dot develops a Kondo resonance at the Fermi level independently of the applied magnetic flux. The transport is characterized by the interference of these two independent Kondo resonances. On the contrary, for large interdot interaction, only one electron can be accommodated onto the double dot system. In this situation, not only the spin can fluctuate but also the orbital degree of freedom (the pseudo-spin). As a result, we find different ground states depending on the value of the applied flux. When $\phi=\pi$ (mod $2\pi$) ($\phi=2\pi\Phi/\Phi_0$, where $\Phi$ is applied flux, and $\Phi_0=h/e$ the flux quantum) the electronic transport can take place via simultaneous correlations in the spin and pseudo-spin sectors, leading to the highly symmetric SU(4) Kondo state. Nevertheless, we find situations with $\phi>0$ (mod $2\pi$) where the pseudo-spin quantum number is not conserved during tunneling events, giving rise to the common SU(2) Kondo state with an enhanced Kondo temperature. We investigate the crossover between both ground states and discuss possible experimental signatures of this physics as a function of the applied magnetic flux., Comment: 12 pages, 3 figures; extended discussions, improved presentation

Various phase transitions in models for coupled charge-density waves are investigated by means of the $\epsilon$-expansion, mean-field theory, and Monte Carlo simulations. At zero temperature the effective action for the system with appropriate commensurability effects is mapped onto the three- or four-dimensional \XY model, depending on spatiotemporal fluctuations, under the corresponding symmetry-breaking fields. It is revealed that the three- and four-dimensional systems display a single transition between the clock order (with broken Z$_M$ symmetry) and disorder. The nature of the phase transition depends crucially on the commensurability factor $M$: For $M \ge 4$, in particular, the transition belongs to the same university class as the \XY model. On the other hand, in the presence of misfit causing frustration in the charge-density wave, the inter-chain coupling is observed to favor either the commensurate state or the incommensurate state depending on the initial configuration; this gives rise to hysteresis around the commensurate-incommensurate transition. Boundaries separating such phases with different symmetries are obtained in the parameter space consisting of the temperature, symmetry-breaking field, fluctuation strength, inter-chain coupling, and misfit., Comment: 14 pages, to be published in Phys. Rev. B

We investigate the supercurrent through a quantum dot for the whole range of couplings using the numerical renormalization group method. We find that the Josephson current switches abruptly from a $\pi$- to a 0-phase as the coupling increases. At intermediate couplings the total spin in the ground state depends on the phase difference between the two superconductors. Our numerical results can explain the crossover in the conductance observed experimentally by Buitelaar \textit{et al.} [Phys. Rev. Lett. \textbf{89}, 256 801 (2002)]., Comment: Fig.2 and corresponding text have been changed; Several other small changes

We establish systematic consolidation of the Aharonov-Bohm and Aharonov-Casher effects including their scalar counterparts. Their formal correspondences in acquiring topological phases are revealed on the basis of the gauge symmetry in non-simply connected spaces and the adiabatic condition for the state of magnetic dipoles. In addition, investigation of basic two-body interactions between an electric charge and a magnetic dipole clarifies their appropriate relative motions and discloses physical interrelations between the effects. Based on the two-body interaction, we also construct an exact microscopic description of the Aharonov-Bohm effect, where all the elements are treated on equal footing, i.e., magnetic dipoles are described quantum-mechanically and electromagnetic fields are quantized. This microscopic analysis not only confirms the conventional (semiclassical) results and the topological nature but also allows one to explore the fluctuation effects due to the precession of the magnetic dipoles with the adiabatic condition relaxed.

In this work we study quantum phase transitions and persistent currents in capacitively coupled one-dimensional Josephson-junction arrays. We will focus particularly on the roles of exciton-like pairs in the strong coupling limit in the presence of external gate charges and magnetic fluxes. We use the numerical density-matrix renormalization group method for the study in the full range of values of gate charge and magnetic flux. To clarify the various effects, we report the pair correlation functions and the exciton densities as welll as the persistent current., Comment: To appear in Phys. Rev. B; title has been changed; a few parts of the text have been change; 12 pages, 17 figures

We consider interacting electrons in a two-dimensional quantum Coulomb glass and investigate by means of the Hartree-Fock approximation the combined effects of the electron-electron interaction and the transverse magnetic field on fluctuations of the inverse compressibility. Preceding systematic study of the system in the absence of the magnetic field identifies the source of the fluctuations, interplay of disorder and interaction, and effects of hopping. Revealed in sufficiently clean samples with strong interactions is an unusual right-biased distribution of the inverse compressibility, which is neither of the Gaussian nor of the Wigner-Dyson type. While in most cases weak magnetic fields tend to suppress fluctuations, in relatively clean samples with weak interactions fluctuations are found to grow with the magnetic field. This is attributed to the localization properties of the electron states, which may be measured by the participation ratio and the inverse participation number. It is also observed that at the frustration where the Fermi level is degenerate, localization or modulation of electrons is enhanced, raising fluctuations. Strong frustration in general suppresses effects of the interaction on the inverse compressibility and on the configuration of electrons., Comment: 15 pages, 18 figures, To appear in Phys. Rev. B

We study the quantum phase transition in a chain of superconducting grains, coupled by Josephson junctions, with emphasis on the effects of gate charges induced on the grains. At zero temperature the system is mapped onto a two-dimensional classical Coulomb gas, where the gate charge plays the role of an imaginary electric field. Such a field is found relevant in the renormalization-group transformation and to change the nature of the superconductor-insulator transition present in the system, tending to suppress quantum fluctuations and helping establish superconductivity. On the basis of this observation, we propose the zero-temperature phase diagram on the plane of the gate charge and the energy ratio., Comment: 2 figures, to appear in Phys. Rev. B

We investigate the quantum phase transitions in two capacitively coupled two-dimensional Josephson-junction arrays with charge frustration. The system is mapped onto the S=1 and $S=1/2$ anisotropic Heisenberg antiferromagnets near the particle-hole symmetry line and near the maximal-frustration line, respectively, which are in turn argued to be effectively described by a single quantum phase model. Based on the resulting model, it is suggested that near the maximal frustration line the system may undergo a quantum phase transition from the charge-density wave to the super-solid phase, which displays both diagonal and off- diagonal long-range order., Comment: 6 pages, 6 figures, to appear in Phys. Rev. B