Oct 2017

Oct 1-Oct 5 Biao Huang, Oct 6-Oct 10 Haiyuan Zou, Oct 11-Oct 15 Zehan Li, Oct 16-Oct 20 Jiansong Pan, Oct 21-Oct 25 Ahmet Keles, Oct 26-Oct 30 Max Arzamasovs, Oct 31- Xuguang Yue

Oct 20
arXiv:1710.06903 [pdf, other]
Yang monopoles and emergent three-dimensional topological defects in interacting bosons
Yangqian Yan, Qi Zhou
Comments: 6 pages (2 figures) + 5 pages (2 figures)
Subjects: Quantum Gases (cond-mat.quant-gas)
Yang monopole as a zero-dimensional topological defect has been well established in multiple fields in physics. However, it remains an intriguing question to understand interaction effects on Yang monopoles. Here, we show that collective motions of many interacting bosons give rise to exotic topological defects that are distinct from Yang monopoles seen by a single particle. Whereas interactions may distribute Yang monopoles in the parameter space or glue them to a single giant one of multiple charges, three-dimensional topological defects also arise from continuous manifolds of degenerate many-body eigenstates. Their projections in lower dimensions lead to knotted nodal lines and nodal rings. Our results suggest that ultracold bosonic atoms can be used to create emergent topological defects and directly measure topological invariant that are not easy to access in solids.



arXiv:1710.07162 [pdf, other]
Anisotropic polarizability of erbium atoms
Jan Hendrik Becher, Simon Baier, Kiyotaka Aikawa, Maxence Lepers, Jean-François Wyart, Olivier Dulieu, Francesca Ferlaino
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
We report on the determination of the dynamical polarizability of ultracold erbium atoms in the ground and in one excited state at three different wavelengths, which are particularly relevant for optical trapping. Our study combines experimental measurements of the light shift and theoretical calculations. In particular, our experimental approach allows us to isolate the different contributions to the polarizability, namely the isotropic scalar and anisotropic tensor part. For the latter contribution, we observe a clear dependence of the atomic polarizability on the angle between the laser-field-polarization axis and the quantization axis, set by the external magnetic field. Such an angle-dependence is particularly pronounced in the excited-state polarizability. We compare our experimental findings with the theoretical values, based on semi-empirical electronic-structure calculations and we observe a very good overall agreement. Our results pave the way to exploit the anisotropy of the tensor polarizability for spin-selective preparation and manipulation.




Oct 18

arXiv:1710.05927 (cross-list from cond-mat.str-el) [pdf, other]
Floquet Supersymmetry
Thomas Iadecola, Timothy H. Hsieh
Comments: 5+2 pages, 3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We introduce the notion of time-reflection symmetry in periodically driven (Floquet) quantum systems, and show that it enables a Floquet variant of quantum-mechanical supersymmetry. In particular, we find Floquet analogues of the Witten index that place lower bounds on the degeneracies of states with quasienergies 0 and \pi. We provide a simple class of disordered, interacting, and ergodic Floquet models with an exponentially large number of states at quasienergies 0,\pi, which are robust as long as the time-reflection symmetry is preserved. Floquet supersymmetry manifests itself in the evolution of certain local observables as a period-doubling effect with dramatic finite-size scaling, providing a clear signature for experiments.

arXiv:1710.06369 [pdf, other]
Momentum Space Josephson Effects
Junpeng Hou, Xi-Wang Luo, Kuei Sun, Thomas Bersano, Vandna Gokhroo, Sean Mossman, Peter Engels, Chuanwei Zhang
Comments: 10 pages, 7 figuresSubjects: Quantum Gases (cond-mat.quant-gas)


The Josephson effect is a prominent phenomenon of quantum supercurrents that has been widely studied in superconductors and superfluids. Typical Josephson junctions consist of two real-space superconductors (superfluids) coupled through a weak tunneling barrier. Here we propose a momentum-space Josephson junction in a spin-orbit coupled Bose-Einstein condensate, where states with two diffferent momenta are coupled through Raman-assisted tunneling. We show that Josephson currents can be induced not only by applying the equivalent of "voltages", but also by tuning tunneling phases. Such tunneling-phase-driven Josephson junctions in momentum space are characterized through both full mean field analysis and a concise two-level model, demonstrating the important role of interactions between atoms. Our scheme provides a platform for experimentally realizing momentum-space Josephson junctions and exploring their applications in quantum-mechanical circuits.

Oct 17

arXiv:1710.05289 (cross-list from cond-mat.stat-mech) [pdf, other]
Entanglement-Spectrum Crossing and Momentum-Time Skyrmions in Quench Dynamics
Zongping Gong, Masahito Ueda
Comments: 6+6 pages, 4+5 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

By studying the quench dynamics in one-dimensional superlattice systems with inversion symmetry, we find robust crossings in single-particle entanglement spectra for quantum quenches between different symmetry-protected topological phases. The physics behind this phenomenon is the emergence of a dynamical Chern number accompanied by unitarily created momentum-time Skyrmions. We also discuss a possible experimental situation based on Bloch-state tomography in ultracold atomic systems. Our work identifies the unique role of topology in quantum dynamics far from equilibrium.




Oct 16
arXiv:1710.04730 [pdf, other]
Bosonic topological phases of matter: bulk-boundary correspondence, SPT invariants and gauging
Apoorv Tiwari, Xiao Chen, Ken Shiozaki, Shinsei Ryu
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th)
We analyze 2+1d and 3+1d Bosonic Symmetry Protected Topological (SPT) phases of matter protected by onsite symmetry group G by using dual bulk and boundary approaches. In the bulk we study an effective field theory which upon coupling to a background flat G gauge field furnishes a purely topological response theory. The response action evaluated on certain manifolds, with appropriate choice of background gauge field, defines a set of SPT topological invariants. Further, SPTs can be gauged by summing over all isomorphism classes of flat G gauge fields to obtain Dijkgraaf-Witten topological G gauge theories. These topological gauge theories can be ungauged by first introducing and then proliferating defects that spoils the gauge symmetry. This mechanism is related to anyon condensation in 2+1d and condensing bosonic gauge charges in 3+1d. In the dual boundary approach, we study 1+1d and 2+1d quantum field theories that have G 't-Hooft anomalies that can be precisely cancelled by (the response theory of) the corresponding bulk SPT. We show how to construct/compute topological invariants for the bulk SPTs directly from the boundary theories. Further we sum over boundary partition functions with different background gauge fields to construct G-characters that generate topological data for the bulk topological gauge theory. Finally, we study a 2+1d quantum field theory with a mixed ZT/R2×U(1) anomaly where ZT/R2 is time-reversal/reflection symmetry, and the U(1) could be a 0-form or 1-form symmetry depending on the choice of time reversal/reflection action. We briefly discuss the bulk effective action and topological response for a theory in 3+1d that cancels this anomaly. This signals the existence of SPTs in 3+1d protected by 0,1-form U(1)×ZT,R2.



Oct 14
arXiv:1710.04421 [pdf, ps, other]
Vortex pinning by the point potential in topological superconductors: a scheme for braiding Majorana bound states
Hai-Dan Wu, Tao Zhou
Comments: 5 pages, 5 figures
Subjects: Superconductivity (cond-mat.supr-con); Quantum Physics (quant-ph)
We propose theoretically an effective scheme for braiding Majorana bound states by manipulating the point potential. The vortex pinning effect is carefully elucidated. This effect may be used to control the vortices and Majorana bound states in topological superconductors. The exchange of two vortices induced by moving the potentials is simulated numerically. The zero-energy state in the vortex core is robust with respect to the strength of the potential. The Majorana bound states in a pinned vortex are identified numerically.


Oct 13
arXiv:1710.04519 [pdf, other]
Interband interference effects at the edge of a multiband chiral p-wave superconductor
Jia-Long Zhang, Wen Huang, Manfred Sigrist, Dao-Xin Yao
Comments: 8 pages, 8 figures
Subjects: Superconductivity (cond-mat.supr-con)
Chiral superconductors support chiral edge modes and potentially spontaneous edge currents at their boundaries. Motivated by the putative multiband chiral p-wave superconductor Sr2RuO4, we study the influence of the interference between different bands at the edges, which may appear in the presence of moderate edge disorder or in edge tunneling measurements. We show that interband interference can strongly modify the measurable quantities at the edges when the order parameter exhibits phase difference between the bands. This is illustrated by investigating the edge dispersion and the edge current distribution in the presence of interband mixing, as well as the conductance at a tunneling junction. The results are discussed in connection with the putative chiral p-wave superconductor Sr2RuO4. In passing, we also discuss similar interference effects in multiband models with other pairing symmetries.

Oct 11
arXiv:1710.03664 [pdf, ps, other]
Flipping-shuttle oscillations of bright one- and two-dimensional solitons in spin-orbit-coupled Bose-Einstein condensates with Rabi mixing
Hidetsugu Sakaguchi, Boris A. Malomed
Comments: Physical Review A, in press
Subjects: Quantum Gases (cond-mat.quant-gas); Pattern Formation and Solitons (nlin.PS)

We analyze a possibility of macroscopic quantum effects in the form of coupled structural oscillations and shuttle motion of bright two-component spin-orbit-coupled striped (one-dimensional, 1D) and semi-vortex (two-dimensional, 2D) matter-wave solitons, under the action of linear mixing (Rabi coupling) between the components. In 1D, the intrinsic oscillations manifest themselves as flippings between spatially even and odd components of striped solitons, while in 2D the system features periodic transitions between zero-vorticity and vortical components of semi-vortex solitons. The consideration is performed by means of a combination of analytical and numerical methods.

Oct 10
arXiv:1710.02788 [pdf, other]
SU(3) Topological Insulators in the Honeycomb Lattice
Ulrike Bornheimer, Christian Miniatura and, Benoît Grémaud
Comments: 12 pages, 8 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We investigate realizations of topological insulators with spin-1 bosons loaded in a honeycomb optical lattice and subjected to a SU(3) spin-orbit coupling - a situation which can be realized experimentally using cold atomic gases. In this paper, we focus on the topological properties of the single-particle band structure, namely Chern numbers (lattice with periodic boundary conditions) and edge states (lattice with strip geometry). While SU(2) spin-orbit couplings always lead to time-reversal symmetric Hubbard models, and thereby to topologically trivial band structures, suitable SU(3) spin-orbit couplings can break time reversal symmetry and lead to topologically non-trivial bulk band structures and to edge states in the strip geometry. In addition, we show that one can trigger a series of topological transitions (i.e. integer changes of the Chern numbers) that are specific to the geometry of the honeycomb lattice by varying a single parameter in the Hamiltonian.
Oct 9
arXiv:1710.02152 [pdf, other]
Controlled parity switch of persistent currents in quantum ladders
Michele Filippone, Charles-Edouard Bardyn, Thierry Giamarchi
Comments: 5 pages, 4 figures + Supplemental Material
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

We investigate the behavior of persistent currents for a fixed number of noninteracting fermions in a periodic quantum ladder threaded by Aharonov-Bohm and transverse magnetic fluxes Φ and χ. We show that the coupling between ladder legs provides a way to effectively change the ground-state fermion-number parity, by varying χ. Specifically, we demonstrate that varying χ by 2π (one flux quantum) leads to an apparent fermion-number parity switch. We find that persistent currents exhibit a robust 4π periodicity as a function of χ, despite the fact that χ→χ+2π leads to modifications of order 1/N of the energy spectrum, where N is the number of sites in each ladder leg. We show that these parity-switch and 4π periodicity effects are robust with respect to temperature and disorder, and outline potential physical realizations using cold atomic gases and, for bosonic analogs of the effects, photonic lattices.

Oct 6

arXiv:1710.02070 [pdf, other]
Dirac and topological phonons with spin-orbital entangled orders
Yan-Qi Wang, Xiong-Jun Liu
Comments: 5 pages, 4 figures, and Supplementary Material
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

We propose to study novel quantum phases and excitations for a 2D spin-orbit (SO) coupled bosonic p-orbital optical lattice based on the recent experiments. The orbital and spin degrees of freedom with SO coupling compete and bring about nontrivial interacting quantum effects. We develop a self-consistent method for bosons and predict a spin-orbital entangled order for the ground phase, in sharp contrast to spinless high-orbital systems. Furthermore, we investigate the Bogoliubov excitations, showing that the Dirac and topological phonons are obtained corresponding to the predicted different spin-orbital orders. In particular, the topological phonons exhibit a bulk gap which can be several times larger than the single-particle gap of p-bands, reflecting the enhancement of topological effect by interaction. Our results highlight the rich new physics predicted in SO coupled high-orbital systems and shall attract experimental efforts in the near future.




Oct 05
Observation of half-integer thermal Hall conductance
Mitali Banerjee, Moty Heiblum, Vladimir Umansky, Dima E. Feldman, Yuval Oreg, Ady Stern
(Submitted on 2 Oct 2017)

Topological states of matter are characterized by topological invariant, which are physical quantities whose values are quantized and do not depend on details of the measured system. Of these, the easiest to probe in experiments is the electrical Hall conductance, which is expressed in units of e2/h (e the electron charge, h the Planck's constant). In the fractional quantum Hall effect (FQHE), fractional quantized values of the electrical Hall conductance attest to topologically ordered states, which are states that carry quasi particles with fractional charge and anyonic statistics. Another topological invariant, which is much harder to measure, is the thermal Hall conductance, expressed in units of κ0T=(π2kB2/3h)T (kB the Boltzmann constant, T the temperature). For the quantized thermal Hall conductance, a fractional value attests that the probed state of matter is non-abelian. Quasi particles in non-abelian states lead to a ground state degeneracy and perform topological unitary transformations among ground states when braided. As such, they may be useful for topological quantum computation. In this paper, we report our measurements of the thermal Hall conductance for several quantum Hall states in the first excited Landau level. Remarkably, we find the thermal Hall conductance of the ν=5/2 state to be fractional, and to equal 2.5κ0T


Oct 03
arXiv:1710.00851 [pdf, other]
Artificial gauge fields in materials and engineered systems
M. Aidelsburger, S. Nascimbene, N. Goldman
Comments: 35 pages (+ references), 19 figures. First version submitted to the journal. Comments are most welcome!
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

Artificial gauge fields are currently realized in a wide range of physical settings. This includes solid-state devices but also engineered systems, such as photonic crystals, ultracold gases and mechanical setups. It is the aim of this review to offer, for the first time, a unified view on these various forms of artificial electromagnetic fields and spin-orbit couplings for matter and light. This topical review provides a general introduction to the universal concept of engineered gauge fields, in a form which is accessible to young researchers entering the field. Moreover, this work aims to connect different communities, by revealing explicit links between the diverse forms and realizations of artificial gauge fields.



Oct 02
arXiv:1710.00717 [pdf, other]
Long-lived 2D Spin-Orbit coupled Topological Bose Gas
Wei Sun, Bao-Zong Wang, Xiao-Tian Xu, Chang-Rui Yi, Long Zhang, Zhan Wu, Youjin Deng, Xiong-Jun Liu, Shuai Chen, Jian-Wei Pan
Comments: 11 pages, 5 figures with appendix 5 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)

To realize high-dimensional spin-orbit (SO) couplings for ultracold atoms is of great importance for quantum simulation. Here we report the observation of a long-lived two-dimensional (2D) SO coupled Bose-Einstein condensate (BEC) of novel band topology and high controllability. Unlike our recent achievement of 2D SO coupling which is restricted in blue-detuned optical lattice and has limitations in controllability and lifetime, in the present report we overwhelm all the previous restrictions and realize the SO coupling with precisely controllable C4 symmetry based on a new scheme. Due to the high symmetry, the present realization suits for both blue- and red-tuned lattices, renders broad topological regions in arbitrary lattice and Raman coupling strengths, and has a lifetime being up to several seconds, one order exceeding that observed in previous experiment. We measure a stable crossover between 1D and 2D SO couplings, map the band structure through spin injection radio-frequency spectroscopy, and observe topological phase boundaries which are well consistent with theoretical predictions. The high controllability and long lifetime of the 2D SO coupled degenerate atom gas pave the way for the further studies of exotic quantum phenomena with novel topology, particularly for the quantum many-body physics and quantum far-from-equilibrium dynamics.







Oct 01
arXiv:1709.10286 [pdf, ps, other]
Thermal Hall effect in a Kitaev spin liquid: A possible signature of Majorana chiral edge current
Y. Kasahara, K. Sugii, T. Ohnishi, M. Shimozawa, M. Yamashita, N. Kurita, H. Tanaka, J. Nasu, Y. Motome, T. Shibauchi, Y. Matsuda
Comments: 7 pages, 6 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

Quantum spin liquids (QSLs) are novel states of matter lacking magnetic order while possessing some special patterns of quantum mechanical entanglement. Among the long-standing experimental challenges associated with the identification of these exotic states, is the detection of fractionalized excitations, which are signatures of topological order inherent to the QSL. The Kitaev spin model on a honeycomb lattice hosts a topological QSL state that displays the fractionalization of spins into itinerant Majorana fermions and Z2 fluxes. Theoretical studies predict that the itinerant Majorana fermions will manifest themselves in the form of a finite thermal Hall effect which is quantized in the zero-temperature limit. This anomalous heat transport is directly linked to the existence of topologically protected chiral edge currents, a feature commonly discussed in topological superconductors. Here we report on thermal Hall conductivity κxy measurements in α-RuCl3, a candidate Kitaev magnet with the two-dimensional honeycomb lattice. In a spin-liquid (Kitaev paramagnetic) state below the temperature characterized by the Kitaev interaction JK/kB∼80\,K, positive κxy develops gradually upon cooling, demonstrating the presence of highly unusual itinerant excitations. We find that the temperature dependence of κxy/T follows the predicted trend of the itinerant Majorana excitations. Remarkably, in the spin-liquid state, κxy/T per two-dimensional layer reaches about a half of the quantization value of (π/12)(k2B/ℏ). Although the zero-temperature property is masked by the magnetic ordering at TN=7\,K, the sign, magnitude, and T-dependence of κxy/T at intermediate temperatures altogether provide a possible signature of Majorana chiral edge current in the Kitaev QSL.


arXiv:1709.10127 [pdf, ps, other]
Topological Phase Transitions from Harper to Fibonacci Crystals
Guy Amit, Itzhack Dana
Comments: 9 pages, 5 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Other Condensed Matter (cond-mat.other); Strongly Correlated Electrons (cond-mat.str-el)

Topological properties of Harper and generalized Fibonacci chains are studied in the crystalline case, i.e., for rational values of the modulation frequency. The Harper and Fibonacci crystals at fixed frequency are connected by an interpolating one-parameter Hamiltonian. As the parameter is varied, one observes topological phase transitions, i.e., changes in the Chern integers of two bands due to the degeneracy of these bands at some parameter value. For small frequency, corresponding to a semiclassical regime, the degeneracies are shown to occur when the average energy of the two bands is approximately equal to the energy of the classical separatrix. Spectral and topological features of the Fibonacci crystal for small frequency leave a clear imprint on the corresponding Hofstadter butterfly for arbitrary frequency.