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arXiv2013

Simulations of non-Abelian gauge theories with optical lattices

Many phenomena occurring in strongly correlated quantum systems still await conclusive explanations. The absence of isolated free quarks in nature is an example. It is attributed to quark confinement, whose origin is not yet understood. The phase diagram for nuclear matter at general temperatures and densities, studied in heavy-ion collisions, is not settled. Finally, we have no definitive theory of high-temperature superconductivity. Though we have theories that could underlie such physics, we lack the tools to determine the experimental consequences of these theories. Quantum simulators may provide such tools. Here we show how to engineer quantum simulators of non-Abelian lattice gauge theories. The systems we consider have several applications: they can be used to mimic quark confinement or to study dimer and valence-bond states (which may be relevant for high-temperature superconductors).

cond-mat.quant-gas↗

arXiv2013

Heterointerface engineered electronic and magnetic phases of NdNiO3 thin films

Mott physics is characterized by an interaction-driven metal-to-insulator transition in a partially filled band. In the resulting insulating state, antiferromagnetic orders of the local moments typically develop, but in rare situations no long-range magnetic order appears, even at zero temperature, rendering the system a quantum spin liquid. A fundamental and technologically critical question is whether one can tune the underlying energetic landscape to control both metal-to-insulator and Néel transitions, and even stabilize latent metastable phases, ideally on a platform suitable for applications. Here we demonstrate how to achieve this in ultrathin films of NdNiO3 with various degrees of lattice mismatch, and report on the quantum critical behaviours not reported in the bulk by transport measurements and resonant X-ray spectroscopy/scattering. In particular, on the decay of the antiferromagnetic Mott insulating state into a non-Fermi liquid, we find evidence of a quantum metal-to-insulator transition that spans a non-magnetic insulating phase.

cond-mat.str-el↗

arXiv2014

Frustration-induced nanometre-scale inhomogeneity in a triangular antiferromagnet

Phase inhomogeneity of otherwise chemically homogenous electronic systems is an essential ingredient leading to fascinating functional properties, such as high-$T_c$ superconductivity in cuprates, colossal magnetoresistance in manganites, and giant electrostriction in relaxors. In these materials distinct phases compete and can coexist due to intertwined ordered parameters. Charge degrees of freedom play a fundamental role, although phase-separated ground states have been envisioned theoretically also for pure spin systems with geometrical frustration that serves as a source of phase competition. Here we report a paradigmatic magnetostructurally inhomogenous ground state of the geometrically frustrated $α$-NaMnO$_2$ that stems from the system's aspiration to remove magnetic degeneracy and is possible only due to the existence of near-degenerate crystal structures. Synchrotron X-ray diffraction, nuclear magnetic resonance and muon spin relaxation show that the spin configuration of a monoclinic phase is disrupted by magnetically short-range ordered nanoscale triclinic regions, thus revealing a novel complex state of matter.

cond-mat.str-el↗

arXiv2014

Ultrafast observation of critical nematic fluctuations and giant magnetoelastic coupling in iron pnictides

Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behavior these materials exhibit. Yet, the origin of the observed anisotropy is unclear. Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe_(1-x)Co_x)_2As_2. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.

cond-mat.str-el↗

arXiv2014

Direct measurement of the upper critical field in a cuprate superconductor

The upper critical field Hc2 is a fundamental measure of the pairing strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. We have used thermal conductivity as a direct probe of Hc2 in the cuprates YBa2Cu3Oy and YBa2Cu4O8 to show that there is no vortex liquid at T = 0, allowing us to use high-field resistivity measurements to map out the doping dependence of Hc2 across the phase diagram. Hc2(p) exhibits two peaks, each located at a critical point where the Fermi surface undergoes a transformation. The condensation energy obtained directly from Hc2, and previous Hc1 data, undergoes a 20-fold collapse below the higher critical point. These data provide quantitative information on the impact of competing phases in suppressing superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2014

Giant magnetic anisotropy and tunnelling of the magnetization in Li2(Li_{1-x}Fe_x)N

Large magnetic anisotropy and coercivity are key properties of functional magnetic materials and are generally associated with rare earth elements. Here we show an extreme, uniaxial magnetic anisotropy and the emergence of magnetic hysteresis in Li2(Li1-xFex)N. An extrapolated, magnetic anisotropy field of 220 Tesla and a coercivity field of over 11 Tesla at 2 Kelvin outperform all known hard-ferromagnets and single-molecule magnets (SMMs). Steps in the hysteresis loops and relaxation phenomena in striking similarity to SMMs are particularly pronounced for x << 1 and indicate the presence of nano-scale magnetic centres. Quantum tunnelling, in form of temperature-independent relaxation and coercivity, deviation from Arrhenius behaviour and blocking of the relaxation, dominates the magnetic properties up to 10 Kelvin. The simple crystal structure, the availability of large single crystals, and the ability to vary the Fe concentration make Li2(Li1-xFex)N (i) an ideal model system to study macroscopic quantum effects at elevated temperatures and (ii) a basis for novel functional magnetic materials.

cond-mat.str-el↗

arXiv2014

Carrier localization and electronic phase separation in a doped spin-orbit driven Mott phase in Sr3(Ir1-xRux)2O7

Interest in many strongly spin-orbit coupled 5d-transition metal oxide insulators stems from mapping their electronic structures to a J=1/2 Mott phase. One of the hopes is to establish their Mott parent states and explore these systems' potential of realizing novel electronic states upon carrier doping. However, once doped, little is understood regarding the role of their reduced Coulomb interaction U relative to their strongly correlated 3d-electron cousins. Here we show that, upon hole-doping a candidate J=1/2 Mott insulator, carriers remain localized within a nanoscale phase separated ground state. A percolative metal-insulator transition occurs with interplay between localized and itinerant regions, stabilizing an antiferromagnetic metallic phase beyond the critical region. Our results demonstrate a surprising parallel between doped 5d- and 3d-electron Mott systems and suggest either through the near degeneracy of nearby electronic phases or direct carrier localization that U is essential to the carrier response of this doped spin-orbit Mott insulator.

cond-mat.str-el↗

arXiv2013

Symmetry protected topological phases from decorated domain walls

Symmetry protected topological (SPT) phases with unusual edge excitations can emerge in strongly interacting bosonic systems and are classified in terms of the cohomology of their symmetry groups. Here we provide a physical picture that leads to an intuitive understanding and wavefunctions for several SPT phases in d=1,2,3 dimensions. We consider symmetries which include a Z_2 subgroup, that allows us to define domain walls. While the usual disordered phase is obtained by proliferating domain walls, we show that SPT phases are realized when these proliferated domain walls are `decorated', i.e. are themselves SPT phases in one lower dimension. For example a d=2 SPT phase with Z_2 and time reversal symmetry is realized when the domain walls that proliferate are themselves in a d=1 Haldane/AKLT state. Similarly, d=3 SPT phases with Z_2 * Z_2 symmetry emerges when domain walls in a d=2 SPT with Z_2 symmetry are proliferated. The resulting ground states are shown to be equivalent to that obtained from group cohomology and field theoretical techniques. The result of gauging the Z_2 symmetry in these phases is also discussed. An extension of this construction where time reversal plays the role of Z_2 symmetry allows for a discussion of several d=3 SPT phases. This construction also leads to a new perspective on some well known d=1 SPT phases, from which exactly soluble parent Hamiltonians may be derived.

cond-mat.str-el↗

arXiv2014

An order parameter for impurity systems at quantum criticality

A quantum phase transition may occur in the ground state of a system at zero temperature when a controlling field or interaction is varied. The resulting quantum fluctuations which trigger the transition produce scaling behavior of various observables, governed by universal critical exponents. A particularly interesting class of such transitions appear in systems with quantum impurities where a non-extensive term in the free energy becomes singular at the critical point. Curiously, the notion of a conventional order parameter which exhibits scaling at the critical point is generically missing in these systems. We here explore the possibility to use the Schmidt gap, which is an observable obtained from the entanglement spectrum, as an order parameter. A case study of the two-impurity Kondo model confirms that the Schmidt gap faithfully captures the scaling behavior by correctly predicting the critical exponent of the dynamically generated length scale at the critical point.

cond-mat.str-el↗

arXiv2014

Spin-orbital Entangled Molecular $j_{\rm eff}$ States in Lacunar Spinel Compounds

The entanglement of the spin and orbital degrees of freedom through the spin-orbit coupling has been actively studied in condensed matter physics. In several iridium-oxide systems, the spin-orbital entangled state, identified by the effective angular momentum $j_{\rm eff}$, can host novel quantum phases with the help of electron correlations. Here, we show that a series of lacunar spinel compounds, Ga$M_4X_8$ ($M$ = Nb, Mo, Ta, and W and $X$ = S, Se, and Te), gives rise to a $\textit{molecular}$ $j_{\rm eff}$ state as a new spin-orbital composite on which the low energy effective Hamiltonian is based. A wide range of electron correlations is accessible by tuning the bandwidth under external and/or chemical pressure, enabling us to investigate the interesting cooperation between spin-orbit coupling and electron correlations. As illustrative examples, a two-dimensional topological insulating phase and an anisotropic spin Hamiltonian are investigated in the weak and strong coupling regimes, respectively. Our finding can provide an ideal platform for exploring $j_{\rm eff}$ physics and the resulting emergent phenomena.

cond-mat.str-el↗

arXiv2013

Artificially engineered superlattices of pnictide superconductor

Significant progress has been achieved in fabricating high quality bulk and thinfilm iron-based superconductors. In particular, artificial layered pnictide superlattices offer the possibility of tailoring the superconducting properties and understanding the mechanism of the superconductivity itself. For high field applications, large critical current densities (Jc) and irreversibility fields (Hirr) are indispensable along all crystal directions. On the other hand, the development of superconducting devices such as tunnel junctions requires multilayered heterostructures. Here we show that artificially engineered undoped Ba-122 / Co doped Ba-122 compositionally modulated superlattices produce ab-aligned nanoparticle arrays. These layer and self-assemble along c-axis aligned defects, and combine to produce very large Jc and Hirr enhancements over a wide angular range. We also demonstrate a structurally modulated SrTiO3 (STO) / Co doped Ba-122 superlattice with sharp interfaces. Success in superlattice fabrication involving pnictides will aid the progress of heterostructured systems exhibiting novel interfacial phenomena and device applications.

cond-mat.supr-con↗

arXiv2013

Persistence of magnetic excitations in La2-xSrxCuO4 from the undoped insulator to the heavily overdoped non-superconducting metal

One of the most intensely studied scenarios of high-temperature superconductivity (HTS) postulates pairing by exchange of magnetic excitations [1]. Indeed, such excitations have been observed up to around optimal doping in the cuprates [2-7]. In the heavily overdoped regime neutron scattering measurements indicate that magnetic excitations have effectively disappeared [8-10], and this was argued to cause the demise of HTS with overdoping [1, 8, 10]. Here we use resonant inelastic x-ray scattering (RIXS), which is sensitive to complementary parts of reciprocal space, to measure the evolution of the magnetic excitations in La2-xSrxCuO4 across the entire phase diagram, from a strongly correlated insulator (x = 0) to a non-superconducting metal (x = 0.40). For x = 0, well-defined magnon excitations are observed [11]. These magnons broaden with doping, but they persist with a similar dispersion and comparable intensity all the way to the non-superconducting, heavily overdoped metallic phase. The destruction of HTS with overdoping is therefore caused neither by the general disappearance nor by the overall softening of magnetic excitations; other factor(s), such as the redistribution of spectral weight, must be considered.

cond-mat.supr-con↗

arXiv2013

Local Electrostatic Imaging of Striped Domain Order in LaAlO3/SrTiO3

The emerging field of complex oxide interfaces is generically built on one of the most celebrated substrates - strontium titanate (SrTiO3). This material hosts a range of phenomena, including ferroelasticity, incipient ferroelectricity, and most puzzlingly, contested giant piezoelectricity. Although these properties may markedly influence the oxide interfaces, especially on microscopic length scales, the lack of local probes capable of studying such buried systems has left their effects largely unexplored. Here we use a scanning charge detector - a nanotube single-electron transistor - to noninvasively image the electrostatic landscape and local mechanical response in the prototypical LaAlO3/SrTiO3 system with unprecedented sensitivity. Our measurements reveal that on microscopic scales SrTiO3 exhibits large anomalous piezoelectricity with curious spatial dependence. Through electrostatic imaging we unravel the microscopic origin for this extrinsic piezoelectricity, demonstrating its direct, quantitative connection to the motion of locally ordered tetragonal domains under applied gate voltage. These domains create striped potential modulations that can markedly influence the two-dimensional electron system at the conducting interface. Our results have broad implications to all complex oxide interfaces built on SrTiO3 and demonstrate the importance of microscopic structure to the physics of electrons at the LaAlO3/SrTiO3 interface.

cond-mat.mes-hall↗

arXiv2013

Topological surface state in the Kondo Insulator Samarium Hexaboride

Strongly correlated electron systems show many exotic properties such as unconventional superconductity, quantum criticality, and Kondo insulating behavior. In addition, the Kondo insulator SmB6 has been predicted theoretically to be a 3D topological insulator with a metallic surface state. We report here transport measurements on doped SmB6, which show that ~3% magnetic and non-magnetic dopants in SmB6 exhibit clearly contrasting behavior, evidence that the metallic surface state is only destroyed when time reversal symmetry is broken. We find as well a quantum percolation limit of impurity concentration which transform the topological insulator into a conventional band insulator by forming impurity band. Our careful thickness dependence results show that SmB6 is the first demonstatrated perfect 3D topological insulator with virtually zero residual bulk conductivity.

cond-mat.str-el↗

arXiv2013

Current-induced skyrmion dynamics in constricted geometries

Magnetic skyrmions, vortex-like swirling spin structures with quantized topological number, observed in chiral magnets, are appealing for potential applications in spintronics because it is possible to control their motion with ultralow current density. In order to realise skyrmion-based spintronic devices, it is essential to understand the skyrmion motions in confined geometries. Here we show by micromagnetic simulations that the current-induced motion of skyrmions in the presence of geometrical boundaries is very different from that in an infinite plane. In a channel with finite width, the transverse confinement results in steady state characteristics of the skyrmion velocity as a function of current similar to those of domain walls in ferromagnets, whereas the transient behaviour depends on the initial distance of the skyrmion from the boundary. In addition, we show that single skyrmions can be created by electric current in a simple constricted geometry of a plate-shaped specimen of suitable size and geometry. These findings could guide the design of skyrmion-based devices, in which skyrmions are used as information carriers.

cond-mat.mes-hall↗

arXiv2014

Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2

Quantum systems in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors and Dirac electrons in graphene. Interest in such systems has also been intensified by the recent discovery of a large enhancement in photoluminescence quantum efficiency and a potential route to valleytronics in atomically thin layers of transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se, Te), which are closely related to the indirect to direct bandgap transition in monolayers. Here, we report the first direct observation of the transition from indirect to direct bandgap in monolayer samples by using angle resolved photoemission spectroscopy on high-quality thin films of MoSe2 with variable thickness, grown by molecular beam epitaxy. The band structure measured experimentally indicates a stronger tendency of monolayer MoSe2 towards a direct bandgap, as well as a larger gap size, than theoretically predicted. Moreover, our finding of a significant spin-splitting of 180 meV at the valence band maximum of a monolayer MoSe2 film could expand its possible application to spintronic devices.

cond-mat.mtrl-sci↗

arXiv2012

Towards a large-scale quantum simulator on diamond surface at room temperature

Strongly-correlated quantum many-body systems exhibits a variety of exotic phases with long-range quantum correlations, such as spin liquids and supersolids. Despite the rapid increase in computational power of modern computers, the numerical simulation of these complex systems becomes intractable even for a few dozens of particles. Feynman's idea of quantum simulators offers an innovative way to bypass this computational barrier. However, the proposed realizations of such devices either require very low temperatures (ultracold gases in optical lattices, trapped ions, superconducting devices) and considerable technological effort, or are extremely hard to scale in practice (NMR, linear optics). In this work, we propose a new architecture for a scalable quantum simulator that can operate at room temperature. It consists of strongly-interacting nuclear spins attached to the diamond surface by its direct chemical treatment, or by means of a functionalized graphene sheet. The initialization, control and read-out of this quantum simulator can be accomplished with nitrogen-vacancy centers implanted in diamond. The system can be engineered to simulate a wide variety of interesting strongly-correlated models with long-range dipole-dipole interactions. Due to the superior coherence time of nuclear spins and nitrogen-vacancy centers in diamond, our proposal offers new opportunities towards large-scale quantum simulation at room temperatures.

quant-ph↗

arXiv2013

Change of pairing symmetry in the iron-based superconductor KFe2As2

The pairing mechanism in iron-based superconductors is the subject of ongoing debate. Proximity to an antiferromagnetic phase suggests that pairing is mediated by spin fluctuations, but orbital fluctuations have also been invoked. The former typically favour a pairing state of extended s-wave symmetry with a gap that changes sign between electron and hole Fermi surfaces (s+-), while the latter yield a standard s-wave state without sign change (s++). Here we show that applying pressure to KFe2As2 induces a change of pairing state. The critical temperature Tc decreases with pressure initially, and then suddenly increases, above a critical pressure Pc. The constancy of the Hall coefficient through Pc rules out a change in the Fermi surface. There is compelling evidence that the pairing state below Pc is d-wave, from bulk measurements at ambient pressure. Above Pc, the high sensitivity to disorder argues for a particular kind of s+- state. The change from d-wave to s-wave is likely to proceed via an unusual s + id state that breaks time-reversal symmetry. The proximity of two distinct pairing states found here experimentally is natural given the near degeneracy of d-wave and s+- states found theoretically. These findings make a compelling case for spin-fluctuation-mediated superconductivity in this key iron-arsenide material.

cond-mat.supr-con↗