The upgrade of the LHCb trigger system

The LHCb experiment will operate at a luminosity of $2\times10^{33}$ cm$^{-2}$s$^{-1}$ during LHC Run 3. At this rate the present readout and hardware Level-0 trigger become a limitation, especially for fully hadronic final states. In order to maintain a high signal efficiency the upgraded LHCb detector will deploy two novel concepts: a triggerless readout and a full software trigger.Comment: Proceedings of the Workshop on Intelligent Trackers, 14-16 May 2014, University of Pennsylvani

Berezinskii-Kosterlitz-Thouless Type Scenario in Molecular Spin Liquid AACr2_2O4_4

The spin relaxation in chromium spinel oxides $A$Cr$_{2}$O$_{4}$ ($A=$ Mg, Zn, Cd) is investigated in the paramagnetic regime by electron spin resonance (ESR). The temperature dependence of the ESR linewidth indicates an unconventional spin-relaxation behavior, similar to spin-spin relaxation in the two-dimensional (2D) chromium-oxide triangular lattice antiferromagnets. The data can be described in terms of a generalized Berezinskii-Kosterlitz-Thouless (BKT) type scenario for 2D systems with additional internal symmetries. Based on the characteristic exponents obtained from the evaluation of the ESR linewidth, short-range order with a hidden internal symmetry is suggested.Comment: 7 pages, 4 figure

Orbital Freezing in FeCr2S4 Studied by Dielectric Spectroscopy

Broadband dielectric spectroscopy has been performed on single-crystalline FeCr2S4 revealing a transition into a low-temperature orbital glass phase and on polycrystalline FeCr2S4 where long-range orbital order is established via a cooperative Jahn-Teller transition. The freezing of the orbital moments is revealed by a clear relaxational behavior of the dielectric permittivity, which allows a unique characterization of the orbital glass transition. The orbital relaxation dynamics continuously slows down over six decades in time, before at the lowest temperatures the glass transition becomes suppressed by quantum tunneling.Comment: 4 pages, 4 figure

Magnetic Excitations in the Multiferroic N\'eel-type Skyrmion Host GaV4_4S8_8

Broadband microwave spectroscopy has been performed on single-crystalline GaV$_4$S$_8$, which exhibits a complex magnetic phase diagram including cycloidal, N\'eel-type skyrmion lattice, as well as field-polarized ferromagnetic phases below 13 K. At zero and small magnetic fields two collective modes are found at 5 and 15 GHz, which are characteristic of the cycloidal state in this easy-axis magnet. In finite fields, entering the skyrmion lattice phase, the spectrum transforms into a multi-mode pattern with absorption peaks near 4, 8, and 15 GHz. The spin excitation spectra in GaV$_4$S$_8$ and their field dependencies are found to be in close relation to those observed in materials with Bloch-type skyrmions. Distinct differences arise from the strong uniaxial magnetic anisotropy of GaV4S8 not present in so-far known skyrmion hosts

On the complexity of spinels: magnetic, electronic, and polar ground states

This review summarizes more than 100 years of research on spinel compounds, mainly focusing on the progress in understanding their magnetic, electronic, and polar properties during the last two decades. Many spinel compounds are magnetic insulators or semiconductors; however, a number of spinel-type metals exists including superconductors and some rare examples of d-derived heavy-fermion compounds. In the early days, they gained importance as ferrimagnetic or even ferromagnetic insulators with relatively high saturation magnetization and high ordering temperatures, with magnetite being the first magnetic mineral known to mankind. However, spinels played an outstanding role in the development of concepts of magnetism, in testing and verifying the fundamentals of magnetic exchange, in understanding orbital-ordering and charge-ordering phenomena. In addition, the A- site as well as the B-site cations in the spinel structure form lattices prone to strong frustration effects resulting in exotic ground-state properties. In case the A-site cation is Jahn-Teller active, additional entanglements of spin and orbital degrees of freedom appear, which can give rise to a spin-orbital liquid or an orbital glass state. The B-site cations form a pyrochlore lattice, one of the strongest contenders of frustration in three dimensions. In addition, in spinels with both cation lattices carrying magnetic moments, competing magnetic exchange interactions become important, yielding ground states like the time-honoured triangular Yafet-Kittel structure. Finally, yet importantly, there exists a long-standing dispute about the possibility of a polar ground state in spinels, despite their reported overall cubic symmetry. Indeed, over the years number of multiferroic spinels were identified.Comment: 118 pages, 60 figure

Spin dynamics of the ordered phase of the frustrated antiferromagnet ZnCr2O4: a magnetic resonance study

We present an elaborate electron-spin resonance study of the low-energy dynamics and magnetization in the ordered phase of the frustrated spinel ZnCr2O4. We observe several resonance modes corresponding to different structural domains and found that the number of domains can be easily reduced by field-cooling the sample through the transition point. To describe the observed antiferromagnetic resonance spectra it is necessary to take into account an orthorhombic lattice distortion in addition to the earlier reported tetragonal distortion which both appear at the antiferromagnetic phase transition

Experimental review of unpolarised nucleon structure functions

Recent results are reviewed on unpolarised structure functions from fixed target experiments at JLAB, NuTeV and from the HERA ep collider experiments H1 and ZEUS.Comment: Invited talk at the 10th International Conference on the Structure of Baryons (Baryons 2004), Palaiseau, France, 25-29 October 2004; 12 pages, 10 figure

Magnetic and vibrational properties of the covalent chain antiferromagnet RbFeS2

Ternary rubidium-iron sulfide, RbFeS2, belongs to a family of quasi-one-dimensional compounds with the general chemical composition AFeCh2 (where A – K, Rb, Cs, Tl; Ch – S, Se). Understanding the magnetic properties of these compounds is a challenge. The controversy concerning the spin-state of the iron ion needs to be resolved to build the proper model of magnetism. Single crystals of RbFeS2 were grown and characterized by powder x-ray diffraction. QD MPMS-5 SQUID magnetometry was used to measure the magnetic susceptibility, and specific heat was measured utilizing QD PPMS-9 setup. Above the transition to three-dimensional antiferromagnetic order at the Néel temperature of TN = 188 K, the susceptibility exhibits unusual quasi-linear increase up to the highest measured temperature of 500 K. The specific heat was measured in the temperature range 1.8 – 300 K. Ab initio phonon dispersion and density-of-states calculations were performed by means of density functional theory (DFT), and the calculated lattice specific heat was subtracted from the measured one giving the magnetic contribution to the specific heat. Our results suggest that the features of the magnetic specific heat are general for the whole family of the covalent chain ternary iron chalcogenides of the AFeCh2 structure and indicate an intermediate S = 3/2 spin state of the iron ion

3D Assembly of All-Inorganic Colloidal Nanocrystals into Gels and Aerogels

We report on an efficient assembly approach to a variety of electrostatically stabilized all-inorganic semiconductor nanocrystals (NCs) via their linking with appropriate ions into multibranched gel networks. These all-inorganic non-ordered 3D assemblies can combine strong interparticle coupling which facilitates charge transport between the NCs with their diverse morphology, composition, size, and functional capping ligands. Moreover, the resulting dry gels (aerogels) are highly porous monolithic structures, which preserve the quantum confinement of their building blocks. The inorganic semiconductor aerogel made of 4.5 nm CdSe colloidal NCs, capped with iodide ions and bridged with Cd2+ ions, exhibited a surface area as high as 146 m2/g

Multiferroic spin-superfluid and spin-supersolid phases in MnCr2S4

Spin supersolids and spin superfluids reveal complex canted spin structures with independent order of longitudinal and transverse spin components. This work addresses the question whether these exotic phases can exhibit spin-driven ferroelectricity. Here we report the results of dielectric and pyrocurrent measurements of MnCr2S4 as function of temperature and magnetic field up to 60 T. This sulfide chromium spinel exhibits a Yafet-Kittel type canted spin structure at low temperatures. As function of external magnetic field, the manganese spins undergo a sequence of ordering patterns of the transverse and longitudinal spin components, which can be mapped onto phases as predicted by lattice-gas models including solid, liquid, super-fluid, and supersolid phases. By detailed dielectric and pyrocurrent measurements, we document a zoo of multiferroic phases with sizable ferroelectric polarization strongly varying from phase to phase. Using lattice-gas terminology, the title compound reveals multiferroic spin-superfluid and spin-supersolid phases, while the antiferromagnetic solid is paraelectric.Comment: 14 pages including 5 figure