Quantum ice: a quantum Monte Carlo study

Ice states, in which frustrated interactions lead to a macroscopic ground-state degeneracy, occur in water ice, in problems of frustrated charge order on the pyrochlore lattice, and in the family of rare-earth magnets collectively known as spin ice. Of particular interest at the moment are "quantum spin-ice" materials, where large quantum fluctuations may permit tunnelling between a macroscopic number of different classical ground states. Here we use zero-temperature quantum Monte Carlo simulations to show how such tunnelling can lift the degeneracy of a spin or charge ice, stabilizing a unique "quantum-ice" ground state-a quantum liquid with excitations described by the Maxwell action of (3+1)-dimensional quantum electrodynamics. We further identify a competing ordered squiggle state, and show how both squiggle and quantum-ice states might be distinguished in neutron scattering experiments on a spin-ice material.     

Antiferromagnetism of ThCr2Si2

Neutron diffraction measurements indicate that the magnetic moments of chromium atoms in the ThCr₂Si₂ compound show long-range order. The Cr magnetic moment equal to 1.20(25)μ_B at 1.5 K lie in the basal plane and form magnetic structure AFl-type.     

A modification of the DIRECT method for Lipschitz global optimization for a symmetric function

In this paper, we consider a global optimization problem for a symmetric Lipschitz continuous function. An efficient modification of the well-known DIRECT (DIviding RECTangles) method called SymDIRECT is proposed for solving this problem. The method is illustrated and tested on several standard test functions. The application of this method to solving complex center-based clustering problems for the data having only one feature is particularly presented.     

Monte Carlo study of half-magnetization plateau and magnetic phase diagram in pyrochlore antiferromagnetic Heisenberg model

The antiferromagnetic Heisenberg model on a pyrochlore lattice under external magnetic field is studied by classical Monte Carlo simulation. The model includes bilinear and biquadratic interactions; the latter effectively describes the coupling to lattice distortions. The magnetization process shows a half-magnetization plateau at low temperatures, accompanied with strong suppression of the magnetic susceptibility. Temperature dependence of the plateau behavior is clarified. Finite-temperature phase diagram under the magnetic field is determined. The results are compared with recent experimental results in chromium spinel oxides.     

Crystal and magnetic structure of Ho2NiGe6

The crystal and magnetic structure of Ho₂NiGe₆ was studied by powder neutron diffraction. The paramagnetic neutron diffraction data confirmed the Ce₂CuGe₆-type crystal structure reported earlier for this compound. Below the Néel temperature equal to 11 K the Ho magnetic moments form a uniaxial antiferromagnetic ordering. The Ho magnetic moments equal to 8.16(7)μ_B at 1.5 K are parallel to the b-axis. The data are compared with those published for HoNi_0.46(6)Ge₂.     

Comprehensive profiling of the complex dendrimeric contrast agent Gadomer using a combined approach of CE, MS, and CE-MS

The complex dendrimeric contrast agent Gadomer has been comprehensively characterized using various mass spectrometric techniques in combination with capillary electrophoresis. The analytical challenges arising from the structure of this analyte, e.g., the presence of a complex isotopic pattern originating from multiple gadolinium chelates could be overcome using this combined approach. Results obtained from initial MALDI-TOF-MS measurements could subsequently be complemented and confirmed by CE-ESI-TOF-MS measurements. Furthermore, high resolution Fourier transform-ion cyclotron resonance-MS (FT-ICR-MS) as a stand alone technique and in combination with CE was performed to obtain mass spectra of high mass accuracy and resolution of various impurities and related dendrimers present in low concentrations in several Gadomer batches. The profile of related dendrimers and impurities of lower molecular weight present in Gadomer could be elucidated and variances in the pattern of related dendrimers present in batches obtained via different synthetic routes could be detected. The qualitative analysis of Gadomer using MS techniques in combination with CE provides the fundamental basis for a framework of analytical methodologies for the characterization of Gadomer. By designing such a framework of methodologies the challenges associated with the introduction of dendrimers into the clinical stage may be overcome and the reproducible quality and safety of these innovative products can be ensured.     

Magnetic structure of TmCu2Ge2

TmCu₂Ge₂ compound crystallizes in the tetragonal ThCr₂Si₂-type crystal structure. The neutron diffraction reveals the presence of an incommensurate antiferromagnetic order below T_N=2.5 K. The Tm magnetic moment of 5.0(1) μ_B at 0.47 K is parallel to the c-axis. The order is described by the propagation vector k=[k_x, k_x, 0], where k_x=0.117(3). The increase of the values of the components k_x near the Néel temperature is observed.     

On the low-temperature properties of TmCo2Ge2

In this paper low-temperature magnetic properties of TmCo₂Ge₂ are reported. The compound was investigated by means of magnetic, electrical transport, heat capacity and neutron diffraction measurements. Clear anomalies were found in all of the bulk characteristics at the onset of antiferomagnetically ordered state below 2.4 K. According to neutron diffraction data, Tm magnetic moments are aligned along the c-axis, forming the AFI-type magnetic structure. In the paramagnetic state, the specific heat reveals a distinct Schottky contribution with the total crystal field splitting of about 150 K.