Quantum mechanics guided by simplicity

Dr Arieh Warshel, distinguished professor of chemistry at the University of Southern California and 2013 Nobel laureate in chemistry, discusses with Nature Computational Science past and current research, his Nobel Prize, and the benefits and challenges of using computational modeling in his work. You have full access to this article...

Making complex systems computable

Dr John Wettlaufer, A. M. Bateman Professor of Geophysics, Mathematics, and Physics at Yale University, research professor at the Nordic Institute for Theoretical Physics, and a member of the Nobel Committee for Physics, discusses the contributions from the laureates of the 2021 Nobel Prize in Physics, his insights into complex system modeling, and his personal experience serving as a Nobel Committee member.

Square-root higher-order Weyl semimetals

The mathematical foundation of quantum mechanics is built on linear algebra, while the application of nonlinear operators can lead to outstanding discoveries under some circumstances, such as the prediction of positron, a direct outcome of the Dirac equation which stems from the square-root of the Klein-Gordon equation. In this article, we propose a model of square-root higher-order Weyl semimetal (SHOWS) by inheriting features from its parent Hamiltonians. It is found that the SHOWS hosts both "Fermi-arc" surface and hinge states that respectively connect the projection of the Weyl points on the side surface and arris. We theoretically construct and experimentally observe the exotic SHOWS state in three-dimensional (3D) stacked electric circuits with honeycomb-kagome hybridizations and double-helix interlayer couplings. Our results open the door for realizing the square-root topology in 3D solid-state platforms.

In control of chaos to engineer high-entropy ceramics

Nature strives for chaos. That's a nice, comforting phrase when yet another coffee cup has toppled over the computer keyboard and you imagine you could wish the sugary, milky brew back into the coffee cup—where it had been just seconds before. But wishing won't work. Because, as mentioned, nature strives for chaos.

Realizing a 1D topological gauge theory in an optically dressed BEC

Topological gauge theories describe the low-energy properties of certain strongly correlated quantum systems through effective weakly interacting models1,2. A prime example is the Chern"“Simons theory of fractional quantum Hall states, where anyonic excitations emerge from the coupling between weakly interacting matter particles and a density-dependent gauge field3. Although in traditional solid-state platforms such gauge theories are only convenient theoretical constructions, engineered quantum systems enable their direct implementation and provide a fertile playground to investigate their phenomenology without the need for strong interactions4. Here, we report the quantum simulation of a topological gauge theory by realizing a one-dimensional reduction of the Chern"“Simons theory (the chiral BF theory5,6,7) in a Bose"“Einstein condensate. Using the local conservation laws of the theory, we eliminate the gauge degrees of freedom in favour of chiral matter interactions8,9,10,11, which we engineer by synthesizing optically dressed atomic states with momentum-dependent scattering properties. This allows us to reveal the key properties of the chiral BF theory: the formation of chiral solitons and the emergence of an electric field generated by the system itself. Our results expand the scope of quantum simulation to topological gauge theories and open a route to the implementation of analogous gauge theories in higher dimensions12.

Qubits measured in a different light

It is well established that optical nonlinearities can be used to transduce light from one frequency to another. When a photonic signal passes through a nonlinear medium and an intense 'pump' of coherent light is applied, then the signal can be upconverted to light with a frequency that is the sum of the signal and pump frequencies. Commonly used in classical photonics, this approach is also possible at the few-photon level. The challenge is to implement it without introducing so much noise to the signal source and the output that fragile quantum effects are lost or destroyed.

Correlated interlayer exciton insulator in heterostructures of monolayer WSe and moiré WS/WSe

Moiré superlattices in van der Waals heterostructures have emerged as a powerful tool for engineering quantum phenomena. Here we report the observation of a correlated interlayer exciton insulator in a double-layer heterostructure composed of a WSe2 monolayer and a WS2/WSe2 moiré bilayer that are separated by ultrathin hexagonal boron nitride. The moiré WS2/WSe2 bilayer features a Mott insulator state when the density of holes is one per moiré lattice site. When electrons are added to the Mott insulator in the WS2/WSe2 moiré bilayer and an equal number of holes are injected into the WSe2 monolayer, a new interlayer exciton insulator emerges with the holes in the WSe2 monolayer and the electrons in the doped Mott insulator bound together through interlayer Coulomb interactions. The interlayer exciton insulator is stable up to a critical hole density in the WSe2 monolayer, beyond which the interlayer exciton dissociates. Our study highlights the opportunities for realizing quantum phases in double-layer moiré systems due to the interplay between the moiré flat band and strong interlayer electron interactions.

Topological flat bands in a kagome lattice multiorbital system

Flat bands and dispersive Dirac bands are known to coexist in the electronic bands in a two-dimensional kagome lattice. Including the relativistic spin-orbit coupling, such systems often exhibit nontrivial band topology, allowing for gapless edge modes between flat bands at several locations in the band structure, and dispersive bands or at the Dirac band crossing. Here, we theoretically demonstrate that a multiorbital system on a kagome lattice is a versatile platform to explore the interplay between nontrivial band topology and electronic interaction. Specifically, here we report that the multiorbital kagome model with the atomic spin"“orbit coupling naturally supports topological bands characterized by nonzero Chern numbers \({{{{{{{\mathcal{C}}}}}}}}\), including a flat band with \(| {{{{{{{\mathcal{C}}}}}}}}| =1\). When such a flat band is 1/3 filled, the non-local repulsive interactions induce a fractional Chern insulating state. We also discuss the possible realization of our findings in real kagome materials.

Classical harmonic three-body system: an experimental electronic realization

The classical three-body harmonic system in \({\mathbb {R}}^d\) (\(d>1\)) with finite rest lengths and zero total angular momentum \(L=0\) is considered. This model describes the dynamics of the \(L=0\) near-equilibrium configurations of three point masses \((m_1,m_2,m_3)\) with arbitrary pairwise potential \(V(r_{ij})\) that solely depends on the relative distances between bodies. It exhibits an interesting mixed regular and chaotic dynamics as a function of the energy and the system parameters. The corresponding harmonic quantum system plays a fundamental role in atomic and molecular physics. In this work we report on a novel electronic experimental realization of the model as a complementary tool to analyze the rich dynamics of the classical system. Our setup allows us to experimentally explore different regions of behavior due to the fact that the intrinsic parameters and initial states of the system are independently set by voltage inputs. Chaotic and periodic motions are characterized employing time series, phase planes, and the largest Lyapunov exponents as a function of the energy and system parameters. The results show an excellent qualitative as well as quantitative agreement between theory and experiment.

Synergistic use of gradient flipping and phase prediction for inline electron holography

Inline holography in the transmission electron microscope is a versatile technique which provides real-space phase information that can be used for the correction of imaging aberrations, as well as for measuring electric and magnetic fields and strain distributions. It is able to recover high-spatial-frequency contributions of the phase effectively but suffers from the weak transfer of low-spatial-frequency information, as well as from incoherent scattering. Here, we combine gradient flipping and phase prediction in an iterative flux-preserving focal series reconstruction algorithm with incoherent background subtraction that gives extensive access to the missing low spatial frequencies. A procedure for optimizing the reconstruction parameters is presented, and results from Fe-filled C nanospheres, and MgO cubes are compared with phase images obtained using off-axis holography.

Direct observation of geometric and sliding ferroelectricity in an amphidynamic crystal

Sliding ferroelectricity is a recently observed polarity existing in two-dimensional materials. However, due to the weak polarization and poor electrical insulation in these materials, existing experimental evidences are indirect and mostly based on nanoscale transport properties or piezoresponse force microscopy. We report the direct observation of sliding ferroelectricity, using a high-quality amphidynamic single crystal (15-crown-5)Cd3Cl6, which possesses a large bandgap and so allows direct measurement of polarization"“electric field hysteresis. This coordination polymer is a van der Waals material, which is composed of inorganic stators and organic rotators as determined by X-ray diffraction and NMR characterization. From density functional theory calculations, we find that after freezing the rotators, an electric dipole is generated in each layer driven by the geometric mechanism, while a comparable ferroelectric polarization originates from the interlayer sliding. The net polarization of these two components can be directly measured and manipulated. Our finding provides insight into low-dimensional ferroelectrics, especially control of the synchronous dynamics of rotating molecules and sliding layers in solids.