Strain-Induced Landau Levels in Arbitrary Dimensions with an Exact Spectrum

Three-dimensional flat Landau levels in an inhomogeneous acoustic crystal

When electrons moving in two dimensions (2D) are subjected to a strong uniform magnetic field, they form flat bands called Landau levels (LLs). LLs can also arise from pseudomagnetic fields (PMFs) induced by lattice distortions. In three-dimensional (3D) systems, there has been no experimental demonstration of LLs  as a type of flat band thus far. Here, we report the experimental realization of a flat 3D LL in an acoustic crystal. Starting from a lattice whose bandstructure exhibits a nodal ring, we design an inhomogeneous distortion corresponding to a specific pseudomagnetic vector potential (PVP). This distortion causes the nodal ring states to break up into LLs, including a zeroth LL that is flat along all three directions. These findings suggest the possibility of using nodal ring materials to generate 3D flat bands, allowing access to strong interactions and other attractive physical regimes in 3D.

Realization of all-band-flat photonic lattices

Flatbands play an important role in correlated quantum matter and have promising applications in photonic lattices. Synthetic magnetic fields and destructive interference in lattices are traditionally used to obtain flatbands. However, such methods can only obtain a few flatbands with most bands remaining dispersive. Here we realize all-band-flat photonic lattices of an arbitrary size by precisely controlling the coupling strengths between lattice sites to mimic those in Fock-state lattices. This allows us to go beyond the perturbative regime of strain engineering and group all eigenmodes in flatbands, which simultaneously achieves high band flatness and large usable bandwidth. We map out the distribution of each flatband in the lattices and selectively excite the eigenmodes with different chiralities. Our method paves a way in controlling band structure and topology of photonic lattices.

Observation of supersymmetric pseudo-Landau levels in strained microwave graphene

Using an array of coupled microwave resonators arranged in a deformed honeycomb lattice, we experimentally observe the formation of pseudo-Landau levels in the whole crossover from vanishing to large pseudomagnetic field strengths. This result is achieved by utilising an adaptable setup in a geometry that is compatible with the pseudo-Landau levels at all field strengths. The adopted approach enables us to observe the fully formed flat-band pseudo-Landau levels spectrally as sharp peaks in the photonic density of states and image the associated wavefunctions spatially, where we provide clear evidence for a characteristic nodal structure reflecting the previously elusive supersymmetry in the underlying low-energy theory. In particular, we resolve the full sublattice polarisation of the anomalous 0th pseudo-Landau level, which reveals a deep connection to zigzag edge states in the unstrained case.

Relativistic mechanism of chiral magnetic current in Weyl semimetals with tilted dispersion

The chiral magnetic effect is a one of the exotic bulk transport properties of the Weyl semimetals. Because of the Nielsen-Ninomiya 'no-go theorem', the total chiral magnetic current is absent in the equilibrium state. One of the mechanisms for generating this current is the chiral anomaly. This phenomenon is the anomalous nonconservation of chiral charge for massless relativistic particles. It can be realized by parallel magnetic and electric fields (∼[Formula: see text]), and it leads to such new transport phenomenon as the negative longitudinal magnetoresistance. Using a simple theory (we consider both a linearized and lattice model), we have shown, that in Weyl metals with tilted dispersion another mechanism of the chiral magnetic current is possible. It is not associated with the chiral anomaly. The new transport mechanism is based on the relativistic effect of electric field on Landau levels. This effect is that an electric field changes the distance between the Landau levels, and also changes the effective velocity along magnetic field. At the presence of a tilt in the spectrum, this velocity renormalization is different for different Weyl points. This leads to a non-zero resulting drift velocity. As a consequence, an electrical current arises along the magnetic field. The induced by this mechanism the electric current is proportional to the pseudoscalar product of the fields ([Formula: see text]) and directed along the magnetic field, that differs it from the Hall current (∼[Formula: see text]). At the same time, the conductivity corresponding to this transport mechanism does not depend on the scattering time like the Hall conductivity. Thus, we have proposed a new anomalous transport mechanism in the Weyl semimetal, which is not associated with the chiral anomaly.

Magnon Landau Levels and Emergent Supersymmetry in Strained Antiferromagnets

Inhomogeneous strain applied to lattice systems can induce artificial gauge fields for particles moving on this lattice. Here we demonstrate how to engineer a novel state of matter, namely an antiferromagnet with a Landau-level excitation spectrum of magnons. We consider a honeycomb-lattice Heisenberg model and show that triaxial strain leads to equally spaced pseudo-Landau levels at the upper end of the magnon spectrum, with degeneracies characteristic of emergent supersymmetry. We also present a particular strain protocol which induces perfectly quantized magnon Landau levels over the whole bandwidth. We discuss experimental realizations and generalizations.

The induced by an electromagnetic field coexistence of types I and II spectra in Weyl semimetals

Due to their unique properties, Weyl semimetals (WSMs) are promising materials for the future electronics. Currently, the two types (I and II) of WSMs are discovered experimentally. These types of WSMs differ from each other in their topological properties. In this paper we showed that a coexistence of types I and II Weyls spectra is possible in some WSMs under crossed magnetic and electric fields. This is possible in systems with non-equivalent Weyl points (WPs). In particular, it is possible in strained WSMs. Such phase, controlled by electromagnetic field, is principally new for topological matter physics. It is obvious, that in this regime new features of electron transport will appear. We showed that this effect can also be considered as a mechanism of strain induced type-I-type-II transition.

Synthetic Landau Levels and Spinor Vortex Matter on a Haldane Spherical Surface with a Magnetic Monopole

We present a flexible scheme to realize exact flat Landau levels on curved spherical geometry in a system of spinful cold atoms. This is achieved by applying the Floquet engineering of a magnetic quadrupole field to create a synthetic monopole field in real space. The system can be exactly mapped to the electron-monopole system on a sphere, thus realizing Haldane's spherical geometry for fractional quantum Hall physics. This method works for either bosons or fermions. We investigate the ground-state vortex pattern for an s-wave interacting atomic condensate by mapping this system to the classical Thompson's problem. The distortion and stability of the vortex pattern are further studied in the presence of dipolar interaction. Our scheme is compatible with the current experimental setup, and may serve as a promising route of investigating quantum Hall physics and exotic spinor vortex matter on curved space.