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1.
Applying the transfer matrix and Green's function methods, we study the valley-resolved transport properties of zigzag graphene nanoribbon (ZGNR) junctions. The width of the left and right ZGNRs are NL and NR, and NLNR. The step/dip positions of the conductance G, the intravalley transmission coefficients (TKK and ${T}_{{K}^{{\prime} }{K}^{{\prime} }}$), and the valley polarization efficiency ${P}_{{{KK}}^{{\prime} }}$ correspond to the subband edges of the right/left ZGNR that are controlled by NR/NL. The intervalley transmission coefficients (${T}_{{{KK}}^{{\prime} }}$ and ${T}_{{K}^{{\prime} }K}$) exhibit peaks at most of the subband edge of the left and right ZGNRs. In the bulk gap of the right ZGNR, ${T}_{{{KK}}^{{\prime} }}$ $={T}_{{K}^{{\prime} }K}$=0, and ${P}_{{{KK}}^{{\prime} }}$ = ±1, the valley polarization is well preserved. As NR increases, the energy region for ${P}_{{{KK}}^{{\prime} }}$ = ±1 decreases. When NL is fixed and NR decreases, G, TKK, ${T}_{{K}^{{\prime} }{K}^{{\prime} }}$ and ${P}_{{{KK}}^{{\prime} }}$ exhibit more and more dips, and the peaks of ${T}_{{{KK}}^{{\prime} }}$ (${T}_{{K}^{{\prime} }K}$) become more and more high, especially when (NLNR)/2 is odd. These characters are quite useful for manipulating the valley dependent transport properties of carriers in ZGNR junctions by modulating NL or NR, and our results are helpful to the design of valleytronics based on ZGNR junctions.  相似文献   

2.
We study the spectrum of the QCD Dirac operator for two colors with fermions in the fundamental representation and for two or more colors with adjoint fermions. For Nf flavors, the chiral flavor symmetry of these theories is spontaneously broken according to SU (2NfSp (2Nf) and SU (NfO (Nf), respectively, rather than the symmetry breaking pattern SU (Nf) × SU (Nf) → SU (Nf) for QCD with three or more colors and fundamental fermions. In this paper we study the Dirac spectrum for the first two symmetry breaking patterns. Following previous work for the third case we find the Dirac spectrum in the domain λ ΛQCD by means of partially quenched chiral perturbation theory. In particular, this result allows us to calculate the slope of the Dirac spectrum at λ = 0. We also show that for λ 1/L2 ΛQCD (wing L the linear size fo the system) the Dirac spectrum is given by a chiral Random Matrix Theory with the symmetries of the Dirac operator.  相似文献   

3.
The magnetic dipole term T appearing in the X-ray magnetic circular dichroism (XMCD) spin sum rule can be eliminated from the analysis within the spin sum rule by angle-dependent XMCD spectroscopy if the effects of spin–orbit coupling are small so that Tx+Ty+Tz≈0. It is shown by the ab initio electron theory for the extreme case of a low-dimensional system, i.e., for a free-standing monatomic Co wire that this relation is strongly violated, indicating that the determination of T by the angle-dependent XMCD is possibly not very reliable for low-dimensional magnetic systems.  相似文献   

4.
The exact perturbation approach is used to derive the elementary correlation lengths ξi and related mass gaps mi of the two-dimensional dilute AL lattice model in regimes 1 and 2 for L odd from the Bethe ansatz solution. In regime 2 the A3 model is the E8 lattice realisation of the two-dimensional Ising model in a magnetic field at T = Tc The calculations for the A3 model in regime 2 start from the eight thermodynamically significant string types found in previous numerical studies. These string types are seen to be consistent in the ordered high field limit. The eight masses obtained reduce with the approach to criticality to the ES masses predicted by Zamolodchikov, thus providing a further direct lattice determination of the Es mass spectrum.  相似文献   

5.
We present a nonperturbative lattice formulation of noncommutative Yang–Mills theories in arbitrary even dimension. We show that lattice regularization of a noncommutative field theory requires finite lattice volume which automatically provides both an ultraviolet and an infrared cutoff. We demonstrate explicitly Morita equivalence of commutative U(p) gauge theory with p·nf flavours of fundamental matter fields on a lattice of size L with twisted boundary conditions and noncommutative U(1) gauge theory with nf species of matter on a lattice of size p·L with single-valued fields. We discuss the relation with twisted large N reduced models and construct observables in noncommutative gauge theory with matter.  相似文献   

6.
The Tc and oxygen content of TlBa2CaCu2Oy have been investigated by quenching experiments, in which the heat-treated samples were dropped into liquid nitrogen. The oxygen loss, v, of TlBa2CaCu2Oyov was determined by thermogravimetry in a nitrogen atmosphere using an oxygen-annealed specimen of TlBa2CaCu2Oyo and also by measurements of the weight differences before and after quenching. Tc increased from 80 K of the oxygen-annealed specimen up to about 110 K with increasing oxygen loss up to v = 0.035 by annealing at 500°C in a nitrogen atmosphere. Judging from the high Tc above 110 K achieved by a small oxygen loss about v = 0.035, the as-sintered oxygen-annealed TlBa2CaCu2Oyo specimen was in the over-doping state and probably has an oxygen vacancy of 7 − yo0.  相似文献   

7.
We present here the detailed analysis of the magnetic behavior of the Co0.53Ga0.47 alloy, especially at temperatures above the freezing temperature Tf = 10 K. Low field static magnetization measurements were performed by using the SQUID magnetometer in the temperature range 5–65 K and magnetic fields up to 100 Oe. The temperature dependence of the field cooled susceptibility πFC(T) at T > Tf has an anomaly, which is displayed in the double change of the curvature near Ts = 24 K. The data of magnetization MFC in an external field H lie on a universal curve MFC(H/T) at temperatures Tf < T < Ts. The plots of π-1FC(T) and non-linear magnetic susceptibility πnlFC(T-3) are linear lines in the temperature range TfTs. The strong deviation of π-1FC(T) and πnlFC(T-3) from straight line, taking place at T Ts, indicates that Ts is an upper temperature limit of the classical superparamagnetic behavior with the constant cluster moment. The results suggest that such phenomena may be fairly universal for spin glasses.  相似文献   

8.
The Axx and Ayy spin correlation coefficients of proton-proton scattering have been measured at Elab = 46.9 MeV and θc.m. = 90° with the same experimental techniques already used at Saclay from 11 to 26 MeV and at Grenoble at 37.2 MeV. For Axx the deviation from the predictions of the energy-dependent phase-shifts analysis of Livermore is significant (≈ 3.7 standard deviations) but not as large as for the 47.5 MeV Tokyo measurement. For Ayy the agreement is very good.  相似文献   

9.
The magnetic ordering of the orthorhombic compound CeSi (Pama space group, FeB type of structure, Z = 4) Tn = 5.6 K was investigated by means of powder neutron diffraction in the temperatures 1.5 and 10 K. Our results show that the magnetic ordering is associated with a three dimensionally amplitude-modulated phase which is incommensurate with the crystal lattice with wave vector q = (qχ,qy,qz). The ordered Ce3+ moments are confined to the plane (a,b) at an angle of 16.7° with the b direction. It follows from the present data analysis that the dominant wave vector component is qz, which approximately corresponds to a transversal modulation, however the incommensurateness involves also the qx and qy components. The amplitude of the sine wave is μ0j = 1.667μB at 1.5 K which corresponds to quite a reduced ordered moment value compared to the Ce3+ free ion value gJ = 2.14μB.  相似文献   

10.
The magnetic ordering of the hexagonal compound TbMn6Ge6 has been studied by neutron diffraction at various temperatures between 1.8 and 500 K. In almost the whole magnetically ordered regime the magnetic structures are incommensurate with the crystal lattice. The corresponding wave vector q1 = (0,0,qz) is strongly temperature dependent and decreases from qz = 0.1307 r.l.u. at 1.8 K to qz = 0.091 r.l.u. at 410 K. The low-temperature range (T < 85 K) is characterised by a triple ferrimagnetic spiral structure with net moment along the c-direction. At Tt = 85 K the net moment disappears and the magnetic ordering changes into a flat spiral structure that probably persists up to the magnetic ordering temperature, TN = 450 K. Before reaching the magnetic ordering temperature, however, an additional ferrimagnetic component perpendicular to the c-direction develops, marking the onset of re-entrant ferrimagnetism. The resulting structure described by two propagation vectors corresponds to a distorted spiral. Various models are discussed. The observed re-entrant ferrimagnetism is in agreement with results of previously reported magnetic measurements.  相似文献   

11.
刘宏波 《中国物理 B》2017,26(11):117701-117701
The influences of specific heat capacity CP, temperature step △T, electric field step △ E, and initial electric field E1 on predicted electrocaloric (EC) temperature △ T of monodomain BaTiO3 are examined by combining the Maxwell equation and phenomenological theory. Since the procedure is similar to indirect measurement of the EC effect, the results can serve as a reference for experiments. The results suggest that (i) it is reasonable to use zero-field CP, (ii) optimized △ T should be 2 K, (iii) it is better to keep △ E<EC, and (iv) E1<EC. Here, EC is the coercive field of material.  相似文献   

12.
王敏  李京 《计算物理》1996,13(1):38-42
用Vlasov-Poisson方程对相对论电子束在单板、双板间的传播过程进行了数值模拟,给出了单板模型空间电荷积累最大的位置,不同位置上的电流J、电子数密度n、电场E的振荡频率随入射电子数密度n0、入射速度v0的变化关系,双板模型空间电荷积累最大的位置,JnE的振荡频率随入射流J0及两板间距离的变化关系。虚阴极位置的数值结果与稳态理论给出的结果相近,它的振荡频率符合经验公式(1~√2π)ωpeb。单板时入射电子数密度按速度服从高斯分布,能散△En/En < 10%时的数值结果给出与单能情况基本相同的结论。  相似文献   

13.
We study the effects of Lorentz non-invariance on the physical pion velocity at the critical temperature Tc in an effective theory of hidden local symmetry (HLS) with the “vector manifestation” fixed point. We match at a “matching scale” ΛM the axial-vector current correlator in the HLS with the one in the operator product expansion for QCD, and present the matching condition to determine the bare pion velocity. We find that the physical pion velocity, which is found to be one at T=Tc when starting from the Lorentz invariant bare HLS, remains close to one with the Lorentz non-invariance, vπ(Tc)=0.83–0.99. This result is quite similar to the pion velocity in dense matter.  相似文献   

14.
A spin one XY ferromagnet with uniaxial anisotropy has been investigated, using Green's function technique in random phase approximation (RPA). The Green functions associated with the anisotropy energy are treated without decoupling. A set of coupled equations have been obtained to find the critical temperature Tc and (SZ)2 at Tc as function of the uniaxial anisotropy parameter D. Tc and (SZ)2 at Tc are found to increase with D. The results are compared with the earlier results obtained in the Narath type of RPA.  相似文献   

15.
We have studied the stationary Josephson effect on YBa2Cu3O7−δ (Tc=90 K) and Bi2Sr2Ca1Cu2 O8 (Tc=80 K and 87 K for two samples of different origin) ceramic based junctions. The temperature dependence of the critical current near Tc has been found as Ic≈(Tc-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi2Sr2Ca1Cu2O8, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/kBTc3.5.  相似文献   

16.
Recent measurements from the HERMES and SMC Collaborations show a remarkably large azimuthal single-spin asymmetries AUL and AUT of the proton in semi-inclusive pion leptoproduction γ*(q)p→πX. We show that final-state interactions from gluon exchange between the outgoing quark and the target spectator system lead to single-spin asymmetries in deep inelastic lepton–proton scattering at leading twist in perturbative QCD; i.e., the rescattering corrections are not power-law suppressed at large photon virtuality Q2 at fixed xbj. The existence of such single-spin asymmetries requires a phase difference between two amplitudes coupling the proton target with Jzp=±1/2 to the same final-state, the same amplitudes which are necessary to produce a nonzero proton anomalous magnetic moment. We show that the exchange of gauge particles between the outgoing quark and the proton spectators produces a Coulomb-like complex phase which depends on the angular momentum Lz of the proton's constituents and is thus distinct for different proton spin amplitudes. The single-spin asymmetry which arises from such final-state interactions does not factorize into a product of distribution function and fragmentation function, and it is not related to the transversity distribution δq(x,Q) which correlates transversely polarized quarks with the spin of the transversely polarized target nucleon.  相似文献   

17.
Photocurrent measurements on oxidized, single-crystal TiO2 undoped and doped with V, Cr, Mn or Fe were made both before and after exposure to white light from a tungsten lamp; they revealed a band of states centered at 2.0 eV below the conduction-band edge Ec that appears to be the band of surface states active in the photoelectrolysis of water. They also revealed a band of bulk states extending from near the valence-band edge Ev to about 1.9 eV below Ec. The ESR signals from V4+, Cr3+ and Fe3+ ions in oxidized samples and from Mn3+ ions in reduced samples were measured before and after exposure to white and monochromatic light; changes in the ESR signal intensities allow placement of the V4+: d1 level about 2.1 eV below Ec, the Cr3+: d3 level about 2.7 eV belo Ec, the Fe3+ level at or just below the valence band edge Ev which is 3.0 eV below Ec, the Fe2+ level approximately at Ec, the Mn3+ level about 1.9 eV below Ec. Comparison of experiments on oxidized and reduced samples suggests that the bulk states are associated with cation vacancies, and measurements with polarized light gave anisotropies that are qualitatively interpretable with a cation-vacancy model for the oxidized samples.  相似文献   

18.
Initial axial kinetic energy (Ez) distributions of Li+ and F secondary ions desorbed from LiF surfaces, bombarded by low current (500 particles/s) N3+ beams in the 0.075–7.5 MeV primary ion energy (EPI) range, were measured by using the time-of-flight technique. In this energy range, the electronic energy loss, Se, increases from 25 to 160 eV/Å, while the nuclear stopping power, Sn, decreases from 8.0 to 0.25 eV/Å. The observed F initial axial kinetic energy distribution can be described by the linear collision cascade theory and no other contribution was found. The F total yield decreases proportionally to Sn. The Li+ distribution presents a remarkable deviation from the cascade prediction, indicating the existence of additional mechanisms related to the electronic energy-loss process. For the range of EPI studied, these mechanisms produce a Li+ energy distribution with a Maxwell-Boltzman-like shape, which vanishes above Ez 10 eV and presents a maximum at Ez 1.2 eV. The Li+ yield is nonlinear with the electronic energy loss, Se. A simple desorption model, based on the spatial distribution of the energy deposited by the projectile and on the effective energy-loss concept, is presented. This spatial distribution of the deposited energy is due to secondary electron cascades and is connected with a ESD-like mechanism on the surface.  相似文献   

19.
We have used electron spin resonance measurements to derive the temperature and frequency dependences of the field-induced magnetization [M(T, f)] and anisotropy field [Han (T)] in a number of amorphous alloys belonging to the series (FepNi1−p)75P16B6Al3. In re-entrant (p > pc, the critical concentration for ferromagnetism) alloys at hi gh frequencies (f = 35 GHz, field ≈ 12 kOe) M reduces as T3/2 at high T and as T below ≈ 40 K, the deviation from T3/2 becoming more marked as pp+c. For p close to pc, lowering the frequency first causes the T term to increase and ultimately ( ≈ 4 GHz) changes the variation of M with T to that discovered previously for concentrated spin glasses, namely M is constant at low T and drops linearly at high temperatures. For the re-entrants, the results are interpreted on the basis of a model which invokes an energy gap in the spin-wave spectrum, introduces a non-zero density of states of the gap energy and takes into consideration a low-q cut-off in the spin-wave integral in thelow-T (T) regime.In the concentrated spin glasses [M (0) - M (T)]/ M (0) is well represented by the function [exp (Δ / T) - 1]-1, where Δ has values close to the corresponding Curie-Weiss temperatures θp but much larger than the respective spin glass transition temperatures TSG. The temperature dependence of Han is largely given by the function (1 - T/T*), where T* is equal to the zero-field freezing temperature for the re-entrants and TSG for the spin glasses, respectively.  相似文献   

20.
We discuss the thermodynamic Bethe ansatz, and explain how it allows one to reduce the infinite-volume thermodynamics of a (1 + 1)-dimensional purely elastic scattering theory to the solution of a set of integral equations for the one-particle excitation energies. The free energy at zero chemical potential(s) and temperature T is related to the ground state energy E0(R) of the theory on a cylinder of circumference R = 1/T. E0(R) determines properties of the CFT describing the UV limit of the given massive theory. These include the central charge (which we investigated in earlier work), the scaling dimension d of the conformal field whose perturbation leads to the massive theory, the coefficients in the conformal perturbation theory (CPT) expansion of E0(R) in powers of R2−d, and the bulk term in the CPT calculation of the ground-state energy. We determine the bulk term analytically, and obtain numerically the first six coefficients in the expansion of E0(R) for many purely elastic scattering theories, including the scaling limit of the T = Tc Ising model in a magnetic field. The perfect agreement with (more limited) direct CPT results provides further strong support for the identification of these theories as specific perturbed CFTs. We suggest that the singularities of E0(R), the first of which is responsible for the finite radius of convergence of CPT, are square-root branch points and related to the zeros of the partition function of the corresponding lattice model.  相似文献   

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