万歆
掺杂系统，介观系统，和强关联系统等各类复杂体系的理论研究
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 姓名：万歆
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学术头衔：
博士生导师
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学科领域：
凝聚态物理学
 研究兴趣：掺杂系统，介观系统，和强关联系统等各类复杂体系的理论研究
万歆， 生于上海，1995年复旦大学物理系本科毕业。2000年美国普林斯顿大学电工系电子材料和器件专业博士。博士论文题为《二维电子气的量子相变研究》，导师R.N.Bhatt教授。2000年至2003年间在美国国家高磁场实验室凝聚态理论组从事博士后研究。2003年至2005年在德国卡尔斯鲁研究中心纳米技术所担任科研职员。2005年5月开始在浙江大学物理系近代物理中心工作，任教授职务。主要研究方向为掺杂系统，介观系统，和强关联系统等各类复杂体系的理论研究。

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13
万歆， Qinghong Cui， Xin Wan， and Kun Yang
PHYSICAL REVIEW B 70, 094506(2004)，0001，（）：
1年11月30日
We present the results of numerical studies of spin quantum Hall transitions in disordered superconductors, in which the pairing order parameter breaks timereversal symmetry. We focus mainly on pwave superconductors in which one of the spin components is conserved. The transport properties of the system are studied by numerically diagonalizing pairing Hamiltonians on a lattice, and by calculating the Chern and Thouless numbers of the quasiparticle states. We find that in the presence of disorder, (spin) current carrying states exist only at discrete critical energies in the thermodynamic limit, and the spin quantum Hall transition driven by an external Zeeman field has the same critical behavior as the usual integer quantum Hall transition of noninteracting electrons. These critical energies merge and disappear as disorder strength increases, in a manner similar to those in lattice models for integer quantum Hall transition.

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万歆， R. N. Bhatt and Xin Wan
，0001，（）：
1年11月30日
We describe a Monte Carlo simulation study of the magnetic phase diagram of diluted magnetic semiconductors doped with shallow impurities in the low concentration regime. We show that because of a wide distribution of interaction strengths, the system exhibits strong quantum effects in the magnetically ordered phase. A discrete spin model, found to closely approximate the quantum system, shows long relaxation times, and the need for specialized cluster algorithms for updating spin configurations. Results for a representative system are presented.

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【期刊论文】Modified spinwave study of random antiferromagneticferromagnetic spin chains
万歆， Xin Wan and Kun Yang， R. N. Bhatt
PHYSICAL REVIEW B 66, 014429(2002)，0001，（）：
1年11月30日
We study the thermodynamics of a onedimensional quantum spin1/2 Heisenberg ferromagnetic system with random antiferromagnetic impurity bonds. In the dilute impurity limit, we generalize the modified spinwave theory for random spin chains, where local chemical potentials for spin waves in ferromagnetic spin segments are introduced to ensure zero magnetization at finite temperature. This approach successfully describes the crossover from the behavior of a pure onedimensional ferromagnet at high temperatures to a distinct Curie behavior due to randomness at low temperatures. We discuss the effects of impurity bond strength and concentration on the crossover and lowtemperature ehavior.

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【期刊论文】Mobility gap in fractional quantum Hall liquids: Effects of disorder and layer thickness
万歆， Xin Wan， ， D. N. Sheng， E. H. Rezayi， Kun Yang， R. N. Bhatt， and F. D. M. Haldane
PHYSICAL REVIEW B 72, 075325(2005)，0001，（）：
1年11月30日
We study the behavior of twodimensional electron gas in the fractional quantum Hall regime in the presence of finite layer thickness and correlated disordered potential. Generalizing the Chern number calculation to manybody systems, we determine the mobility gaps of fractional quantum Hall states based on the distribution of Chern numbers in a microscopic model. We find excellent agreement between experimentally measured activation gaps and our calculated mobility gaps, when combining the effects of both disordered potential and layer thickness. We clarify the difference between mobility gap and spectral gap of fractional quantum Hall states and explain the disorderdriven collapse of the gap and the subsequent transitions from the fractional quantum Hall states to the insulator.

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【期刊论文】Lattice dynamics of zincblende GaN and AlN: I. Bulk phonons
万歆， Jian Zi†， Xin Wan， Guanghong Wei， Kaiming Zhang and Xide Xie
J. Phys.: Condens. Matter 8(1996)63236328，0001，（）：
1年11月30日
The lattice dynamics of zincblende GaN and AlN were studied theoretically using a twoparameter Keating potential together with the longrange Coulomb interactions. Phonon frequencies transformed from those of the wurtzite counterparts were used to determine the two Keating parameters and the effective charge. We present for the first time the phonon dispersion curves for zincblende GaN and AlN. An interesting feature was found for the phonons propagating along the [110] direction, which does not exist in other IIIV semiconductors.

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万歆， Chenggang Zhou， ， Malcolm P. Kennett， Xin Wan， *， Mona Berciu， and R. N. Bhatt
PHYSICAL REVIEW B 69, 144419(2004)，0001，（）：
1年11月30日
Motivated by the recent suggestion of anisotropic effective exchange interactions between Mn spins in Ga12xMnxAs (arising as a result of spinorbit coupling), we study their effects in diluted Heisenberg spin systems. We perform Monte Carlo simulations on several phenomenological model spin Hamiltonians, and investigate the extent to which frustration induced by anisotropic exchanges can reduce the lowtemperature magnetization in these models and the interplay of this effect with disorder in the exchange. In a model with low coordination number and purely ferromagnetic exchanges, we find that the lowtemperature magnetization is gradually reduced as exchange anisotropy is turned on. As the connectivity of the model is increased, the effect of smalltomoderate anisotropy is suppressed, and the magnetization regains its maximum saturation value at low temperatures unless the distribution of exchanges is very wide. To obtain significant suppression of the lowtemperature magnetization in a model with high connectivity, as is found for longrange interactions, we find it necessary to have both ferromagnetic and antiferromagnetic exchanges (e.g., as in the RudermanKittelKasuyaYosida interaction). This implies that disorder in the sign of the exchange interaction is much more effective in suppressing magnetization at low temperatures than exchange anisotropy.

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【期刊论文】Edge reconstruction in the fractional quantum Hall regime
万歆， Xin Wan， E. H. Rezayi， Kun Yang
PHYSICAL REVIEW B 68, 125307(2003)，0001，（）：
1年11月30日
The interplay of electronelectron interaction and confining potential can lead to the reconstruction of fractional quantum Hall edges. We have performed exact diagonalization studies on microscopic models of fractional quantum Hall liquids, in finitesize systems with disk geometry, and found numerical evidence of edge reconstruction under rather general conditions. In the present work we have taken into account effects such as layer thickness and Landaulevel mixing, which are found to be of quantitative importance in edge physics. Due to edge reconstruction, additional nonchiral edge modes arise for both incompressible and compressible states. These additional modes couple to electromagnetic fields and thus can be detected in microwave conductivity measurements. They are also expected to affect the exponent of electron Green's function, which has been measured in tunneling experiments. We have studied in this work the electric dipole spectral function that is directly related to the microwave conductivity measurement. Our results are consistent with the enhanced microwave conductivity observed in experiments performed on samples with an array of antidots at low temperatures, and its suppression at higher temperatures. We also discuss the effects of the edge reconstruction on the singleelectron spectral function at the edge.

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万歆， D. N. Sheng， ， Xin Wan， E. H. Rezayi， Kun Yang， R. N. Bhatt， and F. D. M. Haldane
PHYSICAL REVIEW LETTERS VOLUME 90, NUMBER 25 ，0001，（）：
1年11月30日
We study the v=1/3 quantum Hall state in the presence of random disorder. We calculate the topologically invariant Chern number, which is the only quantity known at present to distinguish unambiguously between insulating and current carrying states in an interacting system. The mobility gap can be determined numerically this way and is found to agree with experimental value semiquantitatively. As the disorder strength increases towards a critical value, both the mobility gap and plateau width narrow continuously and ultimately collapse, leading to an insulating phase.

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【期刊论文】Universality of the EdgeTunneling Exponent of Fractional Quantum Hall Liquids
万歆， Xin Wan， ， F. Evers， and E. H. Rezayi
PHYSICAL REVIEW LETTERS PRL 94, 166804(2005)，0001，（）：
1年11月30日
In a microscopic model of fractional quantum Hall liquids with electronelectron interactions and confinement, we calculate the edge Green's function via exact diagonalization. Our results for υ=1/3 and 2/3 suggest that, in the presence of Coulomb interaction, "extelalal" parameters such as the sharpness of the edge and the strength of the edge confining potential, which can lead to edge reconstruction, may cause deviations from universality in the edgetunneling IV exponent. In particular, we do not find any direct dependence of this exponent on the range of the interaction potential as suggested by recent calculations in contradiction to the topological nature of the edge.

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【期刊论文】Twodimensional Wigner crystal in anisotropic semiconductors
万歆， Xin Wan* and R. N. Bhatt
PHYSICAL REVIEW B, VOLUME 65, 233209，0001，（）：
1年11月30日
We investigate tile effect of mass anisotropy on tile Wigner crvstallization transition in a twodimensional (2D) electron gas. Tile static and dynamical properties of a 2D Wigner crysstal have been calculated for arbitrary 2D Bravais lattices in tile presence of anisotropic mass, as may be obtainable in Si metaloxideselniconductor fieldeffect transistors with (110) surface, as well as tile newly discovered organic senticonductor systems. By studying the stability of all possible lattices, we find significant changes in tile crystal structure and melting density of the electron lattice with tile lowest ground state energy.

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