The structural and electronic properties of ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ alloys are studied as a function of $c$-plane biaxial strain and In ordering by density functional theory with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. A nonlinear variation of the $c$ lattice parameter with In content is observed in biaxial strain and should be taken into account when deducing In content from interplanar distances. From compressive to tensile strain, the character of the top valence-band state changes, leading to a nonlinear variation of the band gap in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$. Interestingly, the well-known bowing of the ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ band gap is largely removed for alloys grown strictly coherently on GaN, while the actual values for band gaps at $x<0.33$ are hardly affected by strain. Ordering plays a minor role for lattice constants but may induce changes of the band gap up to 0.15 eV.

We consider a waveguide formed in a clean graphene monolayer by a spatially inhomogeneous magnetic field. The single-particle dispersion relation for this waveguide exhibits a zero-energy Landau-like flat band, while finite-energy bands have dispersion and correspond, in particular, to snake orbits. For zero-mode states, all matrix elements of the current operator vanish, and a finite conductance can only be caused by virtual transitions to finite-energy bands. We show that Coulomb interactions generate such processes. In stark contrast to finite-energy bands, the conductance is not quantized and shows a characteristic dependence on the zero-mode filling. Transport experiments thereby offer a novel and highly sensitive probe of electron-electron interactions in clean graphene samples. We argue that this interaction-driven zero-mode conductor may also appear in other physical settings and is not captured by the conventional Tomonaga-Luttinger liquid description.

We study the canted magnetic state in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ using fully relativistic density functional theory (DFT) including an on-site Hubbard $U$ correction. A complete magnetic phase diagram with respect to the tetragonal distortion and the rotation of ${\mathrm{IrO}}_6$ octahedra is constructed, revealing the presence of two types of canted to collinear magnetic transitions: a spin-flop transition with increasing tetragonal distortion and a complete quenching of the basal weak ferromagnetic moment below a critical octahedral rotation. Moreover, we put forward a scheme to study the anisotropic magnetic couplings by mapping magnetically constrained noncollinear DFT onto a general spin Hamiltonian. This procedure allows for the simultaneous account and direct control of the lattice, spin, and orbital interactions within a fully ab initio scheme. We compute the isotropic, single site anisotropy and Dzyaloshinskii-Moriya (DM) coupling parameters, and clarify that the origin of the canted magnetic state in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ arises from the structural distortions and the competition between isotropic exchange and DM interactions.

Structural changes of ${\mathrm{Li}}_2{\mathrm{C}}_2$ under pressure were studied by synchrotron x-ray diffraction in a diamond anvil cell under hydrostatic conditions and by using evolutionary search methodology for crystal structure prediction. We show that the high-pressure polymorph of ${\mathrm{Li}}_2{\mathrm{C}}_2$, which forms from the $Immm$ ground-state structure ($Z=2$) at around 15 GPa, adopts an orthorhombic $Pnma$ structure with $Z=4$. Acetylide ${\mathrm{C}}_2$ dumbbells characteristic of $Immm {\mathrm{Li}}_2{\mathrm{C}}_2$ are retained in $Pnma {\mathrm{Li}}_2{\mathrm{C}}_2$. The structure of $Pnma {\mathrm{Li}}_2{\mathrm{C}}_2$ relates closely to the anticotunnite-type structure. ${\mathrm{C}}_2$ dumbbell units are coordinated by nine Li atoms, as compared to eight in the antifluorite structure of $Immm {\mathrm{Li}}_2{\mathrm{C}}_2$. First-principles calculations predict a transition of $Pnma {\mathrm{Li}}_2{\mathrm{C}}_2$ at 32 GPa to a topologically identical phase with a higher $Cmcm$ symmetry. The coordination of ${\mathrm{C}}_2$ dumbbell units by Li atoms is increased to 11. The structure of $Cmcm {\mathrm{Li}}_2{\mathrm{C}}_2$ relates closely to the ${\mathrm{Ni}}_2$ In-type structure. It is calculated that $Cmcm {\mathrm{Li}}_2{\mathrm{C}}_2$ becomes metallic at pressures above 40 GPa. In experiments, however, $Pnma {\mathrm{Li}}_2{\mathrm{C}}_2$ is susceptible to irreversible amorphization.

The electronic properties of graphene are influenced by both geometric confinement and strain. We study the electronic structure of in-plane bent graphene nanoribbons, systems where confinement and strain are combined. To understand its electronic properties, we develop a tight-binding model that has a small computational cost and is based on exponentially decaying hopping and overlap parameters. Using this model, we show that the edge states in zigzag graphene nanoribbons are sensitive to bending and develop an effective dispersion that can be described by a one-dimensional atomic chain model. Because the velocity of the electrons at the edge is proportional to the slope of the dispersion, the edge states become gradually delocalized upon increasing the strength of bending.

We show that independent size and morphology engineering of epitaxial quantum dots can be obtained using a kinetically controlled quantum dot fabrication procedure, namely droplet epitaxy. Due to the far-from-equilibrium droplet epitaxy procedure, which is based on the crystallization, under As flux, of a nanometric droplet of Ga, independent and precise tuning of quantum dot size, aspect ratio, and faceting can be achieved. The dependence of the dot morphology on the growth conditions is interpreted and described quantitatively through a model that takes into account the crystallization kinetics of the Ga stored in the droplet under As flux.

We propose a quantum electronic device based on a strained graphene nanoribbon. Mechanical strain, internal exchange field, and spin-orbit couplings (SOCs) have been exploited as principle parameters to tune physical properties of the device. We predict a remarkable zero-field topological quantum phase transition between the time-reversal-symmetry-broken quantum spin Hall (QSH) and quantum anomalous Hall states, which was previously thought to take place only in the presence of finite magnetic field. We illustrate as intrinsic SOC is tuned, how two different helicity edge states located in the opposite edges of the nanoribbon exchange their locations. Our results indicate that the pseudomagnetic field induced by the strain could be coupled to the spin degrees of freedom through the SOC responsible for the stability of a QSH state. The controllability of this zero-field phase transition with strength and direction of the strain is also demonstrated. Our prediction offers a tempting prospect of strain, electric, and magnetic manipulation of the QSH effect.

In two-dimensional topological systems chiral edges may exhibit a spectral change due to the formation of a Bose condensate and partial confinement in the bulk according to the topological symmetry breaking (TSB) mechanism. We analyze in detail what happens in the bulk as well as on the edge for a set of simple $c=1$ chiral fractional quantum Hall systems. What this paper achieves is an explicit matching of the detailed field theoretic treatments of the bulk and boundary chiral boson and Chern-Simons theories, which allows for a precise interpretation of the TSB mechanism. TSB corresponds to the spontaneous breaking of a global discrete symmetry both in the bulk and on the edge and therefore to the appearance of domain walls in the bulk that may terminate in kinks on the edge. The walls, however, are locally metastable and break if a confined particle-hole pair is created. We list the $c=1$ chiral models for which this type of instability may occur.

Subtle changes in the geometric and electronic properties of supported molecules, with a potential impact on the functioning of molecular devices, can typically be imaged by scanning probe microscopy, but their exact origin and nature often remain unclear. Here we show explicitly that the symmetry reduction of iron phthalocyanine upon adsorption on Cu(111) can be observed not only in scanning tunneling microscopy, but also in core-level spectroscopy, and that it is related to nonisotropic charge transfer into the two principal molecular axes, but in combination with topographic influences.

DNA is perhaps the worlds most controllable nanowire, with potential applications in nanoelectronics and sensing. However, understanding of its charge transport (CT) properties remains elusive, with experiments reporting a wide range of behaviors from insulating to superconductive. We report extensive first-principle simulations that account for DNA's high flexibility and its native solvent environment. The results show that the CT along the DNA's long axis is strongly dependent on DNA's instantaneous conformation varying over many orders of magnitude. In high CT conformations, delocalized conductive states extending over up to 10 base pairs are found. Their low exponential decay constants further indicate that coherent CT, which is assumed to be active only over 2–3 base pairs in the commonly accepted DNA CT models, can act over much longer length scales. We also identify a simple geometrical rule that predicts CT properties of a given conformation with high accuracy. The effect of mismatched base pairs is also considered: while they decrease conductivities of specific DNA conformations, thermally induced conformational fluctuations wash out this effect. Overall, our results indicate that an immobilized partially dried poly(G)-poly(C) B-DNA is preferable for nanowire applications.

A theoretical estimate of the Casimir force of a metallic structure embedded into a cubic cavity is proposed. We demonstrate that by calculating the eigenmodes of the system we can determine the Casimir force, which can be either attractive or repulsive, by simply changing the geometry of the structures relative to the walls of the cavity. In this analysis, several cases of structures are taken into account, from rectangular slabs to chiral “omega” particles, and the predicted data are consistent with recent literature. We demonstrate that the sidewalls of the studied cavity contribute decisively to the repulsive Casimir force between the system and the nearby top surface of the cavity. Finally, we provide evidence that the medium embedded into the studied cavity (and especially its permittivity) can change the intensity of the Casimir force, while its repulsive nature, once established (owing to favorable geometrical features), remains quite robust.

We investigated the magnetoresistive behavior of an ideal Dirac semimetal—high-quality single crystals of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ with a Fermi level very close to the Dirac point. A large linear magnetoresistance (LMR), up to 3100% in a magnetic field of 14 T, was observed at a temperature of $T=2\mathrm{K}$. The detailed field and temperature dependencies reveal that the LMR occurs only when the Zeeman energy surpasses the thermal energy. Our result hints that the field-induced relative shifting of the two Weyl-Fermi surfaces in each Dirac cone in momentum space is likely responsible for the LMR behavior.

We report on magnetotransport measurements in a two-dimensional (2D) electron gas subject to subterahertz radiation in the regime where Shubnikov–de Haas oscillations (SdHOs) and microwave-induced resistance oscillations (MIROs) coexist over a wide magnetic field range, spanning several harmonics of the cyclotron resonance. Surprisingly, we find that the SdHO amplitude is modified by the radiation in a nontrivial way, owing to the oscillatory correction which has the same period and phase as MIROs. This finding challenges our current understanding of microwave photoresistance in 2D electron gas, calling for future investigations.

Non-Abelian gauge potentials are quite relevant in subatomic physics, but they are relatively rare in a condensed matter context. Here we report the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers by scanning tunneling microscopy and spectroscopy. At a magic twisted angle, $\theta \approx {\left(1.11\pm 0.05\right)}^{\circ }$, a pronounced sharp peak, which arises from the nondispersive flat bands at the charge neutrality point, is observed in the tunneling density of states due to the action of the non-Abelian gauge fields. Moreover, we observe confined electronic states in the twisted bilayer, as manifested by regularly spaced tunneling peaks with energy spacing $\delta E\approx v_{\mathrm{F}}/D\approx 70\mathrm{meV}$ (here $v_{\mathrm{F}}$ is the Fermi velocity of graphene and $D$ is the period of the moiré patterns). This indicates that the non-Abelian gauge potentials in twisted graphene bilayers confine low-energy electrons into a triangular array of quantum dots following the modulation of the moiré patterns. Our results also directly demonstrate that the Fermi velocity in twisted bilayers can be tuned from about ${10}^6\mathrm{m}/\mathrm{s}$ to zero by simply reducing the twisted angle of about $2^{\circ }$.

We present a strategy for producing multiqubit gates that promise high fidelity with minimal tuning requirements. Our strategy combines gap protection from the adiabatic theorem with dynamical decoupling in a complementary manner. Energy-level transition errors are protected by adiabaticity and remaining phase errors are mitigated via dynamical decoupling. This is a powerful way to divide and conquer the various error channels. In order to accomplish this without violating a no-go theorem regarding black-box dynamically corrected gates [Phys. Rev. A 80, 032314 (2009)], we require a robust operating point (sweet spot) in control space where the qubits interact with little sensitivity to noise. There are also energy gap requirements for effective adiabaticity. We apply our strategy to an architecture in Si with P donors where we assume we can shuttle electrons between different donors. Electron spins act as mobile ancillary qubits and P nuclear spins act as long-lived data qubits. This system can have a very robust operating point where the electron spin is bound to a donor in the quadratic Stark shift regime. High fidelity single qubit gates may be performed using well-established global magnetic resonance pulse sequences. Single electron-spin preparation and measurement has also been demonstrated. Putting this all together, we present a robust universal gate set for quantum computation.

Different allotropes of phosphorene are possible of which black and blue phosphorus are the most stable. While blue phosphorus has isotropic properties, black phosphorus is strongly anisotropic in its electronic and optical properties due to its anisotropic crystal structure. In this work, we systematically investigated the lattice thermal properties of black and blue phosphorene by using first-principles calculations based on the quasiharmonic approximation approach. Similar to the optoelectronic and electronic properties, we predict that black phosphorene has highly anisotropic thermal properties, in contrast to the blue phase. The linear thermal expansion coefficients along the zigzag and armchair direction differ up to 20% in black phosphorene. The armchair direction of black phosphorene is more expandable as compared to the zigzag direction and the biaxial expansion of blue phosphorene under finite temperature. Our comparative analysis reveals that the inclusion of finite-temperature effects makes the blue phase thermodynamically more stable over the black phase above 135 K.

The two-dimensional electron gas (2DEG) observed at the interface between ${\mathrm{LaAlO}}_3$ (LAO) and ${\mathrm{SrTiO}}_3$ (STO) is known to be very sensitive to the proximity of the ${\mathrm{LaAlO}}_3$ surface and the conditions to which the surface is exposed. We use first-principles calculations to study surface reconstructions on LAO films, taking into account that the LAO surface can be charged. The results for the charged surfaces and for the coupling between the surface and the 2DEG enable us to account not only for the behavior of the 2DEG as a function of thickness of the LAO layer, but simultaneously determine the stable terminations and reconstructions on the LAO surface under a variety of conditions. Our studies of charged surfaces are based on an extension of the methodology of A. Y. Lozovoi et al. [J. Chem. Phys. 115, 1661 (2001)]. From the calculated electronic structure of the unreconstructed (but relaxed) ${\mathrm{AlO}}_2$ and LaO surface terminations of LAO, we find surface states having excess holes (${\mathrm{AlO}}_2$ termination) or excess electrons (LaO termination). This result is central to understanding the mechanism of 2DEG formation, and is consistent with a 2DEG of density $3.3\times {10}^{14}{\mathrm{cm}}^{-2}$ being intrinsic to the LaO-${\mathrm{TiO}}_2$ interface in the LAO/STO system. We explore the effects of the Al-adatom, O-vacancy, and H-adatom surface reconstructions on the 2DEG density, and find that the stability of different reconstructions is tied to the thickness of the LAO layer as well as the surface exposure conditions. We find that including the effects of charging of the surface significantly stabilizes the ${\mathrm{AlO}}_2$ termination versus the LaO termination. Overall, our methodology has the advantage of decoupling first-principles calculations for the interface from those for the charged surface, and constitutes a general approach that can be applied to the commonly occurring problem of charge exchange between the surface and the interface of a thin film with a substrate, or between the surface and defects/impurities in the bulk of a material.

Recent experimental studies showed that formation of a two-dimensional crystal structure of nitrogenated holey graphene (NHG) is possible. Similar to graphene, NHGs have an atomically thin and strong crystal structure. Using first-principles calculations, we investigate the structural, phononic, and thermal properties of monolayer NHG crystal. Our charge analysis reveals that the charged holey sites of NHG provide a reactive ground for further functionalization by adatoms or molecules. We also found that similar to graphene, the NHG structure has quite high-frequency phonon modes and the presence of nitrogen atoms leads to the emergence of additional vibrational modes. Our phonon analysis reveals the presence of three characteristic Raman-active modes of NHG. Furthermore, the analysis of constant-volume heat capacity showed that the NHG structure has a linear temperature dependence in the low-temperature region. The strong lattice structure and unique thermal properties of the NHG crystal structure are desirable in nanoscale device applications.

The transfer or exchange of multipartite quantum states is critical to the realization of large-scale quantum information processing and quantum communication. In this work, we demonstrate that by using a single quantum two-level system—a qubit—as a coupler, arbitrary multipartite quantum states (either entangled or separable) can be transferred or exchanged simultaneously between two sets of qubits. During the entire process, the coupler remains unexcited minimizing the effect of coupler decoherence on the process. This feature allows one to use qubits with rapid frequency tunability and a large range of frequency tuning, such as phase qubits, as couplers. Our findings offer the potential to significantly reduce the resources needed to construct and operate large-scale quantum information networks consisting of many multiqubit registers, memory cells, and processing units.