Anderson localization, the absence of diffusive transport in disordered systems, has been manifested as hopping transport in numerous electronic systems, whereas in recently discovered topological insulators it has not been directly observed. Here we report experimental demonstration of a crossover from diffusive transport in the weak antilocalization regime to variable range hopping transport in the Anderson localization regime with ultrathin (Bi1−xSbx)2Te3 films. As disorder becomes stronger, negative magnetoconductivity due to the weak antilocalization is gradually suppressed, and eventually positive magnetoconductivity emerges when the electron system becomes strongly localized. This works reveals the critical role of disorder in the quantum transport properties of ultrathin topological insulator films, in which theories have predicted rich physics related to topological phase transitions.The concept of Anderson localization has profoundly influenced our understanding of electron conductivity [1]. While examples for disorder driven metal-insulator transition are abundant in three-dimensions (3D), the question of whether Anderson transitions exist in 2D has posed a lot of theoretical and experimental challenges. Scaling theory proposed by Abrahams et al. predicts that there are no truly metallic states in non-interacting 2D electron systems [2,3]. It was, however, discovered later that extended electron states may exist when electronelectron (e-e) interaction, spin-orbit coupling (SOC) or magnetic field comes in to play [4,5]. The 3D topological insulators (TIs) discovered in recent years [6,7] provide novel types of 2D electron systems that are of particular interest for study of the localization-delocalization problem. The Dirac surface states of 3D TIs are believed to be topologically protected from localization due to its special symmetry class [8][9][10][11][12]. Moreover, when a 3D TI thin film is sufficiently thin, the hybridization between the top and the bottom surface states opens an energy gap near the Dirac point [13], and it is suggested theoretically that the hybridization gap would drive the electron system to topologically different phase, such as a quantum spin Hall insulator or a trivial band insulator [14][15][16]. Even though a lot of work has been carried out on electron transport properties of TI thin films [17][18][19][20], the fate of such electron systems under the condition of strong disorder (or in other words, whether Anderson localization could take place), still remains unclear.In this work, we have studied electron transport in a large number of highly gate-tunable TI thin films with various thicknesses and chemical compositions. We found that only in ultrathin TI films in which surface hybridization and disorder effects are significant, hopping transport, a hallmark of strong localization [21][22][23], can be observed. The observed temperature and magnetic field dependences of conductivity suggest that electron transport can be driven from the diffusive transport governed by we...

We report the observation of colossal positive magnetoresistance (MR) in single crystalline, high mobility TaAs2 semimetal. The excellent fit of MR by a single quadratic function of the magnetic field B over a wide temperature range (T = 2-300 K) suggests the semiclassical nature of the MR. The measurements of Hall effect and Shubnikov-de Haas oscillations, as well as band structure calculations suggest that the giant MR originates from the nearly perfectly compensated electrons and holes in TaAs2. The quadratic MR can even exceed 1,200,000% at B = 9 T and T = 2 K, which is one of the largest values among those of all known semi-metallic compounds including the very recently discovered WTe2 and NbSb2. The giant positive magnetoresistance in TaAs2, which not only has a fundamentally different origin from the negative colossal MR observed in magnetic systems, but also provides a nice complemental system that will be beneficial for applications in magnetoelectronic devices

A cooperative Cu/Pd-catalyzed asymmetric three-component reaction of styrenes, B 2 (pin) 2 , and allyl carbonates was reported. This reaction, in the presence of chiral CuOAc/SOP and achiral Pd(dppf)Cl 2 catalysts, occurs smoothly with high enantioselectivities (up to 97% ee) . The allylboration products, which contain alkene (or diene) unite and alkylboron group, are easily functionalized. The utility of this protocol was demonstrated through the synthesis of an antipsychotic drug, (−)-preclamol.M ultifunctional and enantioenriched organoboranes are useful building blocks in the synthesis of natural products and bioactive compounds. 1 Transition metal-catalyzed asymmetric carboboration reaction is an efficient and straightforward approach to access chiral multisubstituted alkyl-or alkenylboranes. 2 For instance, Ito and co-workers developed a successful copper-catalyzed borylative cyclization to prepare optically pure β-aryl or silylated cyclopropylboronates. 3 The related asymmetric carboboration of allenes, alkynes and 1,3-enynes are also applied in the synthesis of chiral di-, tri-, and tetra-substituted alkenylboron esters by Hoveyda 1c,4 and Lin. 5 However, up to date, enantioselective carboboration of alkenes for the construction of enantioenriched and multifunctional alkylborons are less developed. Sporadic examples were reported and limited to Cu-catalyzed borylative aldol carboboration 6 and Pd-catalyzed 1,1-arylboration. 7 Very recently, Semba, Nakao 8 and Brown 9 independently reported a remarkable three-component carboboration of alkenes with bis(pinacolato)diboron (B 2 (pin) 2 ) and aryl or vinyl halides by a Cu/Pd cooperative catalysis. We envisioned using this combination catalysis 10 as a platform to develop enantioselective carboboration of simple alkenes. The proposed procedure involves two cooperative catalytic cycles (Scheme 1), a Cu-catalyzed enantioselective generation of β-borylalkylcopper 11 and a Pd-catalyzed cross-coupling transformation of this enantioenriched reagent. 1a,12 We believe that the stereospecific transmetalation of alkylcopper with XPd(L)R′ in the proposed mechanism, and the compatibility of the chiral L*Cu(I)-complex with the achiral Pd(0)-or Pd(II)-complex would be the key issues. In our previous work, 13 a stereospecific copper− tin transfer was observed in Cu(I)-catalyzed enantioselective alkene stannylboration when using chiral sulfoxide-(P-aryl)-phosphine (SOP) ligand. In this work, we found that P-alkyl sulfoxide-phosphine ligands 14 can effectively promote the aforementioned Cu−Pd transmetalation and realize a highly enantioselective allylboration of alkenes.To test the feasibility, the three-component reaction of styrene (1a), B 2 (pin) 2 , and allylic electrophiles was performed in the presence of (SOP)CuCl catalyst precursor and Pd(dppf)Cl 2 cocatalyst. After screening a series of allyl substrates with different leaving groups (e.g., halides, esters and carbonates), t-butyl allyl carbonate 2a was confirmed as the best electrophile in terms of the re...

A new C(2)-symmetric chiral bis-sulfoxide ligand, (R,R)-1,2-bis(tert-butylsulfinyl)benzene, has been designed and prepared by the reaction of (R)-benzyl tert-butylsulfoxide with (R)-thiosulfinate. This ligand exhibits excellent enantioselectivities in the Rh-catalyzed asymmetric 1,4-addition reaction. In particular, the present work has realized access to optically pure flavanones for the first time through 1,4-addition of arylboronic reagents to chromenones.

A stereodivergent Pd/Cu catalyst system has been developed for the unprecedented dynamic kinetic asymmetric transformation (DyKAT) of racemic unsymmetrical 1,3-disubstituted allylic acetates with prochiral aldimine esters. A series of α,αdisubstituted α-amino acids bearing vicinal stereocenters were easily prepared with excellent enantioselectivities (up to >99% ee) and diastereoselectivities (up to >20:1 dr). Moreover, all four stereoisomers of the product can be readily obtained simply by switching the configurations of the two chiral metal catalysts. Furthermore, the present work highlights the power of synergistic Pd/ Cu catalysis consisting of two common bidentate chiral ligands for stereodivergent synthesis.

Presented is the first enantioselective copper-catalyzed 1,6-conjugate addition of bis(pinacolato)diboron to para-quinone methides. The reaction proceeds with excellent yields and good to excellent enantioselectivities, and provides an attractive approach to the construction of optically active gem-diarylmehtine boronic esters. Additionally, the subsequent conversion of the derived potassium trifluoroborates into triarylmethanes with highly enantiospecificity was realized.

The evolution of the quantum anomalous Hall effect with the thickness of Cr-doped (Bi,Sb)2 Te3 magnetic topological insulator films is studied, revealing how the effect is caused by the interplay of the surface states, band-bending, and ferromagnetic exchange energy. Homogeneity in ferromagnetism is found to be the key to high-temperature quantum anomalous Hall material.

A highly enantioselective thiocarbonylation of styrenes with CO and thiols has been achieved by Pd catalysis, providing highly enantioenriched thioesters in good to excellent yields. Key to the successful execution of this reaction is the use of a chiral sulfoxide‐(P‐dialkyl)‐phosphine (SOP) ligands. This thiocarbonylation proceeds smoothly under mild reaction conditions (1 atm CO and 0 °C) and displays broad substrate scope. Also demonstrated is that this transformation can be conducted using surrogates of CO, greatly increasing the safety aspects of running the reaction. The generality and utility of the method is manifested by its application to the synthetic transformations of thioester products and the direct acylation of cysteine‐containing dipeptides. A primary mechanism was investigated and a plausible catalytic cycle was proposed.