DNA methyltransferase 1 (DNMT1) is the primary enzyme that maintains DNA methylation. We describe a previously unknown mode of regulation of DNMT1 protein stability through the coordinated action of an array of DNMT1-associated proteins. DNMT1 was destabilized by acetylation by the acetyltransferase Tip60, which triggered ubiquitination by the E3 ligase UHRF1, thereby targeting DNMT1 for proteasomal degradation. In contrast, DNMT1 was stabilized by histone deacetylase 1 (HDAC1) and the deubiquitinase HAUSP (herpes virus–associated ubiquitin-specific protease). Analysis of the abundance of DNMT1 and Tip60, as well as the association between HAUSP and DNMT1, suggested that during the cell cycle the initiation of DNMT1 degradation was coordinated with the end of DNA replication and the need for DNMT activity. In human colon cancers, the abundance of DNMT1 correlated with that of HAUSP. HAUSP knockdown rendered colon cancer cells more sensitive to killing by HDAC inhibitors both in tissue culture and in tumor xenograft models. Thus, these studies provide a mechanism-based rationale for the development of HDAC and HAUSP inhibitors for combined use in cancer therapy.

We report the theoretical discovery of a systematic scheme to produce topological flat bands (TFBs) with arbitrary Chern numbers. We find that generically a multi-orbital high Chern number TFB model can be constructed by considering multi-layer Chern number C = 1 TFB models with enhanced translational symmetry. A series of models are presented as examples, including a twoband model on a triangular lattice with a Chern number C = 3 and an N -band square lattice model with C = N for an arbitrary integer N . In all these models, the flatness ratio for the TFBs is larger than 30 and increases with increasing Chern number. In the presence of appropriate inter-particle interactions, these models are likely to lead to the formation of novel Abelian and Non-Abelian fractional Chern insulators. As a simple example, we test the C = 2 model with hardcore bosons at 1/3 filling and an intriguing fractional quantum Hall (FQH) state is observed. Introduction -The experimental fractional quantum Hall effect (FQHE) arises from the highly degenerate Landau levels of continuum 2D electron systems, and is described by variational wave functions, first proposed by Laughlin for the primary FQHE states [1] and later generalized by Jain for composite fermion states [2], which are analytic functions of the 2D spatial coordinates. Many important properties of the FQHE, e.g., the hierarchy structures and fractionalized excitations [3,4], can be understood within this framework. It even leads to the predictions of intriguing non-Abelian FQH states at certain filling fractions [5][6][7][8]. Moreover, based on a classification of the pattern of zeros of symmetric (analytic) polynomials, a systematic way to classify FQH states [9,10] has been proposed. Thus, our current theoretical knowledge of FQHE is based on the analytic structure the 2D Landau level (LL) Hilbert space.

We consider the magnetic field dependence of the chemical potential for parabolically confined quantum dots in a strong magnetic field. Approximate expressions based on the notion that the size of a dot is determined by a competition between confinement and interaction energies are shown to be consistent with exact diagonalization studies for small quantum dots. Fine structure is present in the magnetic field dependence which cannot be explained without a full many-body description and is associated with ground-state level crossings as a function of confinement strength or Zeeman interaction strength. Some of this fine structure is associated with precursors of the bulk incompressible states responsible for the fractional quantum Hall effect.PACS numbers: 73.20.Dx, 73.20.Mf Advances in nanofabrication technology have made it possible to manufacture "quantum dots" in which electrons are confined to a small area within a twodimensional (2D) electron gas [1]. Interest in these systems has grown as a result of recently developed techniques [2,3] which probe them spectroscopically. The quantity which is measured [4] in these experiments is the magnetic field dependence of the "addition spectrum," i.e., the energy to add one electron to a dot. This is given by fi^ = E% -E%_ 11 where E^ is the ground state energy of an Af-electron dot. Addition spectrum measurements have generally been interpreted in terms of "constant interaction" models in which electron-electron interactions are accounted for by including a charging energy which is characterized by a fixed self-capacitance; or, when this fails, by using Hartree or Hartree-Fock approximations. However, especially at strong magnetic fields, quantum dots can have strongly correlated [5,6] ground states, some of which are precursors of the bulk incompressible states responsible for the fractional quantum Hall effect. In this regime a complete interpretation of addition spectra measurements requires an exact treatment of the Coulombic electron-electron interactions.In this Letter we report on numerical exact diagonalization calculations of the addition spectrum for quantum dots in a strong magnetic field. We find that the addition spectrum has a surprisingly rich magnetic field dependence, showing a large number of sharp features superimposed on a smooth background. The background can be accounted for using a simple Hartree approximation. The sharp features are associated with energy-level crossings at fixed N, often between strongly correlated states. The spin degree of freedom has a nontrivial role, in general not consistent with expectations based on the Hartree-Fock approximation. The constant interaction model fails qualitatively for strong magnetic fields.We consider a system of 2D electrons confined by a parabolic potential [7], V(r) = mQ 2 r 2 /2.We confine our attention here to the strong magnetic field limit [8],where B± is the component of the magnetic field perpendicular to the 2D layer.) In this limit [1] the symmetric gauge single-particle eigenstates are...

We introduce a tensor renormalization group scheme for coarse graining a two-dimensional tensor network that can be successfully applied to both classical and quantum systems on and off criticality. The key innovation in our scheme is to deform a 2D tensor network into small loops and then optimize the tensors on each loop. In this way, we remove short-range entanglement at each iteration step and significantly improve the accuracy and stability of the renormalization flow. We demonstrate our algorithm in the classical Ising model and a frustrated 2D quantum model. DOI: 10.1103/PhysRevLett.118.110504 Introduction.-In recent years, the tensor network (TN) approach [1,2] has become a powerful theoretical and computational [8, tool for studying condensed matter systems. Many physical quantities, including the partition function of a classical system, the Euclidean path integral of a quantum system, and the expectation value of physical observables, can be expressed in terms of tensor networks. Evaluating these quantities is reduced to the contraction of a multidimensional tensor network. In the two dimensional case, many algorithms [8,32,[37][38][39][40][41]43,[45][46][47][48][49][50][53][54][55][56][57] have been developed to implement the approximate tensor contractions. Among these, the tensor renormalization group approach introduced by Levin and Nave [38] and its generalizations [8,22,39,[43][44][45][46][47]55,56,61] have unique features: the tensor contraction is based on a fully isotropic coarse-graining procedure. Moreover, when applying the method to a system on a finite torus, the computational cost is lower than those based on matrix product states (MPS) [32,37,41,[48][49][50]53,54].However, the Levin-Nave tensor network renormalization (TRG, also referred as LN-TNR here) [38] is based on the singular value decomposition (SVD) of local tensors, which only minimizes the truncation errors of tree tensor networks. Several improvements [45][46][47] have taken into account the effect of the environments, but they are still essentially based on tree tensor networks. These approaches cannot completely remove short-range entanglements during the coarse graining process. For example, in the 2D TN calculation of a partition function (or a path integral) TNR based on simple SVD cannot simplify the corner-double-line (CDL) tensor [38], despite the CDL tensor describing a product state that should be simplified to a one-dimensional tensor. In Ref.[8], this issue was seriously discussed. The authors pointed out that to further remove short-range entanglement, it is crucial to optimize the tensor configurations that contain a loop. However, due to the computational cost, only a crude iterative method is

We study exactly both the ground-state fidelity susceptibility and bond-bond correlation function in the Kitaev honeycomb model. Our results show that the fidelity susceptibility can be used to identify the topological phase transition from a gapped A phase with Abelian anyon excitations to a gapless B phase with non-Abelian anyon excitations. We also find that the bond-bond correlation function decays exponentially in the gapped phase, but algebraically in the gapless phase. For the former case, the correlation length is found to be 1/ξ = 2 sinh −1 [ √ 2Jz − 1/(1 − Jz)], which diverges around the critical point Jz = (1/2) + .

We discuss the character of states near the Fermi level in Mn-doped GaAs, as revealed by a survey of dc transport and optical studies over a wide range of Mn concentrations. A thermally activated valence-band contribution to dc transport, a midinfrared peak at energy ប Ϸ 200 meV in the ac conductivity, and the hot photoluminescence spectra indicate the presence of an impurity band in low-doped ͑Ӷ1% Mn͒ insulating GaAs:Mn materials. Consistent with the implications of this picture, both the impurity-band ionization energy inferred from the dc transport and the position of the midinfrared peak move to lower energies, and the peak broadens with increasing Mn concentration. In metallic materials with Ͼ2% doping, no traces of Mn-related activated contribution can be identified in dc transport, suggesting that the impurity band has merged with the valence band. No discrepancies with this perception are found when analyzing optical measurements in the high-doped GaAs:Mn. A higher-energy ͑ប Ϸ 250 meV͒ midinfrared feature which appears in the metallic samples is associated with inter-valence-band transitions. Its redshift with increased doping can be interpreted as a consequence of increased screening, which narrows the localized-state valence-band tails and weakens higher-energy transition amplitudes. Our examination of the dc and ac transport characteristics of GaAs:Mn is accompanied by comparisons with its shallow acceptor counterparts, confirming the disordered valence-band picture of high-doped metallic GaAs:Mn material.

We present a theory of the infrared conductivity and absorption coefficients of metallic (III,Mn)V ferromagnetic semiconductors. We find that the conductivity is dominated by inter-valence-band transitions that produce peaks athω ∼ 220meV and obscure the broadened Drude peak. We demonstrate that transverse f-sum rule measurements can be used to extract accurate values for the free carrier density, bypassing the severe characterization difficulties that have till now been created by the large anomalous Hall effect in these materials.Following the discovery of carrier-induced ferromagnetism in (In,Mn)As [1] and (Ga,Mn)As [2] in the early nineties, studies of ferromagnetism in (III,Mn)V diluted magnetic semiconductors (DMS's) have yielded many surprises. The transport and optical properties of these heavily-doped semiconductors are richer than those of conventional itinerant electron ferromagnets because of strong valence-band spin-orbit coupling, and because of the sensitivity of their magnetic state to growth conditions, doping, and external fields. These novel ferromagnets are likely to have a major technological impact [3] if systems with Curie temperatures above room temperature can be created and better control of disorder effects can be achieved. We present a theory of the infrared conductivity of (III,Mn)V ferromagnets which we believe will increase the value of this powerful probe in characterizing present and future DMS ferromagnets. In particular, we demonstrate that infrared conductivity measurements in metallic samples can be used to obtain accurate measurements of the total free carrier density, likely the key to understanding the now firmly established [4] dependence of magnetic properties on growth and post-growth annealing conditions. This experimental possibility is especially important in these semiconductors because they are ferromagnetic and have a large anomalous Hall conductivity [5] which severely complicates Hall-effect carrier-density measurements, [2,6] even when they are performed in very strong magnetic fields.Our conclusions are based on a semi-phenomenological model [7][8][9][10] in which host semiconductor valence band electrons, described by a k.p model, interact with randomly distributed aligned (T = 0) Mn local-moment acceptors via Coulomb and short-range exchange interactions, and via Coulomb interactions only with the compensating charged defects known to be present. Some of our considerations are based on standard linear-response theory expressions for the ac conductivity of weakly disordered metals, in which disorder is included only through finite quasiparticle lifetimes and localization effects are ignored. In the dc limit, with quasiparticle scattering rates evaluated using Born approximation expressions for scattering off the spatial fluctuations of the screened-Coulomb and exchange potentials, these expressions imply mobilities [11] that are consistent with values measured in optimally annealed, high-T c , DMS ferromagnets. Although the predictions discussed below are int...

Using a dynamical quantum Zeno effect, we propose a general approach to control the coupling between a two-level system (TLS) and its surroundings, by modulating the energy level spacing of the TLS with a high frequency signal. We show that the TLS--surroundings interaction can be turned on or off when the ratio between the amplitude and the frequency of the modulating field is adjusted to be a zero of a Bessel function. The quantum Zeno effect of the TLS can also be observed by the vanishing of the photon reflection at these zeros. Based on these results, we propose a quantum switch to control the transport of a single photon in a 1D waveguide. Our analytical results agree well with numerical results using Floquet theory.Comment: 9 pages, 3 figure