Supersolidity in the triangular lattice spin-1/2 XXZ model: A variational perspective

We study the spin-1/2 XXZ model on the triangular lattice with a nearest neighbor antiferromagnetic Ising coupling $J_z>0$ and unfrustrated ($J_\perp0$) kinetic terms in zero magnetic field. Incorporating long-range Jastrow correlations over a mean field spin state, we obtain the variational phase diagram of this model on large lattices for arbitrary $J_z$ and either sign of $J_\perp$. For $J_\perp<0$, we find a $\sqrt{3}\times\sqrt{3}$ supersolid for $J_z/|J_\perp| \gtrsim 4.7$, in excellent agreement with quantum Monte Carlo data. For $J_\perp >0$, a distinct $\sqrt{3}\times\sqrt{3}$ supersolid is found to emerge for $J_z/J_\perp \geq 1$. Both supersolids exhibit a spontaneous density deviation from half-filling. At $J_z/J_\perp=\infty$, the crystalline order parameters of these two supersolids are nearly identical, consistent with exact results.Comment: 4 pages, 4 figures, 1 table, published versio

Effective Action of a Dynamical Dpp-brane with Background Fluxes

We shall construct the Dirac-Born-Infeld like and the Wess-Zumino like actions for a dynamical D$p$-brane with the $U(1)$ gauge potential and the Kalb-Ramond background field. The brane dynamics simultaneously has both tangential and transverse components. Our calculations will be in the context of the type II superstring theory, via the boundary state formalism.Comment: 17 pages, LaTeX2e, no figur

From magnetism to one-dimensional spin liquid in the anisotropic triangular lattice

We investigate the anisotropic triangular lattice that interpolates from decoupled one-dimensional chains to the isotropic triangular lattice and has been suggested to be relevant for various quasi-two-dimensional materials, such as Cs$_2$CuCl$_4$ or $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$, an organic material that shows intriguing magnetic properties. We obtain an excellent accuracy by means of a novel representation for the resonating valence bond wave function with both singlet and triplet pairing. This approach allows us to establish that the magnetic order is rapidly destroyed away from the pure triangular lattice and incommensurate spin correlations are short range. A non-magnetic spin liquid naturally emerges in a wide range of the phase diagram, with strong one-dimensional character. The relevance of the triplet pairing for $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ is also discussed.Comment: 4+epsilon pages, 6 figure

A design technique for geometric optimisation of resonant coil sizes in low to mid frequency inductive power transmission systems.

Wireless power transfer (WPT) is a well-established method of energising electrically-powered devices. Among the different available WPT techniques, Resonant Inductive Power Transfer (RIPT) has been adapted for use in a wide range of applications. The primary reason is the relatively higher Power Transfer Efficiency (PTE) that RIPT can provide. RIPT systems operate on the principle of magnetic resonance coupling between a Transmitter (Tx) and a Receiver (Rx) coil. Maximising the PTE is a key driver for improving the performance of RIPT systems. In a RIPT link the PTE is influenced by three factors: (i) inductive linkage between the Tx and Rx, (ii) terminating circuitry of Tx and Rx sides and (iii) the Tx/Rx coil's geometrical size. In considering these impacting factors, different techniques to improve PTE have been extensively presented in the literature and are comprehensively reviewed in this thesis. The research work undertaken focuses on the geometrical optimisation of Tx/Rx coils to help maximise PTE in RIPT systems for operation over low- and mid-frequency bands (i.e. between few kHz to several MHz). Conventional methods for maximising PTE require defining various design parameters (i.e. figure-of-merits), which assist in finding the optimum Air-Cored Coil (ACC) geometry. However, traditional techniques for working with Figure-of-Merit (FoM) parameters are very time-consuming and process-demanding. In this thesis, the number of required FoMs have been reduced to one and incorporated into a process that will accelerate production of the optimum geometry design. A unique FoM parameter (i.e. Pscf) is developed by consolidating the PTE's impacting factors. Considering the RIPT application and its physical size constraints, a proper selection method for identifying the numerical value of Pscf is investigated. A novel iterative algorithm has been developed to assist in selection of the most favourable Pscf value, which provides the optimum ACC geometry. Theoretical design examples of two RIPT systems - operating at 10 kHz (low-frequency band) and 300 kHz (mid-frequency band) - are used to investigate the functionality of the ACC design approach, for which successful results are achieved. The novel iterative algorithm is also experimentally validated by developing four prototyped Tx/Rx ACC pairs, with real-world applications, which operate over low- and mid-frequency bands: 1:06 MHz, 100 kHz, 50 kHz, 15 kHz. For the designed ACC geometries, maximum PTEs of 85:63% at 1:06 MHz, 83:10% at 100 kHz, 72:85% at 50 kHz and 34:57% at 15 kHz are practically measured in bench top tests. The measured PTE values are in close correlation (within 14%) with the calculated PTEs at these frequency ranges, and thus validate the novel ACC design procedure. The RIPT system's maximum achievable PTE can be further increased by adding ferrite cores to the Tx/Rx ACC pair. In this thesis, an advanced iterative algorithm is also presented to support the design of geometrically optimised coil pairs employing ferrite cores. The advanced iterative algorithm is an extension of the initial work on optimising ACC geometries. Optimum Ferrite-Cored Coil (FCC) geometries, produced using the advanced iterative algorithm, for RIPT systems operating at 10 kHz and 300 kHz have been investigated. In comparing the FCC and ACC geometries designed for these frequencies, it is demonstrated that RIPT systems with ferrite cores reduce the ACC's geometrical size and additionally improve PTE. To validate the performance of the advanced FCC design algorithm over low- and mid-frequency bands, two RIPT systems are physically constructed for operation at 15 kHz (low-frequency) and 50 kHz (mid-frequency). For the prototyped RIPT systems, maximum PTEs of 45:16% at 50 kHz and 50:74% at 15 kHz are practically measured. The calculated and physically measured PTE values are within 2% difference; hence validating the advanced FCC design process

Eccentricity in Tubes - Experimental Development and Simulation-Based Analysis for an Automatically Adjustable Drawing Die

Ein permanentes Ziel der Fertigung ist die Erhöhung der Produktionseffizienz durch Reduzierung der Durchlaufzeiten bei gleichzeitiger Kostenreduktion und Verbesserung der Produktqualität. Auch die Rohrherstellung - eines der ältesten bekannten Fertigungsverfahren - ist diesem permanenten Druck ausgesetzt. Rohre finden ihren weitgestreuten Einsatz mit unterschiedlichsten Anforderungen zum Transport von Feststoffen, Flüssigkeiten und Gasen in den verschiedensten Bereichen wie der Energiewirtschaft, der Automobil-, Luft- und Raumfahrt oder auch der Medizinindustrie. Da in all diesen Bereichen die Rohrqualität eine entscheidende Rolle spielt, ist die Herstellung von qualitativ hochwertigen und teilweise hochpräzisen Rohren in der Rohrfertigung eine Herausforderung. Dies ist die Motivation dieser Arbeit zur Entwicklung einer neuen Vorrichtung zur Herstellung von Hochpräzisionsrohren. Die Entwicklung einer neuen Methode zur gezielten lokalen Veränderung der Wandstärke des Rohres wurde bereits im Vorfeld mit verschiedenen Ansätzen untersucht. Die Modifizierung und Kontrolle der Rohrexzentrizität sind wesentlich für Anwendungen mit engen Toleranzanforderungen. Eine der kürzlich entwickelten Methoden zur Änderung der Exzentrizität von Rohren ist das Kippen der Ziehmatrize. In dieser Arbeit wird diese Entwicklung fortgeführt und zu einer dynamisch in-line verstellbaren Matrize, zur kontrollierten Beeinflussung der Exzentrizität eines Rohres für den industriellen Einsatz, erweitert. Dazu wird in Abhängigkeit von der einlaufenden Exzentrizität des Rohres die Ziehmatrize in die geeignete Position gekippt. Das entsprechende Aggregat wurde im Rahmen eines AiF-Projekts in Zusammenarbeit mit den Firmen Bültmann GmbH (Entwicklung und Bau der Anlage), Fest AG (Steuerung), GE AG (US-Messtechnik) zusammen mit dem Institut für Metallurgie der TU Clausthal entwickelt und getestet. Die drei wesentlichen Schritte zur Entwicklung der dynamisch verstellbaren Matrize sind: • Entwicklung einer dynamisch verstellbaren Matrize, die den Matrizenhalter um 360 °- bis zu 4° transversal zur Ziehrichtung neigen kann. • Untersuchung der effektiven Parameter der Exzentrizität von gezogenen Rohren durch Kippen der Matrize und Entwicklung eines Regressionsmodells zur Vorhersage der Exzentrizität der Rohre nach dem Ziehen. Das Modell wird für die Ermittlung der optimalen Kippwinkel der Matrize verwendet. • Entwicklung eines Steuer- und Regelsystems zur Korrelation der Messdaten des US-In-Line-Messsystems zur Ermittlung der Exzentrizität des Rohres vor dem Ziehen mit der Kippung der Ziehmatrize. In diesem Konzept wird der Ist-Zustand (Wanddicke) des einlaufenden Rohres über sechs US-Sensoren ermittelt und daraus die Exzentrizität berechnet. Mit den entwickelten Algorithmen wird die Kippung der Ziehmatrize, die für diese Exzentrizitätslage zu einer definierten Exzentrizität am auslaufenden Rohr führt, vorgegeben und dynamisch eingestellt. Sowohl das Ziehen mit festem wie auch mit fliegendem Dorn wurde mit dem Kippen kombiniert. Darüber hinaus werden in dieser Arbeit auch der Kippeffekt und das entsprechende Ziehverfahren auf die Eigenspannungsausbildung in den gezogenen Rohren betrachtet.A goal of the manufacturing industry is to increase the efficiency of the production reducing through-put time as well as cost of the production and increasing the product quality. Tube manufacturing, as one of the oldest known manufacturing processes, is also still moving towards this goal. Since tubes are known as products to transport fluids and gases, they find their applications in areas such as energy, automobile, aerospace and medical industries. Due to the fact that the tube quality plays a crucial role in all these areas, producing high quality and high precise tubes still is challenging in tube manufacturing. This challenge is the motivation for the development of a new device to produce high precision tubes, which is the main aim of this study. To change the wall thickness of the tube aiming to modify its eccentricity was studied using different approaches. Modifying and controlling the eccentricity of tubes is important for reaching tight tolerance requirements. One of the recently developed method for changing the eccentricity of tube is tilting the drawing die. Developing a dynamic, in-line adjustable die for controlled influencing the eccentricity of a tube for industrial use is studied in this thesis, aiming on changing the drawing conditions by tilting the die with defined tilting angles and positions using fixed and floating plugs. This device was designed and developed in the framework of an AiF project with cooperation of Bültmann GmbH, Fest AG and GE AG, for developing the mechanical, control and measuring system of the device, respectively. It was tested at the Institute of Metallurgy at Clausthal University of Technology. The three major steps for developing the dynamic adjustable die are summarized in the following: • Developing a dynamic adjustable die, which can tilt the die holder in 360 ° position up to 4 ° transversal to the drawing direction. • Studying the involved effective parameters on the final eccentricity of drawn tubes by tilting the die and developing a regression model for predicting the eccentricity of the tubes after drawing, which is used to find the proper tilting angle of the die during drawing. • Developing a control and regulation system to correlate the movement of the die and the in-line US measuring system, which is integrated for measuring the initial eccentricity of the tube and with these data targeting the tilting position. The incoming wall thickness condition is measured by the US-device and used to control the (exit) eccentricity of the tube by dynamically adjusting the die. Two tube drawing methods - drawing with fixed plug and floating plug - are combined with die tilting. Moreover, the tilting effect and drawing method on the residual stresses of the drawn tubes are investigated in this study

Study of the Static and Dynamic Magnetization across the First Order Phase Transition in FeRh Thin Films

The equiatomic FeRh alloy undergoes a first-order phase transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state at about 370 K with a small thermal hysteresis of about 10 K around the phase transition. The transition is accompanied by a unit cell volume expansion about 1% in the c lattice parameter. During the transition the new phase nucleates in the matrix of the original phase by reaching the critical temperature followed by a growth in size upon increasing temperature further. Therefore, to understand the transition process with more details, it is desirable to investigate the nucleation and growth of both phases within the first order phase transition. In the present thesis the main focus is on the growth of FeRh thin films by means of Molecular Beam Epitaxy (MBE) technique and characterization of the magnetic and structural properties. To develop an understanding of the phase transformation in FeRh thin films the ways in which one can tune it were investigated. The following aspects concerning the FeRh system have been examined here: 1) influence of annealing temperature on the magnetic and structural response, 2) effect of film thickness on the first-order phase transition temperature as well as the saturation magnetization, 3) influence of chemical composition on the magnetic properties and 4) magnetic field-induced phase transition. To get insight to details of the transition process the magnetization dynamic has been addressed by performing Ferromagnetic resonance (FMR) experiment across the phase transition. FMR measurements determined the existence of two areas with different magnetic properties inside the film. A huge temperature difference for the beginning of the phase transition in comparison with the static magnetization measurement was observed for the equiatomic FeRh thin film prepared by MBE. Tuning of the AFM to FM phase transition in the FeRh thin film by means of low-energy/low fluence Ne+ ion irradiation was studied. Ion irradiation technique offers a quantitative control of the degree of chemical disorder by adjusting the ion fluence applied, while the penetration depth of the disordered phase can be adjusted by the ion-energy. The main results of ion irradiation are the shifting of the phase transition temperature to lower temperature and irradiation with 3×1014 ion/cm2 leads to the disappearance the AFM phase completely

Inhomogeneous metallic phase upon disordering a two dimensional Mott insulator

We find that isoelectronic disorder destroys the spectral gap in a Mott-Hubbard insulator in 2D leading, most unexpectedly, to a new metallic phase. This phase is spatially inhomogeneous with metallic behavior coexisting with antiferromagnetic long range order. Even though the Mott gap in the pure system is much larger than antiferromagnetic exchange, the spectral gap is destroyed locally in regions where the disorder potential is high enough to overcome the inter-electron repulsion thereby generating puddles where charge fluctuations are enhanced. With increasing disorder, these puddles expand and concomitantly the states at the Fermi energy get extended leading to a metallic phase. We discuss the implications of our results for experiments.Comment: (4 pages, 5 figures

Persistent supersolid phase of hard-core bosons on the triangular lattice

We study hard-core bosons with unfrustrated hopping ($t$) and nearest neighbour repulsion ($U$) on the triangular lattice. At half-filling, the system undergoes a zero temperature ($T$) quantum phase transition from a superfluid phase at small $U$ to a supersolid at $U_c \approx 4.45$ in units of $2t$. This supersolid phase breaks the lattice translation symmetry in a characteristic $\sqrt{3} \times \sqrt{3}$ pattern, and is remarkably stable--indeed, a smooth extrapolation of our results indicates that the supersolid phase persists for arbitrarily large $U/t$.Comment: 4 pages, 5 figures, two column forma