Measurement of σ(Hνeνeˉ)×BR(H→ZZ∗){\sigma(H\nu_e\bar{\nu_e})\times BR(H\rightarrow ZZ^\ast)} and Higgs production in ZZZZ fusion at a 1.4 TeV CLIC collider

This paper presents the potential measurement at 1.4 TeV CLIC of the cross-section (times branching ratio) of the Higgs production via $WW$ fusion with the Higgs subsequently decaying in $ZZ^\ast$, ${\sigma(H\nu_e\bar{\nu_e})\times BR(H\rightarrow ZZ^\ast)}$, and of the Higgs production via $ZZ$ fusion with the Higgs subsequently decaying in $b\bar{b}$, ${\sigma(He{^+}e{^-})\times BR(H\rightarrow b\bar{b})}$. For the $H\rightarrow ZZ^\ast$ decay the hadronic final state, ${ZZ^\ast\rightarrow q\bar{q}q\bar{q}}$, and the semi-leptonic final state, ${ZZ^\ast\rightarrow q\bar{q}l^+l^-}$, are considered. The results show that ${\sigma(H\nu_e\bar{\nu_e})\times BR(H\rightarrow ZZ^\ast)}$ can be measured with a precision of 18.3% and 6% for the hadronic and semi-leptonic channel, respectively. ${\sigma(He{^+}e{^-})\times BR(H\rightarrow b\bar{b})}$ can be measured with a precision of 1.7%. This measurement also contributes to the determination of the Higgs coupling to the $Z$ boson, $g_{H_{ZZ}}$.Comment: Talk presented at the International Workshop on Future Linear Colliders (LCWS14), Belgrade, Serbia, 6-10 October 201

On field effect studies and superconductor-insulator transition in high-Tc cuprates

We summarize previous field effect studies in high-T c cuprates and then discuss our method to smoothly tune the carrier concentration of a cuprate film over a wide range using an applied electric field. We synthesized epitaxial one-unit-cell thick films of La2−x Sr x CuO4 and from them fabricated electric double layer transistor devices utilizing various gate electrolytes. We were able to vary the carrier density by about 0.08 carriers per Cu atom, with the resulting change in T c of 30 K. The superconductor-insulator transition occurred at the critical resistance very close to the quantum resistance for pairs, R Q = h/(2e)2 = 6.5 kΩ. This is suggestive of a quantum phase transition, possibly driven by quantum phase fluctuations, between a "Bose insulator” and a high-T c superconductor stat

Complex conductance of ultrathin La₂₋xSrxCuO₄ films and heterostructures

We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal (La₁.₅₅Sr₀.₄₅CuO₄) and a cuprate insulator (La₂CuO₄), in which each layer is just one unit cells thick. We have studied the magnetic field and temperature dependence of the complex sheet conductance, σ(ω), of these films. Experiments have been carried out at frequencies between 2–50 MHz using the single-spiral coil technique. We found that: (i) the inductive response starts at ∆T = 3 K lower temperatures than Re σ(T), which in turn is characterized by a peak close to the transition, (ii) this shift is almost constant with magnetic field up to 14 mT; (iii) ∆T increases sharply up to 4 K at larger fields and becomes constant up to 8 T; (iv) the vortex diffusion constant D(T) is not linear with T at low temperatures as in the case of free vortices, but is rather exponential due to pinning of vortex cores, and (v) the dynamic Berezinski–Kosterlitz–Thouless (BKT) transition temperature occurs at the point where Y = (lω/ξ₊)² = 1. Our experimental results can be described well by the extended dynamic theory of the BKT transition and dynamics of bound vortex–antivortex pairs with short separation lengths

Strain and Electronic Nematicity in La₂₋ₓSrₓCuO₄

Electronic nematicity has previously been observed in La2-xSrxCuO4 thin films by the angle-resolved transverse resistivity method with a director whose orientation is always pinned to the crystal axes when the film is grown on an orthorhombic substrate but not when the substrate is tetragonal. Here we report on measurements of thin films grown on (tetragonal) LaSrAlO4 and subsequently placed in an apparatus that allows the application of uniaxial compressive strain. The apparatus applied enough force to produce a 1% orthorhombicity in LaSrAlO4 and yet no change in the electronic nematicity was observed in films under strain compared to when they were unstrained. The lattice effects are weak, and the origin of nematicity is primarily electronic

History dependent magnetoresistance in lightly doped La_{2-x}Sr_{x}CuO_{4} thin films

The in-plane magnetoresistance (MR) in atomically smooth La_{2-x}Sr_{x}CuO_{4} thin films grown by molecular-beam-epitaxy was measured in magnetic fields B up to 9 T over a wide range of temperatures T. The films, with x=0.03 and x=0.05, are insulating, and the positive MR emerges at T<4 K. The positive MR exhibits glassy features, including history dependence and memory, for all orientations of B. The results show that this behavior, which reflects the onset of glassiness in the dynamics of doped holes, is a robust feature of the insulating state.Comment: 4 pages, 4 figures, International School and Workshop on Electronic Crystals (ECRYS-2011); to appear in Physica

The Meissner effect in a strongly underdoped cuprate above its critical temperature

The Meissner effect and the associated perfect "bulk" diamagnetism together with zero resistance and gap opening are characteristic features of the superconducting state. In the pseudogap state of cuprates unusual diamagnetic signals as well as anomalous proximity effects have been detected but a Meissner effect has never been observed. Here we have probed the local diamagnetic response in the normal state of an underdoped La1.94Sr0.06CuO4 layer (up to 46 nm thick, critical temperature Tc' < 5 K) which was brought into close contact with two nearly optimally doped La1.84Sr0.16CuO4 layers (Tc \approx 32 K). We show that the entire 'barrier' layer of thickness much larger than the typical c axis coherence lengths of cuprates exhibits a Meissner effect at temperatures well above Tc' but below Tc. The temperature dependence of the effective penetration depth and superfluid density in different layers indicates that superfluidity with long-range phase coherence is induced in the underdoped layer by the proximity to optimally doped layers; however, this induced order is very sensitive to thermal excitation.Comment: 7 pages, 7 figures + Erratu

Magnetism in the 2D Limit and Interface Superconductivity in Metal-Insulator La(2-x)Sr(x)CuO(4) Superlattices

We show, by means of low-energy muon spin rotation measurements, that few-unit-cells thick La(2)CuO(4) layers synthesized digitally by molecular beam epitaxy synthesis are antiferromagnetically ordered. Below a thickness of about 5 CuO(2) layers the long-range ordered state breaks down, and a magnetic state appears with enhanced quantum fluctuations and a reduced spin stiffness. This magnetic state can exist in close proximity (few Angstrom) to high-temperature superconducting layers, without transmitting supercurrents.Comment: 4 pages, 3 figure