We propose a qubit efficient scheme to study ground state properties of quantum many-body systems on near-term noisy intermediate scale quantum computers. One can obtain a tensor network representation of the ground state using a number of qubits smaller than the physical degrees of freedom. By increasing the qubits number, one can exponentially increase the bond dimension of the tensor network variational ansatz on a quantum computer. Moreover, we construct circuits blocks which respect U(1) and SU(2) symmetries of the physical system and show that they can significantly speed up the training process and alleviate the gradient vanishing problem. To demonstrate the feasibility of the qubit efficient variational quantum eigensolver in a practical setting, we perform first principle classical simulation of differentiable programming of the circuits. Using only 6 qubits one can obtain the ground state of a 4 × 4 square lattice frustrated Heisenberg model with fidelity over 97%. Arbitrarily long ranged correlations can also be measured on the same circuit after variational optimization.

We study quantum spin ice in an external magnetic field applied along a 100 direction. When quantum spin fluctuations are weak, elementary excitations are quantum strings with monopoles at their ends manifested as multiple spin-wave branches in the dynamical structure factor. Strong quantum fluctuations make the string tension negative and give rise to the deconfinement of monopoles. We discuss our results in the light of recent neutron scattering experiments in Yb2Ti2O7.PACS numbers: 75.40.Gb, 75.40.Mg The quest for novel quantum phases and elementary excitations is one of the central themes in condensedmatter physics. The notion of an elementary excitation is conventionally associated with a point-like object, as the term quasiparticle implies. A natural question is whether elementary excitations in quantum materials could resemble strings, rather than particles. String excitations were recently found in spin ice Dy 2 Ti 2 O 7 [1, 2], a frustrated ferromagnet with fractionalized excitations known as magnetic monopoles [3,4]. In an applied magnetic field, excitations are strings of misaligned spins connecting two monopoles of opposite charge.Conventional spin ice is a classical magnet with Ising spins [5]. Therefore, magnetic monopoles and strings in it are classical objects whose dynamics are due to thermal fluctuations. In this letter, we propose that string excitations with inherent quantum dynamics may exist in quantum spin ice, a new family of spin-ice materials exemplified by Tb 2 Ti 2 O 7 and Yb 2 Ti 2 O 7 [6] [7]. In these compounds, spins exhibit substantial quantum fluctuations. We demonstrate that, in a certain regime of coupling constants, elementary excitations of quantum spin ice are strings with quantum dynamics. The calculated dynamical structure factor S(ω, k) reveals multiple branches of excitations that correspond, loosely speaking, to strings of different lengths. As the applied field increases, these branches gradually separate and the lowest one evolves into a magnon. We connect these findings to recent experiments on neutron scattering in Yb 2 Ti 2 O 7 [8,9].We begin with a toy model of quantum spin ice on the two-dimensional checkerboard lattice, Fig. 1. The point of departure is classical spin ice, in which spins have projections S z i = ±1/2 on local directionsẑ i shown in Fig. 1a. Magnetic charge on a crossed plaquette (planar tetrahedron) is defined as Q = − i∈ S z i , with = ±1 for sublattice A (B). The ground states of the classical spin-ice Hamiltonian,obey the Bernal-Fowler rule, Q = 0, on every tetrahedron [5]. Next we apply a weak magnetic field in the ac plane. In the local frames, the perturbation readsHere we chose the local y-axes to be orthogonal to the field and introduced cosines η i ≡ĉ ·ẑ i = (−1) ci / √ 2. The Zeeman term (2) has two effects. Its longitudinal component B breaks the degeneracy of ice states and favors arXiv:1201.5314v3 [cond-mat.str-el]

We study the quantum spin dynamics of a frustrated XXZ model on a pyrochlore lattice by using large-scale quantum Monte Carlo simulation and stochastic analytic continuation. In the low-temperature quantum spin ice regime, we observe signatures of coherent photon and spinon excitations in the dynamic spin structure factor. As the temperature rises to the classical spin ice regime, the photon disappears from the dynamic spin structure factor, whereas the dynamics of the spinon remain coherent in a broad temperature window. Our results provide experimentally relevant, quantitative information for the ongoing pursuit of quantum spin ice materials.

We revisit the description of the low-energy singlet sector of the spin-1/2 Heisenberg antiferromagnet on kagome in terms of an effective quantum dimer model. With the help of exact diagonalizations of appropriate finite-size clusters, we show that the embedding of a given process in its kagome environment leads to dramatic modifications of the amplitudes of the elementary loop processes, an effect not accessible to the standard approach based on the truncation of the Hamiltonian to the nearest-neighbour valence-bond basis. The resulting parameters are consistent with a Z2 spin liquid rather than with a valence-bond crystal, in agreement with the last density matrix renormalization group results.

Pairing symmetry is important to indentify the pairing mechanism. The analysis becomes particularly timely and important for the newly discovered iron-based multi-orbital superconductors. From group theory point of view we classified all pairing matrices (in the orbital space) that carry irreducible representations of the system. The quasiparticle gap falls into three categories: full, nodal and gapless. The nodal-gap states show conventional Volovik effect even for on-site pairing. The gapless states are odd in orbital space, have a negative superfluid density and are therefore unstable. In connection to experiments we proposed possible pairing states and implications for the pairing mechanism.Introduction The newly discovered family of ironbased ReOFeAs(Re = La, Ce, Pr, etc.) high temperature superconductors are raising great interests in the community.[1] The superconductor consists of layers of FeAs which is believed to be the conducting planes. The ReO layers in between the FeAs layers stabilize the structure and donate carriers to the FeAs layers.

We show that the new technique of terahertz 2D coherent spectroscopy is capable of giving qualitatively new information about fractionalized spin systems. For the prototypical example of the transverse field Ising chain, we demonstrate theoretically that, despite the broad continuum of excitations in linear response, the 2D spectrum contains sharp features that are a coherent signature of a "spinon echo", which gives previously inaccessible information such as the lifetime of the two-spinon excited state. The effects of disorder and finite lifetime, which are practically indistinguishable in the linear optical or neutron response, manifest in dramatically different fashion in the 2D spectra. Our results may be directly applicable to model quasi-1D transverse field Ising chain systems such as CoNb2O6, but the concept can be applied to fractionalized spin systems in

Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO’s quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (⊥ b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets.

Second-order topological insulators are crystalline insulators with a gapped bulk and gapped crystalline boundaries, but with topologically protected gapless states at the intersection of two boundaries. Without further spatial symmetries, five of the ten Altland-Zirnbauer symmetry classes allow for the existence of such second-order topological insulators in two and three dimensions. We show that reflection symmetry can be employed to systematically generate examples of second-order topological insulators and superconductors, although the topologically protected states at corners (in two dimensions) or at crystal edges (in three dimensions) continue to exist if reflection symmetry is broken. A three-dimensional secondorder topological insulator with broken time-reversal symmetry shows a Hall conductance quantized in units of e 2 =h.