arXiv subjects

Gang Su

Publications and source records attributed to Gang Su.

At least 163 records · Page 9Linked to original sources

arXiv2014

New strategy to promote conversion efficiency using high-index nanostructures in thin-film solar cells

Nano-scaled metallic or dielectric structures may provide various ways to trap light into thin-film solar cells for improving the conversion efficiency. In most schemes, the textured active layers are involved into light trapping structures that can provide perfect optical benefits but also bring undesirable degradation of electrical performance. Here we propose a novel approach to design high-performance thin-film solar cells. In our strategy, a flat active layer is adopted for avoiding electrical degradation, and an optimization algorithm is applied to seek for an optimized light trapping structure for the best optical benefit. As an example, we show that the efficiency of a flat a-Si:H thin-film solar cell can be promoted close to the certified highest value. It is also pointed out that, by choosing appropriate dielectric materials with high refractive index (>3) and high transmissivity in wavelength region of 350nm-800nm, the conversion efficiency of solar cells can be further enhanced.

physics.optics↗

arXiv2014

Hinge-like structure induced unusual properties of black phosphorus and new strategies to improve the thermoelectric performance

We systematically investigated the geometric, electronic and thermoelectric (TE) properties of bulk black phosphorus (BP) under strain. The hinge-like structure of BP brings unusual mechanical responses such as anisotropic Young's modulus and negative Poisson's ratio. A sensitive electronic structure of BP makes it transform among metal, direct and indirect semiconductors under strain. The maximal figure of merit $ZT$ of BP is found to be 0.72 at $800\,\mathrm{K}$ that could be enhanced to 0.87 by exerting an appropriate strain, revealing BP could be a potential medium-high temperature TE material. Such strain-induced enhancements of TE performance are often observed to occur at the boundary of the direct-indirect band gap transition, which can be attributed to the increase of degeneracy of energy valleys at the transition point. By comparing the structure of BP with SnSe, a family of potential TE materials with hinge-like structure are suggested. This study not only exposes various novel properties of BP under strain, but also proposes effective strategies to seek for better TE materials.

cond-mat.mtrl-sci↗

arXiv2014

Evidence of the side jump mechanism in the anomalous Hall effect in paramagnets

Persistent confusion has existed between the intrinsic (Berry curvature) and the side jump mechanisms of anomalous Hall effect (AHE) in ferromagnets. We provide unambiguous identification of the side jump mechanism, in addition to the skew scattering contribution in epitaxial paramagnetic Ni$_{34}$Cu$_{66}$ thin films, in which the intrinsic contribution is by definition excluded. Furthermore, the temperature dependence of the AHE further reveals that the side jump mechanism is dominated by the elastic scattering.

cond-mat.mtrl-sci↗

arXiv2014

Featureless quantum spin liquid, 1/3-magnetization plateau state and exotic thermodynamic properties of spin-1/2 frustrated Heisenberg antiferromagnet on an infinite Husimi lattice

By utilizing tensor-network-based methods, we investigate the zero- and finite-temperature properties of the spin-1/2 Heisenberg antiferromagnetic (HAF) model on an infinite Husimi lattice that contains 3/2 sites per triangle. The ground state of this model is found to possess vanishing local magnetization and is featureless; the spin-spin and dimer-dimer correlation functions are verified to decay exponentially; and its ground-state energy per site is determined to be $e_0=-0.4343(1)$, which is very close to that [$e_0=-0.4386(5)$] of the intriguing kagome HAF model. The magnetization curve shows the absence of a zero-magnetization plateau, implying a gapless excitation. A $1/3$-magnetization plateau with spin up-up-down state is observed, which is selected and stabilized by quantum fluctuations. A ground state phase diagram under magnetic fields is presented. Moreover, both magnetic susceptibility and the specific heat are studied, whose low-temperature behaviors reinforce the conclusion that the HAF model on the infinite Husimi lattice owns a gapless and featureless spin liquid ground state.

cond-mat.str-el↗

arXiv2013

Superstructures of Cubic and Hexagonal Diamonds Comprising a Family of Novel $sp^3$ Superhard Carbon Allotropes

Superstructures of cubic and hexagonal diamonds (h- and c-diamond) comprising a family of stable diamond-like $sp^3$ hybridized novel carbon allotropes are proposed, which are referred to as U$_n$-carbon where $n \geq 2$ denotes the number of structural layers in a unit cell. The conventional h- and c-diamond are included in this family as members with $n=2$ and 3, respectively. U$_n$-carbon ($n=4-6$), which are unveiled energetically and thermodynamically more stable than h-diamond and possess remarkable kinetic stabilities, are shown to be insulators with indirect gaps of $5.6 \sim 5.8$ eV, densities of $ 3.5 \sim 3.6$ g/cm$^3$, bulk modulus of $4.3 \sim 4.4 \times 10^{2}$ GPa, and Vickers hardness of $92.9 \sim 97.5$ GPa even harder than h- and c-diamond. The simulated x-ray diffraction and Raman spectra are presented for experimental characterization. These new structures of carbon would have a compelling impact in physics, chemistry, materials science and geophysics.

cond-mat.mtrl-sci↗

arXiv2013

Electronic and magnetic properties of twisted graphene nanoribbon and Möbius strips: first-principles calculations

The geometrical, electronic, and magnetic properties of twisted zigzag-edged graphene nanoribbons (ZGNRs) and novel graphene Möbius strips (GMS) are systematically investigated using first-principles density functional calculations. The structures of ZGNRs and GMS are optimized, and their stabilities are examined. The molecular energy levels and the spin polarized density of states are calculated. It is found that for twisted ZGNRs, the atomic bonding energy decreases quadratically with the increase of the twisted angle, and the HOMO-LUMO gap are varying in a sine-like behavior with the twisted angle. The calculated spin densities reveal that the ZGNRs and GMS have antiferromagnetic ground states, which persist during the twisting. The spin flips on the zigzag edges of GMS are observed at some positions.

cond-mat.mtrl-sci↗

arXiv2013

Theory of Network Contractor Dynamics for Exploring Thermodynamic Properties of Two-dimensional Quantum Lattice Models

Based on the tensor network state representation, we develop a nonlinear dynamic theory coined as network contractor dynamics (NCD) to explore the thermodynamic properties of two-dimensional quantum lattice models. By invoking the rank-$1$ decomposition in the multi-linear algebra, the NCD scheme makes the contraction of the tensor network of the partition function be realized through a contraction of a local tensor cluster with vectors on its boundary. An imaginary-time-sweep algorithm for implementation of the NCD method is proposed for practical numerical simulations. We benchmark the NCD scheme on the square Ising model, which shows a great accuracy. Besides, the results on the spin-1/2 Heisenberg antiferromagnet on honeycomb lattice are disclosed in good agreement with the quantum Monte Carlo calculations. The quasi-entanglement entropy $S$, Lyapunov exponent $I^{lya}$ and loop character $I^{loop}$ are introduced within the dynamic scheme, which are found to display the ``nonlocality" near the critical point, and can be applied to determine the thermodynamic phase transitions of both classical and quantum systems.

cond-mat.str-el↗

arXiv2013

Honeycomb Heisenberg Spin Ladder: Unusual Ground State and Thermodynamic Properties

The unusual ground state and thermodynamic properties of spin-1/2 two-leg honeycomb (HC) spin ladder are systematically studied by jointly utilizing various analytical and numerical methods. The HC spin ladder is found to exhibit behaviors dramatically different from those of the conventional square spin ladder. A strong relevant term nJ and a half saturation magnetization plateau that can be attributed to the formation of diluted dimer states are observed in the HC ladder, both of which are absent in the square ladder. The ground state phase diagram of the HC spin ladder is identified, and the thermodynamic properties of the specific heat and susceptibility for different couplings are thoroughly explored, where two kinds of excitations are unveiled. The distinct Wilson ratios for both spin ladders at the lower critical fields are also obtained. Our calculated result is well fitted to the experimental data of the two-leg HC spin ladder compound [Cu2L1(N3)4]n.

cond-mat.str-el↗

arXiv2013

Kosterlitz-Thouless phase transition and reentrance in an anisotropic 3-state Potts model on the generalized Kagome lattice

The unusual reentrant phenomenon is observed in the anisotropic 3-state Potts model on a gen- eralized Kagome lattice. By employing the linearized tensor renormalization group method, we find that the reentrance can appear in the region not only under a partial ordered phase as commonly known but also a phase without a local order parameter, which is uncovered to fall into the uni- versality of the Kosterlitz-Thouless (KT) type. The region of the reentrance depends strongly on the ratios of the next nearest couplings α = J2 /|J1 | and β = J3 /|J1 |. The phase diagrams in the plane of temperature versus β for different α are obtained. Through massive calculations, it is also revealed that the quasi-entanglement entropy can be used to accurately detect the KT transition temperature.

cond-mat.str-el↗

arXiv2012

Low-energy effective theory and two distinct critical phases in a spin-1/2 frustrated three-leg spin tube

Motivated by the crystal structures of [(CuCl2tachH)3Cl]Cl2 and Ca3Co2O6, we develop a low-energy effective theory using the bosonization technique for a spin-1/2 frustrated three-leg spin tube with trigonal prism units in two limit cases. The features obtained with the effective theory are numerically elucidated by the density matrix renormalization group method. Three different quantum phases in the ground state of the system, say, one gapped dimerized phase and two distinct gapless phases, are identified, where the two gapless phases are found to have the conformal central charge c=1 and 3/2, respectively. Spin gaps, spin and dimer correlation functions, and the entanglement entropy are obtained. In particular, it is disclosed that the critical phase with c=3/2 is the consequence of spin frustrations, which might belong to the SU(2)k=2 Wess-Zumino-Witten-Novikov universality class, and is induced by the twist term in the bosonized Hamiltonian density.

cond-mat.str-el↗

arXiv2012

Strain-induced Dirac cone-like electronic structures and semiconductor-semimetal transition in graphdiyne

By means of the first-principles calculations combined with the tight-binding approximation, the strain-induced semiconductor-semimetal transition in graphdiyne is discovered. It is shown that the band gap of graphdiyne increases from 0.47 eV to 1.39 eV with increasing the biaxial tensile strain, while the band gap decreases from 0.47 eV to nearly zero with increasing the uniaxial tensile strain, and Dirac cone-like electronic structures are observed. The uniaxial strain-induced changes of the electronic structures of graphdiyne come from the breaking of geometrical symmetry that lifts the degeneracy of energy bands. The properties of graphdiyne under strains are disclosed different remarkably from that of graphene.

cond-mat.mtrl-sci↗

arXiv2012

Octagraphene as a Versatile Carbon Atomic Sheet for Novel Nanotubes, Unconventional Fullerenes and Hydrogen Storage

We study a versatile structurally favorable periodic $sp^2$-bonded carbon atomic planar sheet with $C_{4v}$ symmetry by means of the first-principles calculations. This carbon allotrope is composed of carbon octagons and squares with two bond lengths and is thus dubbed as octagraphene. It is a semimetal with the Fermi surface consisting of one hole and one electron pocket, whose low-energy physics can be well described by a tight-binding model of $π$-electrons. Its Young's modulus, breaking strength and Poisson's ratio are obtained to be 306 $N/m$, 34.4 $N/m$ and 0.13, respectively, which are close to those of graphene. The novel sawtooth and armchair carbon nanotubes as well as unconventional fullerenes can also be constructed from octagraphene. It is found that the Ti-absorbed octagraphene can be allowed for hydrogen storage with capacity around 7.76 wt%.

cond-mat.mtrl-sci↗

arXiv2012

Optimized Decimation of Tensor Networks with Super-orthogonalization for Two-Dimensional Quantum Lattice Models

A novel algorithm based on the optimized decimation of tensor networks with super-orthogonalization (ODTNS) that can be applied to simulate efficiently and accurately not only the thermodynamic but also the ground state properties of two-dimensional (2D) quantum lattice models is proposed. By transforming the 2D quantum model into a three-dimensional (3D) closed tensor network (TN) comprised of the tensor product density operator and a 3D brick-wall TN, the free energy of the system can be calculated with the imaginary time evolution, in which the network Tucker decomposition is suggested for the first time to obtain the optimal lower-dimensional approximation on the bond space by transforming the TN into a super-orthogonal form. The efficiency and accuracy of this algorithm are testified, which are fairly comparable with the quantum Monte Carlo calculations. Besides, the present ODTNS scheme can also be applicable to the 2D frustrated quantum spin models with nice efficiency.

cond-mat.stat-mech↗

arXiv2012

Matrix Product State and Quantum Phase Transitions in the One-Dimensional Extended Quantum Compass Model

The matrix product state (MPS) is utilized to study the ground state properties and quantum phase transitions (QPTs) of the one-dimensional quantum compass model (QCM). The MPS wavefunctions are argued to be very efficient descriptions of QCM ground states, and are numerically determined by imaginary time projections. The ground state energy, correlations, quantum entanglement and its spectrum, local and nonlocal order parameters, etc., are calculated and studied in details. It is revealed that the bipartite and block entanglement entropies, as well as the nearest neighbor correlation functions can be used to detect the second-order QPTs, but not the first-order ones, while fidelity detections can recognize both. The entanglement spectrum is extracted from the MPS wavefunction, and found to be doubly degenerate in disordered phases of QCM, where non-local string order parameters exist. Moreover, with linearized tensor renormalization group method, the specific heat curves are evaluated and their low temperature behaviors are investigated.

cond-mat.str-el↗

arXiv2012

Phase Diagrams, Distinct Conformal Anomalies and Thermodynamics of Spin-1 Bond-Alternating Heisenberg Antiferromagnetic Chain in Magnetic Fields

The ground state phase diagrams and thermodynamic properties of a spin-1 bond-alternating Heisenberg antiferromagnetic chain with a single-ion anisotropy in longitudinal and transverse magnetic fields are investigated jointly by means of the infinite time evolving block decimation, the linearized tensor renormalization group and the density matrix renormalization group methods. It is found that in the magnetic field-bond alternating ratio plane six phases such as singlet dimer, Haldane, two Tomonaga-Luttinger liquid, 1/2 magnetization plateau and spin polarized phases are identified in a longitudinal field, while in a transverse field there are fives phases including singlet dimer, Haldane, Z$_2$ symmetry breaking, quasi-1/2 magnetization plateau and quasi spin polarized phases. A reentrant behavior of the staggered magnetization in a transverse field is observed. The quantum critical behaviors in longitudinal and transverse magnetic fields are disclosed to fall into different universality classes with corresponding conformal field central charge $c=1$ and 1/2, respectively. The experimental data of the compound NTENP under both longitudinal and transverse magnetic fields are nicely fitted, and the Luttinger liquid behavior of low temperature specific heat experimentally observed is also confirmed.

cond-mat.str-el↗

arXiv2012

Thermoelectric Properties of Silicon Carbide Nanowires with Nitrogen Dopants and Vacancies

The thermoelectric properties of cubic zincblend silicon carbide nanowires (SiCNWs) with nitrogen impurities and vacancies along [111] direction are theoretically studied by means of atomistic simulations. It is found that the thermoelectric figure of merit ZT of SiCNWs can be significantly enhanced by doping N impurities together with making Si vacancies. Aiming at obtaining a large ZT, we study possible energetically stable configurations, and disclose that, when N dopants locate at the center, a small number of Si vacancies at corners are most favored for n-type nanowires, while a large number of Si vacancies spreading into the flat edge sites are most favored for p-type nanowires. For the SiCNW with a diameter of 1.1 nm and a length of 4.6 nm, the ZT value for the n-type is shown capable of reaching 1.78 at 900K. The conditions to get higher ZT values for longer SiCNWs are also addressed.

cond-mat.mes-hall↗

arXiv2012

Charged States and Band-Gap Narrowing in Codoped ZnO Nanowires for Enhanced Photoelectrochemical Responses

By means of first-principles calculations within the density functional theory, we study the structural and optical properties of codoped ZnO nanowires and compare them with those of the bulk and film. It is disclosed that the low negatively charged ground states of nitrogen related defects play a key role in the optical absorption spectrum tail that narrows the band-gap and enhances the photoelectrochemical response significantly. A strategy of uncompensated N, P and Ga codoping in ZnO nanowires is proposed to produce a red-shift of the optical absorption spectra further than the exclusive N doping and to get a proper formation energy with a high defect concentration and a suppressed recombination rate. In this way, the absorption of the visible light can be improved and the photocurrent can be raised. These observations are consistent with the existing experiments, which will be helpful to improve the photoelectrochemical responses for the wide-band-gap semiconductors especially in water splitting applications.

cond-mat.mes-hall↗

arXiv2011

Modified Tucker Decomposition for Tensor Network and Fast Linearized Tensor Renormalization Group Algorithm for Two-Dimensional Quantum Spin Lattice Systems

We propose a novel algorithm with a modified Tucker decomposition for tensor network that allows for efficiently and precisely calculating the ground state and thermodynamic properties of two-dimensional (2D) quantum spin lattice systems, and is coined as the fast linearized tensor renormalization group (fLTRG). Its amazing efficiency and precision are examined by studying the spin-1/2 anisotropic Heisenberg antiferromagnet on a honeycomb lattice, and the results are found to be fairly in agreement with the quantum Monte Carlo calculations. It is also successfully applied to tackle a quasi-2D spin-1/2 frustrated bilayer honeycomb Heisenberg model, where a quantum phase transition from an ordered antiferromagnetic state to a gapless quantum spin liquid phase is found. The thermodynamic behaviors of this frustrated spin system are also explored. The present fLTRG algorithm could be readily extended to other quantum lattice systems.

cond-mat.str-el↗