Q. Q. Ge

Black phosphorus field-effect transistors

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

Electronic structure of Fe1.04Te0.66Se0.34

We report the electronic structure of the iron-chalcogenide superconductor, Fe-1.04(Te0.66Se0.34), obtained with high-resolution angle-resolved photoemission spectroscopy and density-functional calculations. In photoemission measurements, various photon e

Electronic Identification of the Parental Phases and Mesoscopic Phase Separation of KxFe2-ySe2 Superconductors

F. Chen, 1 M. Xu,1 Q. Q. Ge, 1 Y. Zhang, 1, ∗ Z. R. Ye,1 L. X. Yang,1 Juan Jiang, 1 B. P. Xie,1 R. C. Che, 2 M. Zhang, 3 A. F. Wang, 3 X. H. Chen, 3 D. W. Shen, 4 X. M. Xie,4 M. H. Jiang, 4 J. P. Hu, 5 and D. L. Feng1, † 1State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 2Department of Materials Science, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 3Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 4State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 20005 5Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA

Symmetry breaking via orbital-dependent reconstruction of electronic structure in detwinned NaFeAs

The superconductivity discovered in iron pnictides is intimately related to a nematic ground state, where the C-4 rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with polarization-dependent

Doping dependence of the electronic structure in phosphorus-doped ferropnictide superconductor BaFe2(As1−xPx)2studied by angle-resolved photoemission spectroscopy

The superconductivity in high temperature superconductors ordinarily arises when doped with hetero-valent ions that introduce charge carriers [1–4]. However, in ferropnictides, “iso-valent” doping, which is generally believed not to introduce charge carriers, can induce superconductivity as well [5–11]. Moreover, unlike other ferropnictides [12, 13], the superconducting gap in BaFe2(As1−xPx)2 has been found to contain nodal lines [14– 16]. The exact nature of the “iso-valent” doping and nodal gap here are key open issues in building a comprehensive picture of the iron-based high temperature superconductors [17–20]. With angle-resolved photoemission spectroscopy (ARPES), we found that the phosphor substitution in BaFe2(As1−xPx)2 induces sizable amount of holes into the hole Fermi surfaces, while thedxy-originated band is relatively intact. This overturns the previous common belief of “iso-valent” doping, explains why the phase diagram of BaFe2(As1−xPx)2 is similar to those of the holedoped compounds, and rules out theories that explain the nodal gap based on vanishingdxy hole pocket. BaFe2(As1−xPx)2 is a rather unique ferropnictide as its superconductivity is introduced by the iso-valent doping of P for As [5, 6]. Unlike the hetero-valent doping that alters the carrier concentration in Ba 1−xKxFe2As2, BaFe2−xCoxAs2, or LaO1−xFxFeAs [2–4], the iso-valent doping is often considered not to alter the occupation of the Fe 3 d bands, as illustrated by the density functional theory calculations of BaFe2As2 and BaFe2P2 as well [6, 7]. Yet, surprisingly, it has a similar phase diagram just like the hetero-valent dope cases: with P doping, spin density wave (SDW) is suppressed and superconductivity (SC) emerges [6]. Since P anion is smaller than As anion, and thus introduces internal strain or distortion, i.e. chemical pressure, the superconductivity introduced by iso-valent doping is associ ated with the unprecedented pressure dependence of the supercon ducting transition temperature ( Tc) generally observed in ironbased superconductors [21–24]. In fact, it is the largest am ong all superconductors in both relative and absolute scales. F or example, aTc dependency of 2-4K /GPa and sometimes even 10K/GPa is observed in BaFe 2(As1−xPx)2, LaO1−xFxFeAs, etc. [21, 22]; and an increase of Tc from 0 to above 30 K is observed in BaFe 2As2 and FeSe under pressure [23, 24]. However, these remarkable pressure e ffects are still far from understood. Theoretically, P doping is predicted to alter t h band structure and Fermi surface topology dramatically, co nsidering it changes the electron hopping terms [17, 25]. Par ticularly, it is predicted that thedz2-based band would go above the Fermi energy ( EF), while thedxy-based band would move down belowEF with P doping. Several theories further claim that nodes will appear in the superconducting gap when the dxy hole Fermi pocket disappears [17–19]. Figure 1 examines the dependence of the Fermi surfaces on the P concentration in a series of BaFe 2(As1−xPx)2, where the photoemission intensity maps near EF are shown for twokz’s. The features at the zone center ( Γ and Z) are hole pockets, and those at the zone corner (M and A) are electron pockets [26, 28]. As P doping increases, the size of the hole pockets increase significantly, while the electron pockets show neg ligible doping dependence. This indicates that the P doping could induce extra holes into the system, contradicting to t he ordinary picture of iso-valent doping. To understand such extraordinary P doping e ffect, more de-

Surface and bulk electronic structures of LaOFeAs studied by angle resolved photoemission spectroscopy

The electronic structure of LaOFeAs, a parent compound of iron-arsenic superconductors, is studied by angleresolved photoemission spectroscopy. By examining its dependence on photon energy, polarization, sodium dosing and the counting of Fermi surface volume, both the bulk and the surface contributions are identified. We find that a bulk band moves toward high binding energies below structural transition, and shifts smoothly across the spin density wave transition by about 25 meV. Our data suggest the band reconstruction may play a crucial role in the spin density wave transition, and the structural transition is driven by the short range magnetic order. For the surface states, both the LaO-terminated and FeAs-terminated components are revealed. Certain small band shifts are verified for the FeAs-terminated surface states in the spin density wave state, which is a reflection of the bulk electronic structure reconstruction. Moreover, sharp quasiparticle peaks quickly rise at low temperatures, indicating of drastic reduction of the scattering rate. A kink structure in one of the surface band is shown to be possibly related to the electron-phonon interactions.

Surface and bulk electronic structures of LaFeAsO studied by angle-resolved photoemission spectroscopy

Chang Liu, 2 Yongbin Lee, A. D. Palczewski, 2 J. -Q. Yan, Takeshi Kondo, 2 B. N. Harmon, 2 R. W. McCallum, 3 T. A. Lograsso, and A. Kaminski 2 Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA (Dated: June 7, 2010)

Anisotropic but Nodeless Superconducting Gap in the Presence of Spin-Density Wave in Iron-Pnictide Superconductor NaFe1-xCoxAs

The coexisting regime of spin-density wave (SDW) and superconductivity in iron pnictides represents a novel ground state. We have performed high-resolution angle-resolved photoemission measurements on NaFe1-xCoxAs (x = 0.0175) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is nonexclusive. Particularly, we find that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in a SDW-free NaFe1-xCoxAs (x = 0.045), which puts strong constraints on theory. DOI.10.1103/PhysRevX.3.011020

Electronic structure of the BaTi 2 As 2 O parent compound of the titanium-based oxypnictide superconductor

The electronic structure of BaTi2As2O, a parent compound of the newly discovered titanium-based oxypnictide superconductors, is studied by angle-resolved photoemission spectroscopy. The electronic structure shows multi-orbital nature and possible three-dimensional character. An anomalous temperature-dependent spectral weight redistribution and broad lineshape indicate the incoherent nature of the spectral function. At the density-wave-like transition temperature around 200 K, a partial gap opens at the Fermi patches. These findings suggest that BaTi2As2O is likely a charge density wave material in the strong interaction regime.

Weak electronic correlations and absence of heavy-fermion state in KNi2Se2

We have studied the low-lying electronic structure of a new ThCr