Quasi Two-dimensional System Research Articles

Diffusion and Velocity Correlations of the Phase Transitions in a System of Macroscopic Rolling Spheres.

We study an air-fluidized granular monolayer composed of plastic spheres which roll on a metallic grid. The air current is adjusted so that the spheres never lose contact with the grid and so that the dynamics may be regarded as pseudo two dimensional (or two dimensional, if the effects of the sphere rolling are not taken into account). We find two surprising continuous transitions, both of them displaying two coexisting phases. Moreover, in all the cases, we found the coexisting phases display a strong energy non-equipartition. In the first transition, at a weak fluidization, a glass phase coexists with a disordered fluid-like phase. In the second transition, a hexagonal crystal coexists with the fluid phase. We analyze, for these two-phase systems, the specific diffusive properties of each phase, as well as the velocity correlations. Surprisingly, we find a glass phase at a very low packing fraction and for a wide range of granular temperatures. Both phases are also characterized by strong anticorrelated velocities upon a collision. Thus, the dynamics observed for this quasi two-dimensional system unveil phase transitions with peculiar properties, very different from the predicted behavior in well-know theories for their equilibrium counterparts.

Chiral electronic excitations in the quasi-two-dimensional Rashba system BiTeI

The optical transitions between spin-polarized bands of the quasi-two dimensional Rashba system BiTeI are investigated using polarization resolved resonant Raman spectroscopy. We detect chiral excitations between states with opposite helicity and compare spectra to calculations within a three-band model. Using the resonant Raman excitation profile, we deduce the Rashba parameters and band gaps of the higher conduction bands near the Fermi level, and compare the parameters to values obtained by ab initio density function theory (DFT).

Pressure dependence of structural, elastic, electronic, thermodynamic, and optical properties of van der Waals-type NaSn2P2 pnictide superconductor: Insights from DFT study

NaSn2P2 is a recently discovered superconducting system belonging to a particular class of materials with van der Waals (vdW) structure. There is enormous interest in vdW compounds because of their intriguing electrical, optical, chemical, thermal, and superconducting state properties. We have studied the pressure dependent structural, thermo-physical, electronic band structure, and superconducting state properties of this quasi-two dimensional system in details for the first time via ab initio technique. The optical properties are also investigated for different electric field polarizations for the first time. Structural, electronic, and optical properties were explored via density functional theory (DFT) calculations. Thermal properties were investigated using the quasi-harmonic Debye model. NaSn2P2 is found to be mechanically stable in the pressure range 0–3.0 GPa. The elastic anisotropy indices point towards high level of mechanical and bonding anisotropy in NaSn2P2 consistent with its highly layered structure. The elastic constants, moduli, and Debye temperature (θD) show non-monotonic variation with pressure, particularly close to 1.0 GPa. The pressure dependent superconducting transition temperature, Tc, of NaSn2P2 is predicted to vary strongly with the pressure dependent variation of θD. The electronic energy dispersion curves, E(k), reveal high level of direction dependence; the effective masses of charge carries are particularly high for the out-of-plane (c-axis) charge transport. The optical parameters compliment the underlying electronic energy density of states features and are weakly dependent on the polarization of the incident electric field. The reflectivity of NaSn2P2 is very high in the visible region and remains quite high and non-selective over an extended energy range in the ultraviolet region. The absorption coefficient is also high in the mid-ultraviolet band. All these optical features render NaSn2P2 suitable for optoelectronic device applications.

Quantum magneto transport properties of nanostructure multi quantum wells short wave Infrared detectors

In this paper, we investigated the band structure and quantum magneto transport properties of the In0.53Ga0.47As(d1=100Å)/InP(d2=70Å) type I multi quantum wells (MQWs) at low temperature. These studies were based on the envelope function and effective mass formalisms. We calculated the effect of d1, d2, the band valence offset and temperature on the band gap and the cut-off wavelength of detection. The result of the computed density of states and the position of Fermi level indicate that this sample is quasi two-dimensional system with n-type conductivity. The calculated evolution of the cutoff wavelength with temperature predicts this MQWs can be used as a short-infrared detector. Furthermore, we interpreted theoretically the photoluminescence, the Shubnikov de Haas (SdH) and quantum Hall effects observed by Pusep et al.

Counterflow-Induced Inverse Energy Cascade in Three-Dimensional Superfluid Turbulence.

Finite-temperature quantum turbulence is often described in terms of two immiscible fluids that can flow with a nonzero-mean relative velocity. Such out-of-equilibrium state is known as counterflow superfluid turbulence. We report here the emergence of a counterflow-induced inverse energy cascade in three-dimensional superfluid flows by performing extensive numerical simulations of the Hall-Vinen-Bekarevich-Khalatnikov model. As the intensity of the mean counterflow is increased, an abrupt transition, from a fully three-dimensional turbulent flow to a quasi-two-dimensional system exhibiting a split cascade, is observed. The findings of this work could motivate new experimental settings to study quasi-two-dimensional superfluid turbulence in the bulk of three-dimensional experiments. They might also find applications beyond superfluids in systems described by more than one fluid component.

Dipole–dipole interaction induced phases in hydrogen-bonded squaric acid crystal

We study the finite-temperature phase diagram of proton ordering of a quasi-two dimensional hydrogen-bonded system, namely the squaric acid crystal (H2C4O4) using quantum Monte Carlo. We take into account the four-spin plaquette interaction at the zeroth order followed by next nearest neighbor Ising interaction within a plaquette, dipole–dipole interaction and an external transverse magnetic field respectively. Using an improvised loop algorithm within the stochastic series expansion (SSE) quantum Monte Carlo method, we find two distinct phases as we increase the temperature and magnetic-field. One of the phase is the Πf, the phase with long range ferroelectric order and the other being an intermediate state with strong local correlations, i.e, a quantum liquid-like state Πql. The transition to Πf shows a very small anomalous peak in the specific heat with strong dependence of critical temperature on the strength of dipole–dipole interaction. The presence of the small peak is attributed to the absence of macroscopic degeneracy in the presence of dipole–dipole interaction and re-entrance of such degeneracy to some extent at small temperature. The work also discusses an intricate connection of quantum fluctuation and thermal fluctuation in the presence of competing interaction with entropic effects.

Symmetry breaking at the (111) interfaces of SrTiO3 hosting a two-dimensional electron system

We used x-ray absorption spectroscopy to study the orbital symmetry and the energy band splitting of (111) LaAlO${_3}$/SrTiO${_3}$ and LaAlO${_3}$/EuTiO${_3}$/SrTiO${_3}$ heterostructures, hosting a quasi two-dimensional electron system (q2DES), and of a Ti-terminated (111) SrTiO${_3}$ single crystal, also known to form a q2DES at its surface. We demonstrate that the bulk tetragonal Ti-3d D${_4}$${_h}$ crystal field is turned into trigonal D${_3}$${_d}$ crystal field in all cases. The symmetry adapted a${_1}$${_g}$ and e${^\pi_g}$ orbitals are non-degenerate in energy and their splitting, \Delta, is positive at the bare STO surface but negative in the heterostructures, where the a${_1}$${_g}$ orbital is lowest in energy. These results demonstrate that the interfacial symmetry breaking induced by epitaxial engineering of oxide interfaces has a dramatic effect on their electronic properties, and it can be used to manipulate the ground state of the q2DES.

Investigations in electronic quantum transport of quasi two dimensional InxGa1-xAs/InP nanostructure superlattice for infrared detection

We investigated, theoretically, the electronic band structures, transport and magneto-transport properties of In0.53Ga0.47As(d1 = 10 nm)/InP(d2 = 5 nm) lattice matched superlattice (SL) at 1.7 K. These studies were performed in the envelope function formalism with effects of well thickness d1. The SL has a direct band gap of 849 meV and the cut-off wavelength of 1.46 μm indicates that it can be used as a short infrared detector. The computed density of states and position of Fermi level, predict that this sample is a quasi two-dimensional system and exhibits n type conductivity. We have interpreted the oscillations in the magneto-resistance observed by Pusep et al. in this superlattice. These results were compared and discussed with the available data in the literature.

Dynamics of granular segregation in quasi two-dimensional system

Segregation during flow of granular materials is important from an industrial point of view. Considerable work has been done on granular segregation in heap flows by continuous pouring. We studied the flow and segregation of granular mixtures during heap formation in a quasi two-dimensional rectangular bin by intermittent pouring. The heap formed by repeatedly pouring a fixed mass of the mixture. Each feeding results in the formation of a layer of the mixture on the surface of the heap. The system is a simplified model for the feeding of raw materials to a blast furnace, which is widely used for the manufacture of iron and steel. Experiments were carried out to study the dynamics of granular materials during heap formation. The number density, area fraction and average velocity of small and big particles are plotted across the flowing depth with time. Results shows that larger particles are always on top flowing over small particles. During flow small particles easily percolate through the gaps between the large particles. A thin layer of small particles is also observed at the free surface. Here the system never reached a steady state as we are pouring the mixture intermittently and system is closed. The velocity increases initially and then decreases towards the end. The number density (i.e. area fraction) profile changes for small and big particles during flow. Image analysis is done to detect the position of each particle on the side wall. Each experiment is repeated six times to get average data.

Evolution of the vortex state in the BCS–BEC crossover of a quasi two-dimensional superfluid Fermi gas**Project supported by the National Natural Science Foundation of China (Grant Nos. 51331006, 51590883, and 11204321) and the Project of Chinese Academy of Sciences (Grant No. KJZD-EW-M05-3).

We use the path-integral formalism to investigate the vortex properties of a quasi-two dimensional (2D) Fermi superfluid system trapped in an optical lattice potential. Within the framework of mean-field theory, the cooper pair density, the atom number density, and the vortex core size are calculated from weakly interacting BCS regime to strongly coupled while weakly interacting BEC regime. Numerical results show that the atoms gradually penetrate into the vortex core as the system evolves from BEC to BCS regime. Meanwhile, the presence of the optical lattice allows us to analyze the vortex properties in the crossover from three-dimensional (3D) to 2D case. Furthermore, using a simple re-normalization procedure, we find that the two-body bound state exists only when the interaction is stronger than a critical one denoted by Gc which is obtained as a function of the lattice potential’s parameter. Finally, we investigate the vortex core size and find that it grows with increasing interaction strength. In particular, by analyzing the behavior of the vortex core size in both BCS and BEC regimes, we find that the vortex core size behaves quite differently for positive and negative chemical potentials.

Crystal Growth with Oxygen Partial Pressure of the BaCuSi2O6 and Ba1–xSrxCuSi2O6 Spin Dimer Compounds

$BaCuSi_2O_6$ is a quasi-two dimensional spin dimer system and a model material for studying Bose-Einstein condensation (BEC) of magnons in high magnetic fields. The new $Ba_{1-x}Sr_xCuSi_2O_6$ mixed system, which can be grown with x < 0.3, and $BaCuSi_2O_6$, both grown by using a crystal growth method with enhanced oxygen partial pressure, have the same tetragonal structure ($I4_1/acd$) at room temperature. The mixed system shows no structural phase transition, so that the tetragonal structure is stable down to low temperatures. The oxygen partial pressure acts as control parameter for the growth process. A detailed understanding of the crystal structure depending on the oxygen content will enable the study of the spin dynamics of field-induced order states in this model magnetic compound of high current interest with only one type of dimer layers, which shows the same distance between the Cu atoms, in the structure.

Effect of dimensionality on sliding charge density waves: The quasi-two-dimensionalTbTe3system probed by coherent x-ray diffraction

We report on sliding Charge Density Wave (CDW) in the quasi two-dimensional TbTe$_3$ system probed by coherent x-ray diffraction combined with {\it in-situ} transport measurements. We show that the non-Ohmic conductivity in TbTe$_3$ is made possible thanks to a strong distortion of the CDW. Our diffraction experiment versus current shows first that the CDW remains undeformed below the threshold current I$_S$ and then suddenly rotates and reorders by motion above threshold. Contrary to quasi-one dimensional systems, the CDW in TbTe$_3$ does not display any phase shifts below I$_S$ and tolerates only slow spatial variations of the phase above. This is a first observation of CDW behavior in the bulk in a quasi-two dimensional system allowing collective transport of charges at room temperature.

Chiral Unitary Quantum Phase Transition in 2H-Fe $$_x$$ x TaSe $$_2$$ 2

We have observed a metal-insulator transition of a quasi-two dimensional electronic system in transition metal dichalcogenide $2H$-TaSe$_2$ caused by doping iron. The sheet resistance of $2H$-Fe$_x$TaSe$_2$ ($0 \leq x \leq 0.120$) single crystals rises about $10^6$ times with the increasing of $x$ at the lowest temperature. We investigated the temperature dependence of the resistance and found a metal-insulator transition with a critical sheet resistance $11.7 \pm 5.4$ k$\rm{\Omega}$. The critical exponent of the localization length $\nu$ is estimated $0.31 \pm 0.18$. The values of the critical sheet resistance and $\nu$ are accordant to those of the \textit{chiral unitary class} (less than $h/1.49e^2=17.3$ k$\rm{\Omega}$ and $0.35 \pm 0.03$, respectively). We suggest that $2H$-Fe$_x$TaSe$_2$ is classified as the chiral unitary class, not as standard unitary class.

Experimental Investigation of Coke Collapse in Quasi-two Dimensional System for a Blast Furnace

In the iron making industry, the blast furnace is charged with coke and ore (metallic) particles by means of an inclined chute rotating about its vertical axis. For each rotation, the particles fall on the sloping burden and form a circular ring. Charging of this metallic ore burden over the layer of coke causes a portion of the coke layer to get dislodged from its original position. This phenomenon is known as coke collapse. We have studied these phenomena in a quasi two dimensional rectangular bin on a smaller scale. In-order to get the desired density difference between the pouring media, stainless-steel balls (representing the ore) and glass- beads (representing the coke) are used as the model granular materials. The parameters varied in the experiments are the volume of pouring and the size of particles (steel balls). Varying quantities of these particles are poured into the experimental unit and the resulting pattern thus formed is then captured using a Nikon-DX camera. The images are analyzed using computer code to determine the position of each particle (steel ball and glass bead). The results presented here are in terms of concentration of steel balls by making bins of 5x5mm in size. The interface (stock line) between the steel balls and the glass beads has been analyzed separately and plotted for all data sets. Each experiment has been repeated three times in-order to get an average data.

Sliding state of the quasi-two dimensional CDW system TbTe3 probed by coherent x-ray diffraction

Coherent x-ray diffraction experiments have been performed to probe the sliding state of a quasi-two dimensional charge density wave (CDW) in TbTe3. The 2kF satellite reflection associated with the CDW has been observed with respect to external dc currents, on both sides of the threshold current. These measurements illustrate how the periodic lattice distortion associated to the CDW supports the collective motion of a sliding CDW in a quasi-two dimensional system. A comparison with the quasi-one dimensional NbSe3 system is done.

Revealing multiple charge-density-wave orders inTbTe3by optical conductivity and ultrafast pump-probe experiments

Rare-earth tri-tellurium RTe$_3$ is a typical quasi-two dimensional system which exhibits obvious charge density wave (CDW) orders. So far, RTe$_3$ with heavier R ions (Dy, Ho, Er and Tm) are believed to experience two CDW phase transitions, while the lighter ones only hold one. TbTe$_3$ is claimed to belong to the latter. However in this work we present evidences that TbTe$_3$ also possesses more than one CDW order. Aside from the one at 336 K, which was extensively studied and reported to be driven by imperfect Fermi surface nesting with a wave vector $q=(2/7 c^*)$, a new CDW energy gap (260 meV) develops at around 165 K, revealed by both infrared reflectivity spectroscopy and ultrafast pump-probe spectroscopy. More intriguingly, the origin of this energy gap is different from the second CDW order in the heavier R ions-based compounds RTe$_3$ (R=Dy, Ho, Er and Tm).

Interaction effects near constriction of a quasi two-dimensional electron system: an exact diagonalization study

• A quasi 2D few-electron system is studied in a closed 3D geometry with a constriction. • In the exact ground-state the majority spin accumulates at the constriction. • A quasi-bound single-electron island is found for odd electron numbers. • In local potential cavities the opposite spin densities are nearly equal to each other. We present a systematic exact diagonalization study of a quasi two-dimensional few-electron system in a closed three-dimensional cylindrical geometry containing a constriction that models a quantum point contact formed within the two-dimensional electron gas. The calculations that are performed using the configuration interaction approach with multicenter Gaussian basis indicate that the tendency of the majority spin to gather in the area of an increased effective potential – e.g. by constriction of the quantum point contact – is distinct already for weak constrictions and low electron numbers. Opposite effects – nearly equal spin up and spin down densities – are obtained for local cavities of the confinement potential. Formation of a quasi-bound single-spin island within the constriction is also discussed.

Peierls-type structural phase transition in a crystal induced by magnetic breakdown

We predict a new type of phase transition in a quasi-two dimensional system of electrons at high magnetic fields, namely the stabilization of a density wave which transforms a two dimensional open Fermi surface into a periodic chain of large pockets with small distances between them. The quantum tunneling of electrons between the neighboring closed orbits enveloping these pockets transforms the electron spectrum into a set of extremely narrow energy bands and gaps which decreases the total electron energy, thus leading to a magnetic breakdown induced density wave (MBIDW) ground state. We show that this DW instability has some qualitatively different properties in comparison to analogous DW instabilities of Peierls type. E. g. the critical temperature of the MBIDW phase transition arises and disappears in a peculiar way with a change of the inverse magnetic field.

Spin Liquids and Antiferromagnetic Order in the Shastry-Sutherland-Lattice CompoundYb2Pt2Pb

We present measurements of the magnetic susceptibility χ and the magnetization M of single crystals of metallic Yb(2)Pt(2)Pb, where localized Yb moments lie on the dimerized and frustrated Shastry-Sutherland lattice (SSL). Strong magnetic frustration is found in this quasi-two-dimensional system, which orders antiferromagnetically at T(N) = 2.02 K from a paramagnetic liquid of Yb dimers, having a gap Δ = 4.6 K between the singlet ground state and the triplet excited states. Magnetic fields suppress the antiferromagnetic (AF) order, which vanishes at a 1.23 T quantum critical point. The spin gap Δ persists to 1.5 T, indicating that dimer singlets survive the collapse of the B = 0 AF state. Quantized steps are observed in M(B) within the AF state, a signature of SSL systems. Our results show that Yb(2) Pt(2)Pb is unique, both as a metallic SSL system that is close to an AF quantum critical point, and as a heavy fermion compound where quantum frustration plays a decisive role.

Spin response of electrons on the surface of a topological insulator

The surface of a topological insulator hosts a very special form of a quasi-two dimensional metallic system when it is embedded in a topologically trivial medium like the vacuum. The electronic properties of this unusual 2D metal are distinct in many aspects from both the conventional two-dimensional electron gas systems in quantum well heterostructures as well as those of a single layer graphene. In this paper, we study one of these distinct features i.e., the response of the electronic spins to an applied magnetic field perpendicular to the surface. We find an unusual behaviour of the spin magnetization and susceptibility as a function of both the magnetic field and the chemical potential for a generic topological surface. We propose that this behavior could be studied by the recently developed experimental technique called \beta NMR which is highly sensitive to the surface electron spins. We explain how this technique could be used to probe for spontaneous magnetic ordering caused by magnetic dopants or interactions discussed in the recent literature.