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Register a new userHomogeneous superconducting gap in DBCO synthesized by oxide molecular beam epitaxy
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Much of what is known about high-temperature cuprate superconductors stems from studies based on two surface analytical tools, angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging scanning tunneling microscopy (SI-STM). A question of general interest is whether and when the surface properties probed by ARPES and SI-STM are representative of the intrinsic properties of bulk materials. We find this question is prominent in thin films of a rarely studied cuprate DBCO. We synthesize DBCO films by oxide molecular beam epitaxy and study them by in situ ARPES and SI-STM. Both ARPES and SI-STM show that the surface DBCO layer is different from the bulk of the film. It is heavily underdoped, while the doping level in the bulk is close to optimal doping evidenced by bulk-sensitive mutual inductance measurements. ARPES shows the typical electronic structure of a heavily underdoped CuO2 plane and two sets of one-dimensional bands originating from the CuO chains with one of them gapped. SI-STM reveals two different energy scales in the local density of states, with one corresponding to the superconductivity and the other one to the pseudogap. While the pseudogap shows large variations over the length scale of a few nanometers, the superconducting gap is very homogeneous. This indicates that the pseudogap and superconductivity are of different origins.
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Gray tin, also known as {alpha}-Sn, has been attracting research interest recent years due to its topological nontrivial properties predicted theoretically. The Dirac linear band dispersion has been proved experimentally by angle resolved photoemission spectroscopy. We have grown a series of {alpha}-Sn thin film samples in two types with different substrates and thicknesses by molecular beam epitaxy. To explore the possible exotic physical properties related to the topological band structures, we have measured the electrical transport properties of our {alpha}-Sn thin film samples and observed multiple superconducting transitions. We have identified the transitions above 4.5 K, besides the transition maybe related to the b{eta} phase around 3.7 K. The changes of the superconducting properties over time reflect the aging effects in our samples. We have also confirmed the strain effects on the superconducting transitions through altering the relative thickness of our samples.
We report growth of superconducting Sr2RuO4 films by oxide molecular beam epitaxy (MBE). Careful tuning of the Ru flux with an electron beam evaporator enables us to optimize growth conditions including the Ru/Sr flux ratio and also to investigate stoichiometry effects on the structural and transport properties. The highest onset transition temperature of about 1.1 K is observed for films grown in a slightly Ru-rich flux condition in order to suppress Ru deficiency. The realization of superconducting Sr2RuO4 films via oxide MBE opens up a new route to study the unconventional superconductivity of this material.
Hole-doped perovskite bismuthates such as Ba$_{1-x}$K$_x$BiO$_3$ and Sr$_{1-x}$K$_x$BiO$_3$ are well-known bismuth-based oxide high-transition-temperature superconductors. Reported thin bismuthate films show relatively low quality, likely due to their large lattice mismatch with the substrate and a low sticking coefficient of Bi at high temperatures. Here, we report the successful epitaxial thin film growth of the parent compound strontium bismuthate SrBiO$_3$ on SrO-terminated SrTiO$_3$ (001) substrates by molecular beam epitaxy. Two different growth methods, high-temperature co-deposition or recrystallization cycles of low-temperature deposition plus high-temperature annealing, are developed to improve the epitaxial growth. SrBiO$_3$ has a pseudocubic lattice constant $sim$4.25 AA, an $sim$8.8% lattice mismatch on SrTiO$_3$ substrate, leading to a large strain in the first few unit cells. Films thicker than 6 unit cells prepared by both methods are fully relaxed to bulk lattice constant and have similar quality. Compared to high-temperature co-deposition, the recrystallization method can produce higher quality 1-6 unit cell films that are coherently or partially strained. Photoemission experiments reveal the bonding and antibonding states close to the Fermi level due to Bi and O hybridization, in good agreement with density functional theory calculations. This work provides general guidance to the synthesis of high-quality perovskite bismuthate films.
We report a systematic study on the growth conditions of Sn$_{1-x}$In$_x$Te thin films by molecular beam epitaxy for maximization of superconducting transition temperature $T_mathrm{c}$. Careful tuning of the flux ratios of Sn, In, and Te enables us to find an optimum condition for substituting rich In content ($x$ = 0.66) into Sn site in a single phase of Sn$_{1-x}$In$_x$Te beyond the bulk solubility limit at ambient pressure ($x$ = 0.5). $T_mathrm{c}$ shows a dome-shaped dependence on In content $x$ with the highest $T_mathrm{c}$ = 4.20 K at $x$ = 0.55, being consistent to that reported for bulk crystals. The well-regulated Sn$_{1-x}$In$_x$Te films can be a useful platform to study possible topological superconductivity by integrating them into the state-of-the-art junctions and/or proximity-coupled devices.
Superconducting thin films of magnesium diboride (MgB$_2$) were prepared on MgO (001) substrate by a molecular beam epitaxy (MBE) method with the co-evaporation conditions of low deposition rate in ultra-high vacuum. The structural and physical properties of the films were studied by RHEED, XRD, XPS, resistivity and magnetization measurements.All films demonstrated superconductivity without use of any post-annealing process.The highest {it T}$_{c,onset}$ determined by resistivity measurement was about 33K in the present samples.Anisotropic superconducting properties were evaluated by the resistivity and magnetic measurements.We will discuss the anisotropy of superconductivity for as-grown MgB$_2$ thin films.
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