arXiv subjects

Hong Yao

Publications and source records attributed to Hong Yao.

At least 127 records · Page 7Linked to original sources

arXiv2016

Correlated triple-Weyl semimetals with Coulomb interactions

We study interaction effects, including both long-ranged Coulomb and short-range interactions, in three-dimensional topological triple-Weyl semimetals whose triple-Weyl points are protected by crystal symmetries. By performing Wilsonian renormalization group analysis of the low-energy effective field theory of the minimal model with triple-Weyl nodes, we find that the fixed point of noninteracting triple-Weyl fermions is unstable in the presence of Coulomb interactions and flows to a nontrivial stable fixed point representing marginal Fermi liquids with anisotropic screening effects. We further discuss relevant unusual physical consequences due to the novel behavior of correlation effects in this system.

cond-mat.str-el↗

arXiv2016

Fractional charge and emergent mass hierarchy in diagonal two-leg t-J cylinders

We define a new class of "diagonal" t-J ladders rotated by $π$/4 relative to the canonical lattice directions of the square lattice, and study it using density matrix renormalization group (DMRG). Here, we focus on the two-leg cylinder with a doped hole concentration near x = 1/4. At exactly x = 1/4, the system forms a period 4 charge density wave (CDW) and exhibits spin-charge separation. Slightly away from 1/4 doping we observe several topologically distinct types of solitons with well defined fractionalized quantum numbers. Remarkably, given the absence of any obvious small parameter, the effective masses of the emergent solitons differ by several orders of magnitude.

cond-mat.str-el↗

arXiv2016

Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe2

Weyl semimetal is a new quantum state of matter [1-12] hosting the condensed matter physics counterpart of relativisticWeyl fermion [13] originally introduced in high energy physics. The Weyl semimetal realized in the TaAs class features multiple Fermi arcs arising from topological surface states [10, 11, 14-16] and exhibits novel quantum phenomena, e.g., chiral anomaly induced negative mag-netoresistance [17-19] and possibly emergent supersymmetry [20]. Recently it was proposed theoretically that a new type (type-II) of Weyl fermion [21], which does not have counterpart in high energy physics due to the breaking of Lorentz invariance, can emerge as topologically-protected touching between electron and hole pockets. Here, we report direct spectroscopic evidence of topological Fermi arcs in the predicted type-II Weyl semimetal MoTe2 [22-24]. The topological surface states are confirmed by directly observing the surface states using bulk-and surface-sensitive angle-resolved photoemission spectroscopy (ARPES), and the quasi-particle interference (QPI) pattern between the two putative Fermi arcs in scanning tunneling microscopy (STM). Our work establishes MoTe2 as the first experimental realization of type-II Weyl semimetal, and opens up new opportunities for probing novel phenomena such as exotic magneto-transport [21] in type-II Weyl semimetals.

cond-mat.mes-hall↗

arXiv2016

Lorentz-violating type-II Dirac fermions in transition metal dichalcogenide PtTe$_2$

Topological semimetals have recently attracted extensive research interests as host materials to condensed matter physics counterparts of Dirac and Weyl fermions originally proposed in high energy physics. These fermions with linear dispersions near the Dirac or Weyl points obey Lorentz invariance, and the chiral anomaly leads to novel quantum phenomena such as negative magnetoresistance. The Lorentz invariance is, however, not necessarily respected in condensed matter physics, and thus Lorentz-violating type-II Dirac fermions with strongly tilted cones can be realized in topological semimetals. Here, we report the first experimental evidence of type-II Dirac fermions in bulk stoichiometric PtTe$_2$ single crystal. Angle-resolved photoemission spectroscopy (ARPES) measurements and first-principles calculations reveal a pair of strongly tilted Dirac cones along the $Γ$-A direction under the symmetry protection, confirming PtTe$_2$ as a type-II Dirac semimetal. The realization of type-II Dirac fermions opens a new door for exotic physical properties distinguished from type-I Dirac fermions in condensed matter materials.

cond-mat.mtrl-sci↗

arXiv2016

Charge-4e superconductors: a Majorana quantum Monte Carlo study

Many features of charge-4e superconductors remain unknown because even the "mean-field Hamiltonian" describing them is an interacting model. Here we introduce an interacting model to describe a charge-4e superconductor (SC) deep in the superconducting phase, and explore its properties using quantum Monte Carlo (QMC) simulations. The QMC is sign-problem-free, but only when a Majorana representation is employed. As a function of the chemical potential we observe two sharply-distinct behaviors: a "strong" quarteting phase in which charge-4e quartets are tightly bound (like molecules) so that charge-2e pairing does not occur even in the temperature $T \to 0$ limit, and a "weak" quarteting phase in which a further transition to a charge-2e superconducting phase occurs at a lower critical temperature. Analogous issues arise in a putative Z4 spin-liquid with a pseudo-Fermi surface and in characterizing charge-2e SCs with "odd frequency pairing" (or equivalently with a "composite order parameter") and metallic spin-nematics. Under certain circumstances, we also identified a stable T=0 charge-4e SC phase with gapless nodal quasiparticles. We further discuss possible relevance of our results to various experimental observations in 1/8-doped LBCO.

cond-mat.supr-con↗

arXiv2016

Topological photonic crystal with equifrequency Weyl points

Weyl points in three-dimensional photonic crystals behave as monopoles of Berry flux in momentum space. Here, based on general symmetry analysis, we show that a minimal number of four symmetry-related (consequently equifrequency) Weyl points can be realized in time-reversal invariant photonic crystals. We further propose an experimentally-feasible way to modify double-gyroid photonic crystals to realize four equifrequency Weyl points, which is explicitly confirmed by our first-principle photonic band-structure calculations. Remarkably, photonic crystals with equifrequency Weyl points are qualitatively advantageous in applications including angular selectivity, frequency selectivity, invisibility cloaking, and three dimensional imaging.

cond-mat.mes-hall↗

arXiv2016

What makes the Tc of monolayer FeSe on SrTiO3 so high: a sign-problem-free quantum Monte Carlo study

Monolayer FeSe films grown on SrTiO3 (STO) substrate show superconducting gap-opening temperatures (Tc) which are almost an order of magnitude higher than those of the bulk FeSe and are highest among all known Fe-based superconductors. Angle-resolved photoemission spectroscopy (ARPES) observed "replica bands" suggesting the importance of the interaction between FeSe electrons and STO phonons. These facts rejuvenated the quest for Tc enhancement mechanisms in iron-based, especially iron-chalcogenide, superconductors. Here, we perform the first numerically-exact sign-problem-free quantum Monte Carlo simulations to iron-based superconductors. We (i) study the electronic pairing mechanism intrinsic to heavily electron doped FeSe films, and (ii) examine the effects of electron-phonon interaction between FeSe and STO as well as nematic fluctuations on Tc. Armed with these results, we return to the question "what makes the Tc of monolayer FeSe on SrTiO3 so high?" in the conclusion and discussions.

cond-mat.supr-con↗

arXiv2016

Ideal Weyl semimetals in the chalcopyrites CuTlSe2, AgTlTe2, AuTlTe2 and ZnPbAs2

Weyl semimetals are new states of matter which feature novel Fermi arcs and exotic transport phenomena. Based on first-principles calculations, we report that the chalcopyrites CuTlSe2, AgTlTe2, AuTlTe2 and ZnPbAs2 are ideal Weyl semimetals, having largely separated Weyl points (~ 0.05/A) and uncovered Fermi arcs that are amenable to experimental detections. We also construct a minimal effective model to capture the low-energy physics of this class of Weyl semimetals. Our discovery is a major step toward a perfect playground of intriguing Weyl semimetals and potential applications for low-power and high-speed electronics.

cond-mat.mes-hall↗

arXiv2016

Detecting monopole charge in Weyl semimetals via quantum interference transport

Topological Weyl semimetals can host Weyl nodes with monopole charges in momentum space. How to detect the signature of the monopole charges in quantum transport remains a challenging topic. Here, we reveal the connection between the parity of monopole charge in topological semimetals and the quantum interference corrections to the conductivity. We show that the parity of monopole charge determines the sign of the quantum interference correction, with odd and even parity yielding the weak anti-localization and weak localization effects, respectively. This is attributed to the Berry phase difference between time-reversed trajectories circulating the Fermi sphere that encloses the monopole charges. From standard Feynman diagram calculations, we further show that the weak-field magnetoconductivity at low temperatures is proportional to $+\sqrt{B}$ in double-Weyl semimetals and $-\sqrt{B}$ in Weyl semimetals, respectively, which could be verified experimentally.

cond-mat.mes-hall↗

arXiv2016

Visualizing the elongated vortices in $γ$-Ga nanostrips

We study the magnetic response of superconducting $γ$-Ga via low temperature scanning tunneling microscopy and spectroscopy. The magnetic vortex cores rely substantially on the Ga geometry, and exhibit an unexpectedly-large axial elongation with aspect ratio up to 40 in rectangular Ga nano-strips (width $l$ $<$ 100 nm). This is in stark contrast with the isotropic circular vortex core in a larger round-shaped Ga island. We suggest that the unusual elongated vortices in Ga nanostrips originate from geometric confinement effect probably via the strong repulsive interaction between the vortices and Meissner screening currents at the sample edge. Our finding provides novel conceptual insights into the geometrical confinement effect on magnetic vortices and forms the basis for the technological applications of superconductors.

cond-mat.supr-con↗

arXiv2016

Bulk Fermi surface of charge-neutral excitations in SmB6 or not: a heat-transport study

Recently there have been increasingly hot debates on whether a bulk Fermi surface of charge-neutral excitations exists in the topological Kondo insulator SmB6. To unambiguously resolve this issue, we performed the low-temperature thermal conductivity measurements of a high-quality SmB6 single crystal down to 0.1 K and up to 14.5 T. Our experiments show that the residual linear term of thermal conductivity at zero field is zero, within the experimental accuracy. Furthermore, the thermal conductivity is insensitive to magnetic field up to 14.5 T. These results exclude the existence of fermionic charge-neutral excitations in bulk SmB6, such as scalar Majorana fermions or spinons, thus put a strong constraint on the explanation of the quantum oscillations observed in SmB6.

cond-mat.str-el↗

arXiv2016

Symmetry-protected ideal Weyl semimetal in HgTe-class materials

Ideal Weyl semimetals with all Weyl nodes exactly at the Fermi level and no coexisting trivial Fermi surfaces in the bulk, similar to graphene, could feature deep physics such as exotic transport phenomena induced by the chiral anomaly. Here, we show that HgTe and half-Heusler compounds, under a broad range of in-plane compressive strain, could be materials in nature realizing ideal Weyl semimetals with four pairs of Weyl nodes and topological surface Fermi arcs. Generically, we find that the HgTe-class materials with nontrivial band inversion and noncentrosymmetry provide a promising arena to realize ideal Weyl semimetals. Such ideal Weyl semimetals could further provide a unique platform to study emergent phenomena such as the interplay between ideal Weyl fermions and superconductivity in the half-Heusler compound LaPtBi.

cond-mat.mes-hall↗

arXiv2015

The nature of effective interaction in cuprate superconductors: a sign-problem-free quantum Monte-Carlo study

Superconductivity is an emergent phenomena in the sense that the energy scale associated with Cooper pairing is generically much lower than the typical kinetic energy of electrons. Addressing the mechanism of Cooper pairing amounts to determine the effective interaction that operates at low energies. Deriving such an interaction from a bottom-up approach has not been possible for any superconductor, especially strongly correlated ones. Top-down approaches, where one assumes an effective interaction, is plagued with the difficulty of extracting the implied electronic instabilities without uncontrolled approximations. These facts severely hinder our ability to determine the pairing mechanism for high temperature superconductors. Here we perform large-scale sign-problem-free quantum Monte-Carlo simulations on an effective theory, featured with antiferromagnetic and nematic fluctuations, to study the intertwined antiferromagnetic, superconducting, and charge density wave instabilities of the cuprates. Our results suggest the inclusion of nematic fluctuations is essential in order to produce the observed type of charge density wave ordering. Interestingly we find that the d-wave Cooper pairing is enhanced by nematic fluctuations.

cond-mat.supr-con↗

arXiv2015

Topological p+ip superconductivity in doped graphene-like single-sheet materials BC$_3$

We theoretically study exotic superconducting phases in graphenelike single-sheet material BC$_3$ doped to its type-II Van Hove singularity whose saddle point momenta are {\it not} time-reversal-invariant. From combined renormalization group analysis and RPA calculations, we show that the dominant superconducting instability induced by weak repulsive interactions is in the time-reversal-invariant $p+ip$ pairing channel because of the interplay among dominant ferromagnetic fluctuations, subleading spin fluctuations at finite momentum, and spin-orbit coupling. Such time-reversal-invariant $p+ip$ superconductivity has nontrivial $\mathbb{Z}_2$ topological invariant. Our results show that doped BC$_3$ provides a promising route to realize a genuine 2D helical $p+ip$ superconductor.

cond-mat.str-el↗

arXiv2015

Double-semion topological order from exactly solvable quantum dimer models

We construct a generalized quantum dimer model on two-dimensional nonbipartite lattices including the triangular lattice, the star lattice and the kagome lattice. At the Rokhsar-Kivelson (RK) point, we obtain its exact ground states that are shown to be a fully gapped quantum spin liquid with the double-semion topological order. The ground-state wave function of such a model at the RK point is a superposition of dimer configurations with a nonlocal sign structure determined by counting the number of loops in the transition graph. We explicitly demonstrate the double-semion topological order in the ground states by showing the semionic statistics of monomer excitations. We also discuss possible implications of such double-semion resonating valence bond states to candidate quantum spin-liquid systems discovered experimentally and numerically in the past few years.

cond-mat.str-el↗

arXiv2015

Fermion-sign-free Majarana-quantum-Monte-Carlo studies of quantum critical phenomena of Dirac fermions in two dimensions

Quantum critical phenomena may be qualitatively different when massless Dirac fermions are present at criticality. Using our recently-discovered fermion-sign-free Majorana quantum Monte Carlo (MQMC) method introduced by us in Ref. [1], we investigate the quantum critical phenomena of {\it spinless} Dirac fermions at their charge-density-wave (CDW) phase transitions on the honeycomb lattice having $N_s=2L^2$ sites with largest $L=24$. By finite-size scaling, we accurately obtain critical exponents of this so-called Gross-Neveu chiral-Ising universality class of {\it two} (two-component) Dirac fermions in 2+1D: $η=0.45(2)$, $ν=0.77(3)$, and $β=0.60(3)$, which are qualitatively different from the mean-field results but are reasonably close to the ones obtained from renormalization group calculations.

cond-mat.str-el↗

arXiv2015

Correlated double-Weyl semimetals with Coulomb interactions: possible applications to HgCr2Se4 and SrSi2

We study the fate of double-Weyl fermions in three-dimensional systems in the presence of long-range Coulomb interactions. By employing the momentum-shell renormalization group approach, we find that the fixed point of noninteracting double-Weyl fermions is unstable against Coulomb interactions and flows to a nontrivial stable fixed point with anisotropic screening effect. Moreover, experimentally measurable quantities such as specific heat obtain unusual logarithmic corrections. We also discuss implications of our results to three-dimensional materials HgCr2Se4 and SrSi2, candidate materials of hosting double-Weyl fermions.

cond-mat.str-el↗

arXiv2015

Topological Odd-Parity Superconductivity at Type-II 2D Van Hove Singularities

We study unconventional superconductivity induced by weak repulsive interactions in 2D electronic systems at Van Hove singularity (VHS) where density of states is logarithmically divergent. We define two types of VHS. For systems at type-I VHS, weak repulsive interactions generically induce unconventional singlet pairing. However and more interestingly, for type-II VHS renormalization group (RG) analysis shows that weak repulsive interactions favor triplet pairing (e.g. p-wave) when the Fermi surface is not sufficiently nested. For type-II VHS systems respecting tetragonal symmetry, topological superconductivity (either chiral p+ip pairing or time-reversal invariant $Z_2$ p+ip pairing) occurs generally. We shall also discuss relevance of our study to materials including recently discovered superconductors LaO$_{1-x}$F$_x$BiS$_2$ which can be tuned to type-II VHS by doping.

cond-mat.supr-con↗