125 research outputs found
[Viewpoint:] Postponing Heat Death in Periodically Driven Systems
An exponential suppression of heating has been observed in a periodically driven optical lattice, opening up an opportunity to engineer new states of matter
Creating topological interfaces and detecting chiral edge modes in a 2D optical lattice
We propose and analyze a general scheme to create chiral topological edge
modes within the bulk of two-dimensional engineered quantum systems. Our method
is based on the implementation of topological interfaces, designed within the
bulk of the system, where topologically-protected edge modes localize and
freely propagate in a unidirectional manner. This scheme is illustrated through
an optical-lattice realization of the Haldane model for cold atoms, where an
additional spatially-varying lattice potential induces distinct topological
phases in separated regions of space. We present two realistic experimental
configurations, which lead to linear and radial-symmetric topological
interfaces, which both allows one to significantly reduce the effects of
external confinement on topological edge properties. Furthermore, the
versatility of our method opens the possibility of tuning the position, the
localization length and the chirality of the edge modes, through simple
adjustments of the lattice potentials. In order to demonstrate the unique
detectability offered by engineered interfaces, we numerically investigate the
time-evolution of wave packets, indicating how topological transport
unambiguously manifests itself within the lattice. Finally, we analyze the
effects of disorder on the dynamics of chiral and non-chiral states present in
the system. Interestingly, engineered disorder is shown to provide a powerful
tool for the detection of topological edge modes in cold-atom setups.Comment: 18 pages, 21 figure
Pressure tuning of light-induced superconductivity in K3C60
Optical excitation at terahertz frequencies has emerged as an effective means
to manipulate complex solids dynamically. In the molecular solid K3C60,
coherent excitation of intramolecular vibrations was shown to transform the
high temperature metal into a non-equilibrium state with the optical
conductivity of a superconductor. Here we tune this effect with hydrostatic
pressure, and we find it to disappear around 0.3 GPa. Reduction with pressure
underscores the similarity with the equilibrium superconducting phase of K3C60,
in which a larger electronic bandwidth is detrimental for pairing. Crucially,
our observation excludes alternative interpretations based on a high-mobility
metallic phase. The pressure dependence also suggests that transient, incipient
superconductivity occurs far above the 150 K hypothesised previously, and
rather extends all the way to room temperature.Comment: 33 pages, 17 figures, 2 table
Topological Floquet engineering of twisted bilayer graphene
We investigate the topological properties of Floquet-engineered twisted bilayer graphene above the so-called magic angle driven by circularly polarized laser pulses. Employing a full Moiré-unit-cell tight-binding Hamiltonian based on first-principles electronic structure, we show that the band topology in the bilayer, at twisting angles above 1.05∘, essentially corresponds to the one of single-layer graphene. However, the ability to open topologically trivial gaps in this system by a bias voltage between the layers enables the full topological phase diagram to be explored, which is not possible in single-layer graphene. Circularly polarized light induces a transition to a topologically nontrivial Floquet band structure with the Berry curvature analogous to a Chern insulator. Importantly, the twisting allows for tuning electronic energy scales, which implies that the electronic bandwidth can be tailored to match realistic driving frequencies in the ultraviolet or midinfrared photon-energy regimes. This implies that Moiré superlattices are an ideal playground for combining twistronics, Floquet engineering, and strongly interacting regimes out of thermal equilibrium
Superconducting fluctuations observed far above T<sub>c</sub> in the isotropic superconductor K<sub>3</sub>C<sub>60</sub>
Alkali-doped fullerides are strongly correlated organic superconductors that exhibit high transition temperatures, exceptionally large critical magnetic fields and a number of other unusual properties. The proximity to a Mott insulating phase is thought to be a crucial ingredient of the underlying physics, and may also affect precursors of superconductivity in the normal state above T. We report on the observation of a sizeable magneto-thermoelectric (Nernst) effect in the normal state of KC, which displays the characteristics of superconducting fluctuations. The anomalous Nernst effect emerges from an ordinary quasiparticle background below a temperature of 80K, far above T = 20K. At the lowest fields and close to T, the scaling of the effect is captured by a model based on Gaussian fluctuations. The temperature up to which we observe fluctuations is exceptionally high for a three-dimensional isotropic system, where fluctuation effects are usually suppressed
Microscopic theory for the light-induced anomalous Hall effect in graphene
We employ a quantum Liouville equation with relaxation to model the recently
observed anomalous Hall effect in graphene irradiated by an ultrafast pulse of
circularly polarized light. In the weak-field regime, we demonstrate that the
Hall effect originates from an asymmetric population of photocarriers in the
Dirac bands. By contrast, in the strong-field regime, the system is driven into
a non-equilibrium steady state that is well-described by topologically
non-trivial Floquet-Bloch bands. Here, the anomalous Hall current originates
from the combination of a population imbalance in these dressed bands together
with a smaller anomalous velocity contribution arising from their Berry
curvature. This robust and general finding enables the simulation of electrical
transport from light-induced Floquet-Bloch bands in an experimentally relevant
parameter regime and creates a pathway to designing ultrafast quantum devices
with Floquet-engineered transport properties
Floquet dynamics in light-driven solids
We demonstrate how the properties of light-induced electronic Floquet states
in solids impact natural physical observables, such as transport properties, by
capturing the environmental influence on the electrons. We include the
environment as dissipative processes, such as inter-band decay and dephasing,
often ignored in Floquet predictions. These dissipative processes determine the
Floquet band occupations of the emergent steady state, by balancing out the
optical driving force. In order to benchmark and illustrate our framework for
Floquet physics in a realistic solid, we consider the light-induced Hall
conductivity in graphene recently reported by J.~W.~McIver, et al., Nature
Physics (2020). We show that the Hall conductivity is estimated by the Berry
flux of the occupied states of the light-induced Floquet bands, in addition to
the kinetic contribution given by the average band velocity. Hence, Floquet
theory provides an interpretation of this Hall conductivity as a
geometric-dissipative effect. We demonstrate this mechanism within a master
equation formalism, and obtain good quantitative agreement with the
experimentally measured Hall conductivity, underscoring the validity of this
approach which establishes a broadly applicable framework for the understanding
of ultrafast non-equilibrium dynamics in solids
Giant resonant enhancement for photo-induced superconductivity in KC
Photo-excitation at terahertz and mid-infrared frequencies has emerged as a
new way to manipulate functionalities in quantum materials, in some cases
creating non-equilibrium phases that have no equilibrium analogue. In
KC, a metastable zero-resistance phase was documented with optical
properties and pressure dependences compatible with non-equilibrium high
temperature superconductivity. Here, we report the discovery of a dominant
energy scale for this phenomenon, along with the demonstration of a giant
increase in photo-susceptibility near 10 THz excitation frequency. At these
drive frequencies a metastable superconducting-like phase is observed up to
room temperature for fluences as low as ~400 . These findings shed
light on the microscopic mechanism underlying photo-induced superconductivity.
They also trace a path towards steady state operation, currently limited by the
availability of a suitable high-repetition rate optical source at these
frequencies.Comment: 35 pages, 13 figures, including supplementar
Creating, moving and merging Dirac points with a Fermi gas in a tunable honeycomb lattice
Dirac points lie at the heart of many fascinating phenomena in condensed
matter physics, from massless electrons in graphene to the emergence of
conducting edge states in topological insulators [1, 2]. At a Dirac point, two
energy bands intersect linearly and the particles behave as relativistic Dirac
fermions. In solids, the rigid structure of the material sets the mass and
velocity of the particles, as well as their interactions. A different, highly
flexible approach is to create model systems using fermionic atoms trapped in
the periodic potential of interfering laser beams, a method which so far has
only been applied to explore simple lattice structures [3, 4]. Here we report
on the creation of Dirac points with adjustable properties in a tunable
honeycomb optical lattice. Using momentum-resolved interband transitions, we
observe a minimum band gap inside the Brillouin zone at the position of the
Dirac points. We exploit the unique tunability of our lattice potential to
adjust the effective mass of the Dirac fermions by breaking inversion symmetry.
Moreover, changing the lattice anisotropy allows us to move the position of the
Dirac points inside the Brillouin zone. When increasing the anisotropy beyond a
critical limit, the two Dirac points merge and annihilate each other - a
situation which has recently attracted considerable theoretical interest [5-9],
but seems extremely challenging to observe in solids [10]. We map out this
topological transition in lattice parameter space and find excellent agreement
with ab initio calculations. Our results not only pave the way to model
materials where the topology of the band structure plays a crucial role, but
also provide an avenue to explore many-body phases resulting from the interplay
of complex lattice geometries with interactions [11, 12]
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