We propose an efficient three-qubit gate of singlet-triplet states in quantum dots based on capacitative coupling. This scheme can be used to generate the maximally entangled Greenberger-Horne-Zeilinger state with one simple gate operation. Our simulations using a realistic microscopic model combined with our detailed analysis for the gate operation can be used to extract the actual experimental pulse sequence needed to realize this.

The Si/Ag(110) interface is attracting increasing interest since the recent claim that the one-dimensional (1D) nanostructures formed at this surface would be silicene nanoribbons. Lately, the synthesis of multilayer silicene nanoribbons was reported as well. In this paper, we report on the scanning tunneling microscopy, low-energy electron diffraction, and density functional theory study of the 1D nanostructures forming on Ag(110) upon Si deposition at temperatures above 460 K. Two different kinds of nanostructures are here described: nanodikes and nanotrenches, respectively protruding from and engraved on the Ag(110) substrate. The first kind of structure was recently reported to be constituted by multistacks of silicene nanoribbons. We show here that nanodikes and nanotrenches are the result of silver faceting stabilized by silicon atoms. Indeed, such nanostructures share the very same facet crystallographic features, namely inclination and reconstruction: the top and base parts are Ag(110) planes, while their sides are Ag(221) and Ag(22$\overline{1}$) facets stabilized by Si atoms. Finally, density functional theory calculations suggest that Si atoms may substitute Ag atoms in such facets, casting doubts on the usual assumption that Ag is a suitable substrate for silicene growth because it is inert against silicon.

We study theoretically the relative importance of short-range disorder in determining the low-temperature two-dimensional (2D) mobility in GaAs-based structures with respect to Coulomb disorder, which is known to be the dominant disorder in semiconductor systems. We give results for unscreened and screened short-range disorder effects on 2D mobility in quantum wells and heterostructures, comparing with the results for Coulomb disorder and finding that the asymptotic high-density mobility is always limited by short-range disorder which, in general, becomes effectively stronger with increasing “carrier density”, in contrast to Coulomb disorder. We also predict an intriguing reentrant metal-insulator transition at very high carrier densities in Si metal-oxide-semiconductor field-effect transistors driven by the short-range disorder associated with surface-roughness scattering.

The methylammonium lead iodide perovskites at the core of recently proposed solar cells with exceptionally large quantum conversion efficiency are studied by first-principles methods. Large absorption coefficients (0.03–0.04 nm${}^{-1}$ for wavelength $\sim$500 nm) and small effective masses suited for both $n$-type and $p$-type transport are obtained as a consequence of their peculiar structural and electronic characteristics. In particular, the presence of a direct gap between highly dispersed Pb(6$s$)-I($5p$) valence bands and Pb($6p$) conduction bands is the key ingredient at the basis of their excellent performance in photovoltaic applications.

Resonant x-ray diffraction was used to study the proton jumps in hydrogen-bonded rubidium dihydrogen phosphate (RDP) crystals. In the paraelectric RDP phase, hydrogen is delocalized between two crystallographically equivalent positions. At lower temperatures, this symmetry can be broken, which defines the processes that lead to the para- to ferroelectric phase transition. We have measured the energy spectra of the forbidden reflections 006 and 550 at incident radiation energies close to the Rb $K$ edge in a wide temperature range, down to the temperature of the ferroelectric phase transition. In the paraelectric phase, we observed a growth of integrated intensity for both forbidden reflections with temperature. This behavior is opposite to conventional nonresonant Bragg reflections, where intensity decreases in accordance with the Debye-Waller factor. The developed theoretical model explains this effect with the thermal motion induced (TMI) scattering mechanism and also confirms the adiabatic approximation stating that electrons instantly follow the nuclei movements. In the 550 energy spectra, we have observed an additional contribution to the resonant structure factor, which could be associated with the presence of transient Slater-type proton configurations (PC) in the half-filled hydrogen position.

We investigate the $N$-leg spin-$S$ Heisenberg ladders by using the density matrix renormalization group method. We present estimates of the spin gap ${\Delta }_s$ and of the ground-state energy per site $e_{\infty }^N$ in the thermodynamic limit for ladders with widths up to six legs and spin $S\le \frac{5}{2}$. We also estimate the ground-state energy per site $e_{\infty }^{2\mathrm{D}}$ for the infinite two-dimensional spin-$S$ Heisenberg model. Our results support that for ladders with semi-integer spins the spin excitation is gapless for $N$ odd and gapped for $N$ even, whereas for integer spin ladders the spin gap is nonzero, independent of the number of legs. Those results agree with the well-known conjectures of Haldane and Sénéchal-Sierra for chains and ladders, respectively. We also observe edge states for ladders with $N$ odd, similar to what happens in spin chains.

In order to assess the phase transition nature in Sr${}_3$Ir${}_4$Sn${}_{13}$, we have carried out the electrical resistivity, Hall coefficient, Seebeck coefficient, thermal conductivity, specific heat, and ${}^{119}$Sn nuclear magnetic resonance (NMR) measurements, mainly focusing on the signatures around the phase transition temperature $T^{*}=147$ K. The phase transition has been characterized by marked features near $T^{*}$ in all measured physical quantities. In particular, the Hall measurement reveals the abrupt change in both magnitude and sign of the charge carriers below $T^{*}$, providing strong evidence for the Fermi surface reconstruction associated with this phase transition. Moreover, the NMR observations indicate the presence of the splitting of the resonance lines below $T^{*}$ which could be accounted for by the local distortion of the Sn2 icosahedra within the $Pm\overline{3}n$ phase. The NMR Knight shift analysis further provides microscopic evidence for the reduction in both Sn 5$s$ and Ir 5$d$ electronic states near the Fermi surfaces of Sr${}_3$Ir${}_4$Sn${}_{13}$. With respect to these observations, it suggests that the strong interplay between electronic and structural instability is responsible for the peculiar phase transition in Sr${}_3$Ir${}_4$Sn${}_{13}$.

We investigate the effect of disorder on the heat transport properties of the $S=\frac{1}{2}$ Heisenberg spin chain compound Sr${}_2$CuO${}_3$ upon chemically substituting Sr by increasing concentrations of Ca. As Ca occupies sites outside but near the Cu–O–Cu spin chains, bond disorder, i.e., a spatial variation of the exchange interaction $J$, is expected to be realized in these chains. We observe that the magnetic heat conductivity (${\kappa }_{\mathrm{mag}}$) due to spinons propagating in the chains is gradually but strongly suppressed with increasing amounts of Ca, where the doping dependence can be understood in terms of increased scattering of spinons due to Ca-induced disorder. This is also reflected in the spinon mean-free path which can be separated in a doping-independent but temperature-dependent scattering length due to spinon-phonon scattering, and a temperature-independent but doping-dependent spinon-defect scattering length. The latter spans from very large ($>$1300 lattice spacings) to very short ($\sim$12 lattice spacings) and scales with the average distance between two neighboring Ca atoms. Thus, the Ca-induced disorder acts as an effective defect within the spin chain, and the doping scheme allows us to cover the whole doping regime between the clean and dirty limits. Interestingly, at maximum impurity levels, we observe in Ca-doped Sr${}_2$CuO${}_3$ an almost linear increase of ${\kappa }_{\mathrm{mag}}$ at temperatures above 100 K which reflects the intrinsic low-temperature behavior of heat transport in a Heisenberg spin chain. These findings are quite different from that observed for the Ca-doped double spin chain compound, SrCuO${}_2$, where the effect of Ca seems to saturate already at intermediate doping levels.

We study the nearest-neighbor exchange ($J_1$) and second-nearest-neighbor exchange ($J_2$) $XY$ antiferromagnet on the honeycomb lattice using ground state series expansions around Néel, columnar, and dimer phases. The conventional two-sublattice $XY$ Néel order at small $J_2$ vanishes at $J_2/J_1=0.22\pm 0.01$ in agreement with results from density matrix renormalization group (DMRG) studies. Near the transition, we find evidence for an approximate emergent symmetry between $XY$ and Ising degrees of freedom, namely the nearest-neighbor Ising and $XY$ spin correlations become nearly equal. This suggests that the system is close to a bicritical point separating $XY$ and Ising orders. At still larger $J_2/J_1$ the columnar and dimer energies are found to be nearly degenerate. At even larger $J_2$ the columnar phase is obtained. The ground state energies in all three phases are in good agreement with the values found in the DMRG studies.

We study spin-half fermions in a one-dimensional extended Hubbard chain at low filling. We identify three triplet and one singlet pairing channels in the system, which are independently tunable as a function of nearest-neighbor charge and spin interactions. In a large-size system with translational invariance, we derive gap equations for the corresponding pairing gaps and obtain a Bogoliubov–de Gennes Hamiltonian with its nontrivial topology determined by the interplay of these gaps. In an open-end system with a fixed number of particles, we compute the exact many-body ground state and identify the dominant pairing revealed by the pair density matrix. Both cases show competition between the four pairing states, resulting in broad regions for each of them and relatively narrow regions for mixed-pairing states in the parameter space. Our results enable the possibility of tuning a nanowire between singlet and triplet pairing states without breaking time-reversal or $\text{SU}\left(2\right)$ symmetry, accompanied by a change in the system's topology.

The Kitaev-Hubbard model of interacting fermions is defined on the honeycomb lattice and, at strong coupling, interpolates between the Heisenberg model and the Kitaev model. It is basically a Hubbard model with ordinary hopping $t$ and spin-dependent hopping $t^{\prime }$. We study this model in the weak to intermediate coupling regime, at half filling, using the cellular dynamical impurity approximation (CDIA), an approach related to dynamical mean field theory but based on Potthoff's variational principle. We identify four phases in the $(U,t^{\prime })$ plane: two semimetallic phases with different numbers of Dirac points, an antiferromagnetic insulator, and an algebraic spin liquid. The last two are separated by a first-order transition. These four phases all meet at a single point and could be realized in cold atom systems.

We have investigated the pressure-induced spin-state transition in Co${}^{2+}$ systems in terms of a competition between Hund's exchange energy ($J$) and crystal-field splitting (${\Delta }_{\mathrm{CF}}$). First, we show the universal metastability of the low-spin state in octahedrally coordinated Co${}^{2+}$ systems. Then we present the strategy to search for a Co${}^{2+}$ system, for which the mechanism of spin-state and metal-insulator transitions is governed not by Mott physics but by $J$ versus ${\Delta }_{\mathrm{CF}}$ physics. Using CoCl${}_2$ as a prototypical Co${}^{2+}$ system, we have demonstrated the pressure-induced spin-state transition from high-spin to low-spin, which is accompanied with insulator-to-metal and antiferromagnetic to half-metallic ferromagnetic transitions. Combined with the metastable character of Co${}^{2+}$ and the high compressibility nature of CoCl${}_2$, a transition pressure as low as 27 GPa can be identified on the basis of $J$ versus ${\Delta }_{\mathrm{CF}}$ physics.

Even with renewed interest in Ge as a competitor to Si in field-effect transistors, several key features of the surface electronic structure of Ge(001) have remained controversial. Notably, the origin of strong Fermi-level pinning in Ge has been heavily debated. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) and first-principles hybrid density functional theory calculations, we compare and unambiguously establish the critical differences between the electronic structure of the Si and Ge (001) surfaces. We explicitly show that the surface state that determines the charge neutrality level, and thus the Schottky barrier height in Si, is actually a surface resonance in Ge. It means that the evanescent states near the Ge surface play an essential role in the strong Fermi-level pinning. Additionally, we identify the origin of a number of highly debated ARPES features for Ge(001) and Si(001).

In this paper, the electronic properties of as-deposited and N${}_2$-exposed CuPc/F${}_{16}$CuPc interface, a prototype system for organic photovoltaic applications, are investigated by using ultralow background, high-sensitivity photoemission spectroscopy. It is found that (i) N${}_2$ exposure significantly modifies the energy level alignment (ELA) at the interface between CuPc and F${}_{16}$CuPc layer and (ii) the direction of the N${}_2$-induced energy level shift of the CuPc depends on the position of the Fermi level ($E_F$) in the CuPc highest occupied molecular orbital-lowest unoccupied molecular orbital gap of the as-deposited film. These observations are related to the changes in the density of gap states (DOGS) produced by structural imperfections in the molecular packing geometry, as introduced by the N${}_2$ penetration into the CuPc layer. This result demonstrates the key role of structure-induced DOGS in controlling the ELA at organic/organic interfaces.

We report on the observation of the cyclotron-resonance-assisted photon drag effect. Resonant photocurrent is detected in InSb/InAlSb quantum well structures subject to a static magnetic field and excited by terahertz radiation at oblique incidence. The developed theory based on Boltzmann's kinetic equation is in good agreement with the experimental findings. We show that the resonant photocurrent originates from the transfer of photon momentum to free electrons drastically enhanced at the cyclotron resonance.

Auger parameter (Δα) measurements have been employed to determine the extent to which initial- and final-state effects govern surface core-level shifts in x-ray photoelectron spectroscopy (XPS) measurements of Pd atoms confined between a bilayer SiO${}_2$ film and its Ru(0001) support. For atoms bound in this manner, we note negative binding energy shifts (ΔBEs) of ∼0.3 eV, relative to the Pd 3$d$ peak position in the bulk, and attribute these shifts to large variations in the initial-state orbital energies of the supported atoms (∼1.1 eV towards $E_F$), coupled with decreased final-state relaxation contributions (∼0.8 eV). Theoretical calculations reveal that, despite small partial positive charges and decreased final-state screening, the decreased 4$d$-5sp hybridization of the undercoordinated Pd atoms results in large enough upward 3$d$ orbital-energy shifts to yield the net-negative ΔBE noted by XPS.

We derive an efficient and unbiased method for computing order parameters in correlated electron systems with competing instabilities. Charge, magnetic, and pairing fluctuations above the energy scale of spontaneous symmetry breaking are taken into account by a functional renormalization group flow, while the formation of order below that scale is treated in mean-field theory. The method captures fluctuation-driven instabilities such as $d$-wave superconductivity. As a first application we study the competition between antiferromagnetism and superconductivity in the ground state of the two-dimensional Hubbard model.

Polarization-controlled synchrotron radiation was used to map the electronic structure of buried conducting interfaces of LaAlO${}_3$/SrTiO${}_3$ in a resonant angle-resolved photoemission experiment. A strong polarization dependence of the Fermi surface and band dispersions is demonstrated, highlighting different Ti $3d$ orbitals involved in two-dimensional (2D) conduction. Measurements on samples with different doping levels reveal different band occupancies and Fermi-surface areas. The photoemission results are directly compared with advanced first-principles calculations, carried out for different $3d$-band filling levels connected with the 2D mobile carrier concentrations obtained from transport measurements, with indication of charge localization at the interface.