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Using the density matrix renormalization group technique, we study the ground state phase diagram and other low-energy properties of an isotropic antiferromagnetic spin-half chain with both dimerization and frustration, i.e., an alternation δ of the nearest neighbor exchanges and a next-nearest-neighbor exchange J 2 . For δ = 0, the system is gapless for J 2 < J 2c and has a gap for J 2 > J 2c where J 2c is about 0.241.For J 2 = J 2c , the gap above the ground state grows as δ to the power 0.667 ± 0.001. In the J 2 − δ plane, there is a disorder line 2J 2 + δ = 1. To the left of this line, the peak in the static structure factor S(q) is at q max = π (Neel phase), while to the right of the line, q max decreases from π to π/2 as J 2 is increased to large values (spiral phase). For δ = 1, the system is equivalent to two coupled chains as on a ladder and it is gapped for all values of the interchain coupling.

Anorganische Schichten: Graphenartiges MoS2 und WS2 wurde durch drei verschiedene chemische Methoden hergestellt. Mikroskopische Untersuchungen offenbarten, dass die Strukturen aus einer oder wenigen Schichten aufgebaut sind (siehe TEM‐Aufnahme von WS2‐Schichten), und ein atomar aufgelöstes TEM‐Bild zeigt, dass schichtförmiges MoS2 eine hexagonale Anordnung von Mo‐ und S‐Atomen aufweist (siehe Einschub).

We perform first-principles calculations based on density functional theory to study quasi onedimensional edge-passivated (with hydrogen) zigzag graphene nanoribbons (ZGNRs) of various widths with chemical dopants, boron and nitrogen, keeping the whole system isoelectronic. Gradual increase in doping concentration takes the system finally to zigzag boron nitride nanoribbons (ZBNNRs). Our study reveals that, for all doping concentrations the systems stabilize in antiferromagnetic ground states. Doping concentrations and dopant positions regulate the electronic structure of the nanoribbons, exhibiting both semiconducting and half-metallic behaviors as a response to the external electric field. Interestingly, our results show that ZBNNRs with terminating polyacene unit exhibit half-metallicity irrespective of the ribbon width as well as applied electric field, opening a huge possibility in spintronics device applications.Nanomaterials of carbon, like nanotubes, fullerenes, etc., have been of great interest in condensed-matter and material science because of their novel low-dimensional properties [1,2]. Over past few decades, cutting edge research has been carried out for advanced device integration, exploring the electronic and mechanical properties of these systems. The recent addition in this journey is graphene: a strictly two-dimensional flat monolayer of carbon atoms tightly packed into a honeycomb lattice [3,4]. Since its innovation [5,6,7], it has made possible the understanding of various properties in two dimensions, by simple experiments and has opened up huge possibilities for electronic device fabrications [8,9,10]. A large number of theoretical and experimental groups all over the world have gathered on this two dimensional platform to search for the "plenty of room" at this reduced dimension [11].Electronic properties of low dimensional materials are mainly governed by their size and geometry. Recent experimental sophistications permit the preparation of finite size quasi one dimensional graphene, named as graphene nanoribbons (GNRs) of varying widths, either by cutting mechanically exfoliated graphenes and patterning by lithographic techniques [12,13] or by tuning the epitaxial growth of graphenes [14,15]. Different geometrical terminations of the graphene monolayer give rise to two different edge geometries of largely varying electronic properties, namely, zigzag and armchair graphene. Several theoretical models, e.g., tight-binding model within Schrodinger [16,17,18], Dirac formalism for mass less fermions [19,20,21], density functional theory (DFT) etc. have been applied to explore the electronic and band structure properties of GNRs. There exists a few many-body studies, exploring the electronic and magnetic properties of these systems [22,23].DFT studies suggest that, the anti-ferromagnetic quasi one-dimensional (1D) zigzag edge graphene nanoribbons (ZGNRs) show half-metallicity at a finite external elec- tric field across the ribbon width within both local density approximation (LDA) [24] and g...

We report spin wave and DMRG studies of the ground and low-lying excited states of uniform and dimerized alternating spin chains. The DMRG procedure is also employed to obtain low-temperature thermodynamic properties of the system. The ground state of a 2N spin system with spin-1 and spin-1 2 alternating from site to site and interacting via an antiferromagnetic exchange is found to be ferrimagnetic with total spin s G = N/2 from both DMRG and spin wave analysis. Both the studies also show that there is a gapless excitation to a state with spin s G − 1 and a gapped excitation to a state with spin s G + 1. Surprisingly, the correlation length in the ground state is found to be very small from both the studies for this gapless system. For this very reason, we show that the ground state can be described by a variational ansatz of the product type. DMRG analysis shows that the chain is susceptible to a conditional spin-Peierls' instability. The DMRG studies of magnetization, magnetic susceptibility (χ) and specific heat show strong magnetic-field dependence. The product χT shows a minimum as a function of temperature(T ) at low-magnetic fields and the minimum vanishes at high-magnetic fields. This lowfield behaviour is in agreement with earlier experimental observations. The specific heat shows a maximum as a function of temperature and the height of the maximum increases sharply at high-magnetic fields. It is hoped that these studies will motivate experimental studies at high-magnetic fields.

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Graphene samples prepared by the exfoliation of graphitic oxide and conversion of nanodiamond exhibit good hydrogen uptake at 1 atm, 77 K, the uptake going up to 1.7 wt %. The hydrogen uptake varies linearly with the surface area, and the extrapolated value of hydrogen uptake by single-layer graphene works out to be just above 3 wt %. The H 2 uptake at 100 atm and 298 K is found to be 3 wt % or more, suggesting thereby the single-layer graphene would exhibit much higher uptakes. Equally interestingly, the graphene samples prepared by us show high uptake of CO 2 , the value reaching up to 35 wt % at 1 atm and 195 K. The firstprinciples calculations show that hydrogen molecules sit alternately in parallel and perpendicular orientation on the six-membered rings of the graphene. Up to 7.7 wt % of hydrogen can be accommodated on singlelayered graphene. CO 2 molecules sit alternatively in a parallel fashion on the rings, giving use to a maximum uptake of 37.93 wt % in single-layer graphene. The presence of more than one layer of graphene in our samples causes a decrease in the H 2 uptake.

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