Thermodynamics of the Spin-Chain Compounds Cs2CoCl4 and Cu(C4H4N2)(NO3)2

In this thesis the thermodynamic properties of the spin-1/2 chain compounds Cs2CoCl4 and Cu(C4H4N2)(NO3)2 (CuPzN) are investigated. Measurements of the specific heat, the thermal expanison, the magnetization and the magnetocaloric effect in a temperature range from 0.25 to 30 K and in magnetic fields up to 17 T are compared to model calculations. In Cs2CoCl4 a crystal field anisotropy of about 7 K leads to a splitting of the orbital spin-3/2 states of the magnetic Co^{2+} into Kramers doublets. It is shown that the inclusion of excited crystal field states in the application of a spin-1/2 XXZ model to the system is essential for a consistent description of the thermodynamics of Cs2CoCl4. The anisotropy Delta~0.12 of the spin chain is smaller than previously assumed in literature. Below 2 K and for magnetic fields smaller than 4 T the magnetism of Cs2CoCl4 is well described by the XXZ model in a transverse magnetic field. Signatures of a quantum phase transition at 2.0 T are found in the thermodynamic data. Below T_N=0.22 K magnetic order arises. The magnetic phase diagrams for different field directions are derived from the experimental data and the microscopic origins of the appearing phases are discussed. Bordering antiferromagnetism a low-temperature phase is identified that possibly is not a spin-liquid phase as previously suggested in literature. For fields along the crystallographic b axis an additional two-stage spin-flop transition arises. In the mixed compounds Cs_2CoCl_{4-x}Br_x and Cs_3CoCl_{5-y}Br_y a site-selective doping is indicated by structural and thermodynamic investigations. While in the first system an easy-plane type magnetism, similar to the parent compound is found, the latter shows an easy-axis anisotropy and a non-continuous evolution of the low-temperature magnetism. The magnetism of Cu(C4H4N2)(NO3)2 is highly isotropic and well described by the spin-1/2 Heisenberg chain, which shows a quantum phase transition as a function of an external field. In the thesis, the thermodynamics of Cu(C4H4N2)(NO3)2 are compared to exact results of the Heisenberg model including magnetic fields up to and above the critical field of the spin chain. All investigated thermodynamic quantities are described by the model with a high precision and show clear signatures of a quantum phase transition. A coupling constant of 10.60 K is found and all data consistently indicate a critical field of the spin chain of 13.90 T. Approaching the critical field, the specific heat acquires the expected power law, the magnetocaloric effect diverges and a universal scaling law is obeyed

Giant magnetochiral anisotropy from quantum-confined surface states of topological insulator nanowires

Wireless technology relies on the conversion of alternating electromagnetic fields into direct currents, a process known as rectification. Although rectifiers are normally based on semiconductor diodes, quantum mechanical non-reciprocal transport effects that enable a highly controllable rectification were recently discovered1,2,3,4,5,6,7,8,9. One such effect is magnetochiral anisotropy (MCA)6,7,8,9, in which the resistance of a material or a device depends on both the direction of the current flow and an applied magnetic field. However, the size of rectification possible due to MCA is usually extremely small because MCA relies on inversion symmetry breaking that leads to the manifestation of spin–orbit coupling, which is a relativistic effect6,7,8. In typical materials, the rectification coefficient γ due to MCA is usually ∣γ∣ ≲ 1 A−1 T−1 (refs. 8,9,10,11,12) and the maximum values reported so far are ∣γ∣ ≈ 100 A−1 T−1 in carbon nanotubes13 and ZrTe5 (ref. 14). Here, to overcome this limitation, we artificially break the inversion symmetry via an applied gate voltage in thin topological insulator (TI) nanowire heterostructures and theoretically predict that such a symmetry breaking can lead to a giant MCA effect. Our prediction is confirmed via experiments on thin bulk-insulating (Bi1−xSbx)2Te3 (BST) TI nanowires, in which we observe an MCA consistent with theory and ∣γ∣ ≈ 100,000 A−1 T−1, a very large MCA rectification coefficient in a normal conductor

Prox1 Is Required for Granule Cell Maturation and Intermediate Progenitor Maintenance During Brain Neurogenesis

The transcription factor Prox1 plays a crucial role in intermediate progenitor maintenance and hippocampal neuron differentiation during adult neurogenesis in the dentate gyrus region of the hippocampus

Effects of Anacetrapib in Patients with Atherosclerotic Vascular Disease

BACKGROUND: Patients with atherosclerotic vascular disease remain at high risk for cardiovascular events despite effective statin-based treatment of low-density lipoprotein (LDL) cholesterol levels. The inhibition of cholesteryl ester transfer protein (CETP) by anacetrapib reduces LDL cholesterol levels and increases high-density lipoprotein (HDL) cholesterol levels. However, trials of other CETP inhibitors have shown neutral or adverse effects on cardiovascular outcomes. METHODS: We conducted a randomized, double-blind, placebo-controlled trial involving 30,449 adults with atherosclerotic vascular disease who were receiving intensive atorvastatin therapy and who had a mean LDL cholesterol level of 61 mg per deciliter (1.58 mmol per liter), a mean non-HDL cholesterol level of 92 mg per deciliter (2.38 mmol per liter), and a mean HDL cholesterol level of 40 mg per deciliter (1.03 mmol per liter). The patients were assigned to receive either 100 mg of anacetrapib once daily (15,225 patients) or matching placebo (15,224 patients). The primary outcome was the first major coronary event, a composite of coronary death, myocardial infarction, or coronary revascularization. RESULTS: During the median follow-up period of 4.1 years, the primary outcome occurred in significantly fewer patients in the anacetrapib group than in the placebo group (1640 of 15,225 patients [10.8%] vs. 1803 of 15,224 patients [11.8%]; rate ratio, 0.91; 95% confidence interval, 0.85 to 0.97; P=0.004). The relative difference in risk was similar across multiple prespecified subgroups. At the trial midpoint, the mean level of HDL cholesterol was higher by 43 mg per deciliter (1.12 mmol per liter) in the anacetrapib group than in the placebo group (a relative difference of 104%), and the mean level of non-HDL cholesterol was lower by 17 mg per deciliter (0.44 mmol per liter), a relative difference of -18%. There were no significant between-group differences in the risk of death, cancer, or other serious adverse events. CONCLUSIONS: Among patients with atherosclerotic vascular disease who were receiving intensive statin therapy, the use of anacetrapib resulted in a lower incidence of major coronary events than the use of placebo. (Funded by Merck and others; Current Controlled Trials number, ISRCTN48678192 ; ClinicalTrials.gov number, NCT01252953 ; and EudraCT number, 2010-023467-18 .)

Opportunities in topological insulator devices

Topological insulators (TIs) are expected to be a promising platform for novel quantum phenomena, whose experimental realizations require sophisticated devices. In this Technical Review, we discuss four topics of particular interest for TI devices: topological superconductivity, quantum anomalous Hall insulator as a platform for exotic phenomena, spintronic functionalities, and topological mesoscopic physics. We also discuss the present status and technical challenges in TI device fabrications to address new physics.Comment: Technical Review commissioned by Nature Reviews Physics; 11 pages, 4 figure

Anomalous Fraunhofer Patterns in Gated Josephson Junctions Based on the Bulk-Insulating Topological Insulator BiSbTeSe2

One-dimensional Majorana modes are predicated to form in Josephson junctions based on three-dimensional topological insulators (TIs). While observations of supercurrents in Josephson junctions made on bulk-insulating TI samples have been reported recently, the Fraunhofer patters observed in such Tl-based Josephson junctions, which sometimes present anomalous features, are still not well-understood. Here, we report our study of highly gate-tunable Tl-based Josephson junctions made of one of the most bulk-insulating TI materials, BiSbTeSe2, and Al. The Fermi level can be tuned by gating across the Dirac point, and the high transparency of the Al-BiSbTeSe2 interface is evinced by a high characteristic voltage and multiple Andreev reflections, with peak indices reaching 12 Anomalous Fraunhofer patterns with missing lobes were observed in the entire range of gate voltage. We found that, by employing an advanced fitting procedure to use the maximum entropy method in a Monte Carlo algorithm, the anomalous Fraunhofer patterns are explained as a result of inhomogeneous supercurrent distributions on the TI surface in the junction. Besides establishing a highly promising fabrication technology, this work clarifies one of the important open issues regarding Tl-based Josephson junctions

Gigantic negative magnetoresistance in the bulk of a disordered topological insulator

With the recent discovery of Weyl semimetals, the phenomenon of negative magnetoresistance (MR) is attracting renewed interest. Large negative MR is usually related to magnetism, but the chiral anomaly in Weyl semimetals is a rare exception. Here we report a mechanism for large negative MR which is also unrelated to magnetism but is related to disorder. In the nearly bulk-insulating topological insulator TlBi0.15Sb0.85Te2, we observed gigantic negative MR reaching 98% in 14 T at 10 K, which is unprecedented in a nonmagnetic system. Supported by numerical simulations, we argue that this phenomenon is likely due to the Zeeman effect on a barely percolating current path formed in the disordered bulk. Since disorder can also lead to non-saturating linear MR in Ag-2 (+) Se-delta, the present finding suggests that disorder engineering in narrow-gap systems is useful for realizing gigantic MR in both positive and negative directions