1D Character Research Articles

Lanthanum-induced quasi-one-dimensional reconstructions on Si(111)

Various quasi-one-dimensional (1D) structures on Si(111) surface induced by submonolayer La are investigated by scanning tunneling microscopy/spectroscopy (STM/STS) and density-functional theory (DFT) calculations. Three types of reconstructions, 5 × 2, (2 m + 1) × 6 (m > 2, integer) and 2 × 3, in which rows of La adatoms are separated by Si honeycomb chain-channels (HCCs) or Seiwatz chains (SCs), are formed and their stability can be controlled by the La coverage. In 5 × 2 reconstruction, the Si HCCs and SCs are arranged alternatively with La atoms in between, while the 2 × 3 reconstruction consists of SCs only. The (2 m + 1) × 6 is an intermediate phase, in which each unit cell consists of a single HCC and (m−1) SCs depending on the La coverage. The weak inter-chain interactions in 5 × 2 and 2 × 3 evidence the 1D character of the reconstructions. STS results and DFT calculations indicate these chains are semiconducting. The thermodynamic evolution of the surface is also investigated, providing important information for the growth of LaSi2 films on Si during the initial stage.

Synthesis, structure and magnetic behaviors of a new spin-1/2 chain compound Na4CuTeO6

A new tellurate compound Na4CuTeO6 was synthesized by a conventional high-temperature solid state reaction. The titled compound crystallizes in the monoclinic system of a space group C2/c and exhibits a unique spin-chain structure built by planar CuO4 squares and TeO6 octahedra along the [101] direction. Magnetic measurements confirm that Na4CuTeO6 possesses an antiferromagnetic ordering at TN = ∼2.5 K. The intrachain and interchain interactions are estimated by a spin-chain model, showing a less pronounced 1D character in the system.

Emulating Majorana fermions and their braiding by Ising spin chains

We analyze the control of Majorana zero-energy states by mapping the fermionic system onto a chain of Ising spins. Although the topological protection is lost for the Ising system, the mapping provides additional insight into the nature of the quantum states. By controlling the local magnetic field, one can separate the Ising chain into ferromagnetic and paramagnetic phases, corresponding to topological and nontopological sections of the fermionic system. In this paper we propose (topologically nonprotected) protocols performing the braiding operation, and in fact also more general rotations. We first consider a $T$-junction geometry, but we also propose a protocol for a purely one-dimensional (1D) system. Both setups rely on an extra spin-$\frac{1}{2}$ coupler. By including the extra spin in the $T$-junction geometry, we overcome limitations due to the 1D character of the Jordan--Wigner transformation. In the 1D geometry the coupler, which controls one of the Ising links, should be manipulated once the ferromagnetic (topological) section of the chain is moved far away. We also propose experimental implementations of our scheme. One is based on a chain of flux qubits which allows for all needed control fields. We also describe how to translate our scheme for the 1D setup to a chain of superconducting wires hosting each a pair of Majorana edge states.

Quasiparticle interference and strong electron–mode coupling in the quasi-one-dimensional bands of Sr2RuO4

The single-layered ruthenate Sr$_2$RuO$_4$ has attracted a great deal of interest as a spin-triplet superconductor with an order parameter that may potentially break time reversal invariance and host half-quantized vortices with Majorana zero modes. While the actual nature of the superconducting state is still a matter of controversy, it has long been believed that it condenses from a metallic state that is well described by a conventional Fermi liquid. In this work we use a combination of Fourier transform scanning tunneling spectroscopy (FT-STS) and momentum resolved electron energy loss spectroscopy (M-EELS) to probe interaction effects in the normal state of Sr$_2$RuO$_4$. Our high-resolution FT-STS data show signatures of the \beta-band with a distinctly quasi-one-dimensional (1D) character. The band dispersion reveals surprisingly strong interaction effects that dramatically renormalize the Fermi velocity, suggesting that the normal state of Sr$_2$RuO$_4$ is that of a 'correlated metal' where correlations are strengthened by the quasi 1D nature of the bands. In addition, kinks at energies of approximately 10meV, 38meV and 70meV are observed. By comparing STM and M-EELS data we show that the two higher energy features arise from coupling with collective modes. The strong correlation effects and the kinks in the quasi 1D bands may provide important information for understanding the superconducting state. This work opens up a unique approach to revealing the superconducting order parameter in this compound.

Effect of surface disorder on the domain structure of PLZT ceramics

ABSTRACTPb1-xLax(Zr0.65Ti0.35)1-x/4O3 (PLZT x/65/35) ceramics were studied by Piezoresponse Force Microscopy in order to understand the origin of domain structure as a function of La content. We show that the domain topology is mainly determined by the composition and grain size. The characteristic correlation length decreases with increasing La content, being sensitive also to the synthesis method. The behavior of the correlation length is linked to the macroscopic properties, showing a strong increase of disorder with La doping. The roughness exponent for the domain wall in PLZT 9/65/35 is close to 2/3 indicating 1D character of domain walls in relaxors.

Hybrid Fibrillar Xerogels with Unusual Magnetic Properties.

We report on the preparation of a hybrid nanomaterial made up of 1D filaments of an antiferromagnetic self-assembling bicopper complex encapsulated in polymer nanofibrils. The encapsulation process is achieved through the heterogeneous nucleation of the growth of polymer fibrils obtained by thermoreversible gelation as shown by calorimetry experiments. Neutron scattering experiments confirm that the filaments of a bicopper complex retain their 1D character after encapsulation in the fibrils. Superconducting quantum interference device experiments show that the bicopper complex, originally in the gapped spin state in the 3D bulk mesophase, displays a gapless behavior once encapsulated. Extended absorption fine structure and infrared results further highlight the difference in the molecular arrangement of the bicopper complex between the bulk mesophase and the encapsulated state, which may account for the magnetic behavior. This material, which is largely disordered, differs totally from the usual magnetic systems where this effect is observed only on highly crystalline systems with long-range order. Also, this hybrid material is very easy to prepare from its basic constituents and can be further processed in many ways.

Edge states of graphene wrinkles in single-layer graphene grown on Ni(111)

As quasi-one-dimensional (1D) structures with characteristic widths of nanometer scale, graphene wrinkles (GWs) have been widely observed in graphene grown by chemical vapor deposition. Similar to conventional 1D graphene-based nanostructures, e.g., carbon nanotubes and graphene nanoribbons, 1D electron confinement has been observed in the GWs. However, it remains an open question whether the GWs have effective edges and exhibit corresponding edge states. Here, we report on the edge states of the GWs in single-layer graphene grown on Ni(111) by means of low temperature scanning tunneling microscopy and spectroscopy. We show that the GWs are decoupled from the substrate, while the surrounding planar graphene are strongly coupled with the substrate. The different graphene-substrate coupling leads to effective edges and 1D character of the GWs. The chiral edges of the GWs give rise to pronounced edge states around the Fermi level in the density of states.

Noncollinear exchange interaction in transition metal dichalcogenide edges

We study the Ruderman-Kittel-Kasuya-Yosida effective exchange interaction between magnetic impurities embedded on the edges of transition-metal dichalcogenide flakes, using a three-orbital tight-binding model. Electronic states lying midgap of the bulk structure have strong one-dimensional (1D) character, localized on the edges of the crystallite. This results in exchange interactions with $1/r$ (or slower) decay with distance $r$, similar to other 1D systems. Most interestingly, however, the strong spin-orbit interaction in these materials results in sizable non-collinear Dzyaloshinskii-Moriya interactions between impurities, comparable in size to the usual Ising and in-plane components. Varying the relevant Fermi energy by doping or gating may allow one to modulate the effective interactions, controlling the possible helical ground state configurations of multiple impurities.

(Invited) Aligning Organic Dipolar Molecules in Carbon Nanotubes for Nonlinear Optics

The 1D character of single-walled carbon nanotubes (SWCNT) combined with their mechanical robustness, chemical inertness and wide range of diameters makes them ideal nanocontainers for various molecules. Here we show that by encapsulating elongated dipolar molecules in the 1D internal channel of SWCNTs, Coulomb interactions naturally favour a polar head-to-tail alignment of the molecular dipoles, resulting in a cooperative enhancement of directional properties, such as the dipole moment and the second-order nonlinear optical (NLO) response.[1] This principle is demonstrated for a prototypical dipolar molecule encapsulated in various SWCNT samples. The encapsulation is evidenced by extensive wavelength-dependent fluorescence-excitation and resonant Raman experiments on bile-salt solubilised dye-filled SWCNTs[2], revealing the effect of encapsulation on the vibrational and electronic properties of the SWCNTs and the encapsulated molecules.[3-6] The polar alignment is demonstrated by wavelength-dependent hyper Rayleigh scattering experiments (HRS)[7-9] (i.e. second harmonic light scattering). The newly synthesized organic nanohybrids possess a giant total dipole moment and NLO response, corresponding to ~70 identically aligned molecules (HRS intensity scaling quadratically with this number). These nanohybrids form solution processible building blocks highly interesting for the development of electro-optic modulators and switches.

Disorder effects in the S=1 antiferromagnetic spin ladder CaV2O4

We study the physical properties of the antiferromagnetic spin ladder CaV2O4 (CVO) and the Y-doped related compound Ca0.9Y0.1V2O4. In the latter, X-ray diffraction demonstrates the segregation of a small amount of a vanadium–perovskite impurity phase, leading to the formation of V vacancies within the main CVO-type structure. The 1D character of this calcium–vanadite enhances the influence of the vacancies on the electric and magnetic properties of Ca0.9Y0.1V2O4. Electrical transport is characterized by a variable-range hopping mechanism determined by the charging energy of nm-sized segments of V chains delimited by V vacancies, i.e. a Coulomb gap is formed at the Fermi level. These vacancies also locally affect the magnetic correlations, breaking the long-range AFM order observed in CaV2O4 and producing exchange bias when the Y-doped sample is cooled with an applied magnetic field.

Quasi-one-dimensional nature of the Rashba states of Au wires on Si(5 5 7) surface

The electronic band structure of Si(557) surface adsorbed with Au wires is studied by means of first-principles density-functional-theory calculations. The spin–orbit split metallic bands are associated with the Au–Si zigzag chain and their asymmetric electron density around the Au atoms due to the on-site orbital hybridization explain large Rashba-type splitting and out-of-plane spin polarization. It is found that the Rashba metallic states have strong one-dimensional (1D) character along the Au wires but non-negligible two-dimensional distribution of the wave functions, implying not exactly genuine, only quasi 1D states.

Valence band and core-level photoemission of Au/Ge(001): Band mapping and bonding sites

We have used photoemission spectroscopy in order to investigate the electronic states and chemical bonding related to Au induced atomic chains on the Ge(001) surface. Angle-resolved photoemission reveals two types of dispersions around the Fermi level whose intensities strongly depend on the incident photon energy. Around $h\ensuremath{\nu}=100\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, the band structure is dominated by an electronlike band of mainly one-dimensional (1D) character, which shows Tomonaga-Luttinger-like power-law behavior in the $k$-integrated spectral function. In contrast, lower photon energies reveal a metallic holelike dispersion which resembles the Ge bulk structure with its heavy-hole, light-hole, and split-off branches. The Au $4f$ core-level spectra show two doublets indicating two different Au bonding sites, whereas the Ge $3d$ core-level shows two surface components and one bulk component.

Bandgap engineering of carbon allotropes

Starting from the graphene layer, the bandgap engineering of carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) is described by applying an appropriate boundary condition. Linear E-k relationship of graphene transforms to a parabolic one as momentum vector in the tube direction is reduced to dimensions smaller than inverse of the tube diameter of a CNT. Similar transition is noticeable for narrow width of a GNR. In this regime, effective mass and bandgap expressions are obtained. A CNT or GNR displays a distinctly 1D character suitable for applications in quantum transport.

Enhanced upper critical fields in a new quasi-one-dimensional superconductor Nb2PdxSe5

We report the discovery of superconductivity with Tc = 5.5 K in Nb2PdxSe5, in which one-dimensional (1D) Nb–Se chains existing along the b-direction hybridize each other to form the conducting b–c* plane. The magnetic susceptibility and specific heat data in both single crystal and polycrystal show evidence of bulk superconductivity. The resistivity, Hall coefficient and magneto-resistance data all indicate the presence of an energy scale T* = ∼ 50 K, which becomes systematically lowered under hydrostatic pressure and competes with the stabilization of superconductivity. Combined with the band calculation results showing the Fermi surfaces with 1D character, we postulate that the energy scale T* is related to the formation of a density wave or a stabilization of low-dimensional electronic structure. The zero-temperature upper critical field, Hc2(0), of the single crystal is found to be 10.5, 35 and 22 T in the a′, b and c* directions, respectively. While the linearly increasing Hc2(T) for H//c* indicates the multi-band effect, Hc2(0) for H//b and c* are found to be much bigger than the Bardeen–Cooper–Schrieffer (BCS) Pauli limiting field, 1.84 Tc ∼ 9 T. The suppressed Pauli paramagnetic effect points to the possibility of enhanced spin–orbit scattering related to the low-dimensional electronic structure or the presence of heavy elements such as Pd.

Confinement effects in π-bonded chains at group IV semiconductor (111) surfaces

The degree of 1D character of surface chains at group IV (111)-2 × 1 reconstructed surfaces is established by surface sensitive optical spectroscopy. Optical experiments on a diamond C(111)-2 × 1 surface show that the absorption peak related to dangling-bond transitions exhibits a marked blueshift upon oxygen exposure of the clean surface. Such behaviour is analogous to that observed on a clean Si(111)-2 × 1 surface. For both surfaces the experimental finding is interpreted in terms of quantum confinement of surface electrons in quasi-one-dimensional π-bonded chains, whose length decreases with oxygen uptake. A different behaviour is observed in Ge(111)-2 × 1, where only a very slight blueshift of the surface-state optical transition is detected upon oxidation. The almost negligible blueshift in Ge(111)-2 × 1 is consistent with a significant coupling between the π-bonded chains resulting in a much less pronounced one-dimensional character of Ge(111)-2 × 1 surface electrons compared to diamond and silicon reconstructed surfaces.

Bulk-Sensitive Angle-Resolved Photoemission Spectroscopy on TTF-TCNQ

TTF-TCNQ is a quasi-1D molecular conductor exhibiting metal-insulator transition at 54K accompanied by chargedensity-wave (CDW) formation. Its electronic properties in relation to the Peierls instability and CDW transitions are well investigated through the several decades. Past ARPES studies indicate the band dispersion consisting of TCNQ molecules (namely the TCNQ-band) shows the signature of spin–charge separation, together with the suppression of the quasiparticle excitation at the Fermi level. These observations have been discussed in terms of the 1D character of TTF-TCNQ, nevertheless, there are several mysteries remained. One is the band dispersions obtained by ARPES commonly indicating the band-width (or the band group velocity) larger by a factor of 2 with respect to band theory. To account for this discrepancy, the possibility of relaxed tilting of the topmost molecules and/or strong correlation effect have been raised until now. The second is the anomalous suppression of the spectral weight near the Fermi level (EF), which gives negligible amount of spectral intensity at EF even in the highly-conductive state above >60K. The electronic structure of the metallic quasi-1D state has been thus remained to be elucidated. To investigate the precise electronic structure of TTFTCNQ, here we utilized the laser-excited ARPES with h 1⁄4 6:994 eV. Owing to the relatively low photon-energy as compared with past ARPES studies (h 1⁄4 20{40 eV), the bulk-sensitive measurement becomes available. The effect of photoirradiation-induced damage peculiar to molecular compounds is also considered to be weaker compared to the measurements using higher-energy photons. The high cross section of s and p electrons for this photon-energy provides some advantage in studying the light-element based organic conductors. ARPES measurements were performed using a system constructed with VG-Scienta R4000 electron analyzer and an ultraviolet (h 1⁄4 6:994 eV) laser. He II (h 1⁄4 40:8 eV) light source was also used to check the correspondence with past reports. Single crystals of TTF-TCNQ grown by the diffusion method were cleaved in-situ to obtain fresh surfaces. The pressure was below 5 10 11 Torr throughout all the measurements. The energy resolution was E 1⁄4 4:0meV. The Fermi level EF of the sample was referred to that of a Au film evaporated on the sample substrate, with an accuracy of 0:1meV. Figure 1(a) shows the mapping of the ARPES intensity (h 1⁄4 6:994 eV) at the Fermi level, obtained by integrating the ARPES intensity in the energy window of 50meV. The shape of the Fermi surface thus observed makes a straight line at kF 1⁄4 0:24 0:01 A 1 1⁄4 ð0:29 0:01Þb , indicative of 1D electronic structure. The image of band dispersions along –Z direction obtained by h 1⁄4 6:994 and 40.8 eV are shown in Fig. 1(b,c). The peak positions of the ARPES intensity estimated from momentum distribution curve (MDC) and energy distribution curve (EDC) are plotted by circle and cross markers, respectively. The red and blue curves, on the other hand, represent the band dispersions consisting of TTF and TCNQ molecules, obtained by the first-principles calculation. By comparing with the band calculation, we can notice that the h 1⁄4 6:994 eV data is dominated by the signal from TTF bands, whereas the h 1⁄4 40:8 eV data shows both TTF and TCNQ bands that are well in accord with past reports. This h -dependence, which should be due to the photoionization cross section and/or matrix element effect, also explains the slight difference of the present ‘‘Fermi-surface’’ shape from that previously reported. The Fermi momentum obtained by ARPES, kF 1⁄4 0:24 0:01 A , is somewhat smaller compared with the band calculation, reflecting the overestimation of the charge transfer between TTF and TCNQ inevitably arising in the calculated data. Here, let us focus on the gradient of the TTF band dispersion. In the h 1⁄4 40:8 eV ARPES spectrum, the gradient of the TTF band dispersion is about 2 times larger as compared to the calculation. It shows that the observed High

Static and Dynamical Aspects of a Phonon‐Induced Spin‐Crossover Transition – Local Equilibrium Approach

AbstractWe present a dynamical extension of the phonon‐induced spin‐crossover (SC) transition model, which allows obtaining the time evolution of the metastable high spin fraction at low temperature. The model is based on 1D lattices of two level atoms coupled by springs. This model is somehow isomorphic with an Ising‐like model in which the interactions between the spins take place through lattice distortions. The thermodynamic properties are studied in the frame of a self‐consistent phonon approach, which gives an elegant variational formalism in which the 1D character of the system is preserved. Obviously, this kind of mean‐field approach allows the existence of phase transitions. Thus, we found that first‐order and two‐step SC transitions can be obtained, depending on the choice of the elastic constants between the atoms. The dynamical version of this model is also investigated by using the master equation formalism. The equations of motion of the high‐spin (HS) fraction and equal‐time two‐spin correlation are derived analytically in the frame of a local equilibrium approach, in which we consider that the phonon bath equilibrates instantaneously, in contrast to the dynamics of the macrovariables such as the high‐spin fraction. Sigmoidal curves with long relaxation tails are obtained for the time dependence of the HS fraction in the ferroelastic case. The flow diagrams and the time dependence of the configuration entropy upon relaxation are analyzed, and the obtained results are discussed in relation to those of the dynamical mean‐field theory, which is the approach most used to describe the experimental results, although it neglects short‐range fluctuations.

Dimensional Reduction and Odd-Frequency Pairing of the Checkerboard-Lattice Hubbard Model at 1/4-Filling

The ferromagnetism of the checkerboard lattice Hubbard model at quarter filling is one of the few exact ferromagnetic ground states known in the family of Hubbard models. When the nearest neighbor hopping, t1, is negligible compared with the second neighbor one, t2, the system reduces to a collection of Hubbard chains. We find that the 1D character is surprisingly robust as long as t1 < t2. This phenomenon of dimensional reduction due to the geometrical frustration leads to peculiar magnetic orders with 1D character for intermediate U and is responsible for odd-frequency superconducting states close to the magnetic boundary.

Silver(ii) triflate with one-dimensional [Ag(ii)(SO3CF3)4/2]∞ chains hosting antiferromagnetism

Silver(II) triflate, previously reported by Leung et al. in 1979 (Can. J. Chem., 1979, 57, 326–329), crystallizes in a triclinic P cell with a = 4.9117(11) A, b = 5.1136(10) A, c = 11.033(3) A, α = 79.955(14)°, β = 75.771(16)°, γ = 61.571(17)° and V = 235.68(10) A3 and is isomorphous to Cu(SO3CF3)2. The compound has a layered structure with an interlayer separation of 10.67 A; the van der Waals gaps open between the CF3 groups from neighbouring sheets. Ag(II) is coordinated by six O atoms in the form of an elongated octahedron; the adjacent Ag2+ cations are linked via –OSO– bridges; direct –O– bridges are absent. The [Ag(II)(SO3CF3)2]∞ layers consist of one-dimensional chains which interact weakly with each other via longer Ag⋯OSO–Ag contacts. This results in a 1D rather than 2D antiferromagnetic ordering, which can be described via the Bonner–Fisher model with a superexchange constant, Jintra-chain, of 104 K (9.0 meV) per pair of interacting Ag2+ cations. The magnetic ordering persists even at room temperature leading to a broad ESR signal with g = 2.199. DFT calculations confirm the 1D character of electronic structure and antiferromagnetism residing within the [Ag(II)(SO3CF3)4/2]∞ chains with a |Jinter-chain|/|Jintra-chain| ratio of ∼10−3 to 10−2. The calculated indirect bandgap at the Fermi level of ∼1 eV opens between rather flat valence and conduction bands, which are predominated by contributions from Ag and O atoms. Ag(SO3CF3)2 is extremely sensitive to moisture and decomposes instantly when exposed to atmosphere. When dry, it rapidly decomposes thermally above 120 °C, but its slow exothermic decay to AgSO3CF3 takes place even at room temperature. Silver(II) triflate is also photosensitive and irradiating it with a 632.8 nm laser radiation at a power greater than 0.17 mW leads to its decomposition to Ag(I) triflate and Ag(I)2S2O7.

Semianalytical approach to the design of photonic crystal cavities

Most current methods for the engineering of photonic crystal (PhC) cavities rely on cumbersome, computationally demanding trial-and-error procedures. In the present work, we take a different approach to the problem of cavity design, by seeking to establish a direct, semianalytic relationship between the target electromagnetic field distribution and the dielectric constant of the PhC structure supporting it. We find that such a relationship can be derived by expanding the modes of ${L}_{N}$-type cavities as a linear combination of the one-dimensional (1D) Bloch eigenmodes of a PhC W1 waveguide. Thanks to this expansion, we can also ascertain the presence of a well-defined 1D character in the modes of relatively short cavities (e.g., ${L}_{9--15}$), thus confirming recent theoretical predictions and experimental findings. Finally, we test our method through the successful design of a cavity supporting a mode with Gaussian envelope function and ultralow radiative losses (quality factor of $17.5\ifmmode\times\else\texttimes\fi{}{10}^{6}$).