Showing 1–21 of 21 results for author: Wagner, K

Exploring Exoplanets with Interferometry

Authors: Sascha P. Quanz, Bertrand Mennesson, Charles Beichman, Jonah T. Hansen, Felix A. Dannert, Andrea Fortier, Michael Ireland, Nicholas Beltsten, Eleonora Alei, Leonid Pogorelyuk, William O. Balmer, Denis Defrère, Gautam Vasisht, Malcolm Fridlund, Romain Laugier, Tiffany Kataria, Eugene Serabyn, Steve Ertel, Hélène Rousseau, Kevin Wagner, Rhonda Morgan, Gerard T. van Belle, Gail H. Schaefer, Jean-Philippe Berger, Taro Matsuo , et al. (5 additional authors not shown)

Abstract: (Extract from the Executive Summary) Humanity stands at the threshold of answering one of its most profound questions: Does life exist beyond Earth? Ongoing and upcoming space missions, together with powerful ground-based instruments, have prepared the way for a transformational next step - the detailed characterization of Earth analogs orbiting Sun-like and other stars and the search for atmosphe… ▽ More (Extract from the Executive Summary) Humanity stands at the threshold of answering one of its most profound questions: Does life exist beyond Earth? Ongoing and upcoming space missions, together with powerful ground-based instruments, have prepared the way for a transformational next step - the detailed characterization of Earth analogs orbiting Sun-like and other stars and the search for atmospheric biosignatures that may indicate life. Within this context, the European Space Agency's Voyage 2050 process has identified the direct detection of thermal emission from temperate terrestrial exoplanets in the mid-infrared (mid-IR) as a top scientific priority. The Large Interferometer For Exoplanets (LIFE) - a space-based, mid-IR nulling interferometer - is designed to meet this goal. LIFE will be capable of detecting climate-relevant gases such as CO$_2$ and H$_2$O, identifying classical biosignatures like O$_3$ and CH$_4$, and probing additional, non-classical biosignatures. It will also provide key data for determining planetary radius, albedo, and temperature, which are essential for assessing habitability. In parallel, the U.S. National Academy has recommended a complementary mission now called the Habitable Worlds Observatory (HWO) - a ~6-meter space telescope equipped with advanced coronagraphs to suppress starlight by a factor of ~10$^{10}$ across the visible and possibly into the near-infrared and near-ultraviolet. Together, LIFE and HWO offer synergistic capabilities, enabling a comprehensive and robust assessment of the prevalence of life-bearing exoplanets in our galactic neighbourhood - a first in human history. By uniting an international and interdisciplinary community of scientists and engineers, LIFE offers a credible pathway toward the direct detection and characterization of potentially habitable - and even inhabited - worlds. △ Less

Submitted 8 June, 2026; originally announced June 2026.

Comments: KISS (Keck Institute for Space Studies) Workshop Study Report; Study Leads: S.P. Quanz, B. Mennesson, C. Beichmann; Participant/co-author list in arbitrary order

A van der Waals material exhibiting room temperature broken inversion symmetry with ferroelectricity

Authors: Fabia F. Athena, Cooper A. Voigt, Mengkun Tian, Anjan Goswami, Emily Toph, Moses Nnaji, Fanuel Mammo, Brent K. Wagner, Sungho Jeon, Wenshan Cai, Eric M. Vogel

Abstract: Since the initial synthesis of van der Waals two-dimensional indium selenide was first documented in 1957, five distinct polymorphs and their corresponding polytypes have been identified. In this study, we report a unique phase of indium selenide via Scanning Transmission Electron Microscopy (STEM) analysis in the synthesized large-area films -- which we have named the $β^\text{p}$ phase. The quin… ▽ More Since the initial synthesis of van der Waals two-dimensional indium selenide was first documented in 1957, five distinct polymorphs and their corresponding polytypes have been identified. In this study, we report a unique phase of indium selenide via Scanning Transmission Electron Microscopy (STEM) analysis in the synthesized large-area films -- which we have named the $β^\text{p}$ phase. The quintuple layers of the $β^\text{p}$ phase, characterized by a unique zigzag atomic configuration with unequal indium-selenium bond lengths from the middle selenium atom, are distinct from any other previously reported phase of indium selenide. Cross-sectional STEM analysis has revealed that the $β^\text{p}$ layers exhibit intralayer shifting. We found that indium selenide films with $β^\text{p}$ layers display electric-field-induced switchable polarization characteristic of ferroelectric materials, suggesting the breaking of the inversion symmetry. Experimental observations of nonlinear optical phenomena -- Second Harmonic Generation (SHG) responses further support this conclusion. This study reports a $β^\text{p}$ phase of indium selenide showing ferroelectricity over large areas at room temperature in a low-dimensional limit. △ Less

Submitted 4 October, 2025; originally announced October 2025.

Reproducibility of fixed-node diffusion Monte Carlo across diverse community codes: The case of water-methane dimer

Authors: Flaviano Della Pia, Benjamin X. Shi, Yasmine S. Al-Hamdani, Dario Alfè, Tyler A. Anderson, Matteo Barborini, Anouar Benali, Michele Casula, Neil D. Drummond, Matúš Dubecký, Claudia Filippi, Paul R. C. Kent, Jaron T. Krogel, Pablo López Ríos, Arne Lüchow, Ye Luo, Angelos Michaelides, Lubos Mitas, Kosuke Nakano, Richard J. Needs, Manolo C. Per, Anthony Scemama, Jil Schultze, Ravindra Shinde, Emiel Slootman , et al. (8 additional authors not shown)

Abstract: Fixed-node diffusion quantum Monte Carlo (FN-DMC) is a widely-trusted many-body method for solving the Schrödinger equation, known for its reliable predictions of material and molecular properties. Furthermore, its excellent scalability with system complexity and near-perfect utilization of computational power makes FN-DMC ideally positioned to leverage new advances in computing to address increas… ▽ More Fixed-node diffusion quantum Monte Carlo (FN-DMC) is a widely-trusted many-body method for solving the Schrödinger equation, known for its reliable predictions of material and molecular properties. Furthermore, its excellent scalability with system complexity and near-perfect utilization of computational power makes FN-DMC ideally positioned to leverage new advances in computing to address increasingly complex scientific problems. Even though the method is widely used as a computational gold standard, reproducibility across the numerous FN-DMC code implementations has yet to be demonstrated. This difficulty stems from the diverse array of DMC algorithms and trial wave functions, compounded by the method's inherent stochastic nature. This study represents a community-wide effort to assess the reproducibility of the method, affirming that: Yes, FN-DMC is reproducible (when handled with care). Using the water-methane dimer as the canonical test case, we compare results from eleven different FN-DMC codes and show that the approximations to treat the non-locality of pseudopotentials are the primary source of the discrepancies between them. In particular, we demonstrate that, for the same choice of determinantal component in the trial wave function, reliable and reproducible predictions can be achieved by employing the T-move (TM), the determinant locality approximation (DLA), or the determinant T-move (DTM) schemes, while the older locality approximation (LA) leads to considerable variability in results. These findings demonstrate that, with appropriate choices of algorithmic details, fixed-node DMC is reproducible across diverse community codes-highlighting the maturity and robustness of the method as a tool for open and reliable computational science. △ Less

Submitted 1 September, 2025; v1 submitted 22 January, 2025; originally announced January 2025.

Journal ref: J. Chem. Phys. 163, 104110 (2025)

Ensemble variational Monte Carlo for optimization of correlated excited state wave functions

Authors: William A. Wheeler, Kevin G. Kleiner, Lucas K. Wagner

Abstract: Variational Monte Carlo methods have recently been applied to the calculation of excited states; however, it is still an open question what objective function is most effective. A promising approach is to optimize excited states using a penalty to minimize overlap with lower eigenstates, which has the drawback that states must be computed one at a time. We derive a general framework for constructi… ▽ More Variational Monte Carlo methods have recently been applied to the calculation of excited states; however, it is still an open question what objective function is most effective. A promising approach is to optimize excited states using a penalty to minimize overlap with lower eigenstates, which has the drawback that states must be computed one at a time. We derive a general framework for constructing objective functions with minima at the the lowest $N$ eigenstates of a many-body Hamiltonian. The objective function uses a weighted average of the energies and an overlap penalty, which must satisfy several conditions. We show this objective function has a minimum at the exact eigenstates for a finite penalty, and provide a few strategies to minimize the objective function. The method is demonstrated using ab initio variational Monte Carlo to calculate the degenerate first excited state of a CO molecule. △ Less

Submitted 1 December, 2023; originally announced December 2023.

Comments: 7 pages, 4 figures

PyQMC: an all-Python real-space quantum Monte Carlo module in PySCF

Authors: William A. Wheeler, Shivesh Pathak, Kevin Kleiner, Shunyue Yuan, João N. B. Rodrigues, Cooper Lorsung, Kittithat Krongchon, Yueqing Chang, Yiqing Zhou, Brian Busemeyer, Kiel T. Williams, Alexander Muñoz, Chun Yu Chow, Lucas K. Wagner

Abstract: We describe a new open-source Python-based package for high accuracy correlated electron calculations using quantum Monte Carlo (QMC) in real space: PyQMC. PyQMC implements modern versions of QMC algorithms in an accessible format, enabling algorithmic development and easy implementation of complex workflows. Tight integration with the PySCF environment allows for simple comparison between QMC cal… ▽ More We describe a new open-source Python-based package for high accuracy correlated electron calculations using quantum Monte Carlo (QMC) in real space: PyQMC. PyQMC implements modern versions of QMC algorithms in an accessible format, enabling algorithmic development and easy implementation of complex workflows. Tight integration with the PySCF environment allows for simple comparison between QMC calculations and other many-body wave function techniques, as well as access to high accuracy trial wave functions. △ Less

Submitted 2 December, 2022; originally announced December 2022.

Quantification of electron correlation for approximate quantum calculations

Authors: Shunyue Yuan, Yueqing Chang, Lucas K. Wagner

Abstract: State-of-the-art many-body wave function techniques rely on heuristics to achieve high accuracy at an attainable cost to solve the many-body Schrödinger equation. By far the most common property used to assess accuracy has been the total energy; however, total energies do not give a complete picture of electron correlation. In this work, the authors assess the von Neumann entropy of the one-partic… ▽ More State-of-the-art many-body wave function techniques rely on heuristics to achieve high accuracy at an attainable cost to solve the many-body Schrödinger equation. By far the most common property used to assess accuracy has been the total energy; however, total energies do not give a complete picture of electron correlation. In this work, the authors assess the von Neumann entropy of the one-particle reduced density matrix (1-RDM) to compare selected configuration interaction (CI), coupled cluster, variational Monte Carlo, and fixed-node diffusion Monte Carlo for benchmark hydrogen chains. A new algorithm, the circle reject method is presented which improves the efficiency of the evaluation of the von Neumann entropy using quantum Monte Carlo by several orders of magnitude. The von Neumann entropy of the 1-RDM and the eigenvalues of the 1-RDM are shown to distinguish between the dynamic correlation introduced by the Jastrow and static correlation introduced by determinants with large weights, confirming some of the lore in the field concerning the difference between the selected CI and Slater-Jastrow wave functions. △ Less

Submitted 5 August, 2022; originally announced August 2022.

Excited states in variational Monte Carlo using a penalty method

Authors: Shivesh Pathak, Brian Busemeyer, João N. B. Rodrigues, Lucas K. Wagner

Abstract: The authors present a technique using variational Monte Carlo to solve for excited states of electronic systems. The technique is based on enforcing orthogonality to lower energy states, which results in a simple variational principle for the excited states. Energy optimization is then used to solve for the excited states. This technique is applied to the well-characterized benzene molecule, in wh… ▽ More The authors present a technique using variational Monte Carlo to solve for excited states of electronic systems. The technique is based on enforcing orthogonality to lower energy states, which results in a simple variational principle for the excited states. Energy optimization is then used to solve for the excited states. This technique is applied to the well-characterized benzene molecule, in which $\sim$10,000 parameters are optimized for the first 12 excited states. Agreement within approximately 0.2 eV is obtained with higher scaling coupled cluster methods; small disagreements with experiment are likely due to vibrational effects. △ Less

Submitted 14 October, 2021; v1 submitted 28 September, 2020; originally announced September 2020.

Verniered Optical Phased Arrays for Grating Lobe Suppression and Extended FOV

Authors: Nathan Dostart, Bohan Zhang, Michael Brand, Daniel Feldkhun, Miloš Popović, Kelvin Wagner

Abstract: Optical phased arrays (OPAs) which beam-steer in 2D have so far been unable to pack emitting elements at $λ/2$ spacing, leading to grating lobes which limit the field-of-view, introduce signal ambiguity, and reduce optical efficiency. Vernier schemes, which use paired transmitter and receiver phased arrays with different periodicity, deliberately misalign the transmission and receive patterns so t… ▽ More Optical phased arrays (OPAs) which beam-steer in 2D have so far been unable to pack emitting elements at $λ/2$ spacing, leading to grating lobes which limit the field-of-view, introduce signal ambiguity, and reduce optical efficiency. Vernier schemes, which use paired transmitter and receiver phased arrays with different periodicity, deliberately misalign the transmission and receive patterns so that only a single pairing of transmit/receive lobes permit a signal to be detected. A pair of OPAs designed to exploit this effect thereby effectively suppress the effects of grating lobes and recover the system's field-of-view, avoid potential ambiguities, and reduce excess noise. Here we analytically evaluate Vernier schemes with arbitrary phase control to find optimal configurations, as well as elucidate the manner in which a Vernier scheme can recover the full field-of-view. We present the first experimental implementation of a Vernier scheme and demonstrate grating lobe suppression using a pair of 2D wavelength-steered OPAs. These results present a route forward for addressing the pervasive issue of grating lobes, significantly alleviating the need for dense emitter pitches. △ Less

Submitted 22 July, 2020; originally announced August 2020.

Comments: 11 pages, 5 figures. Supplementary material included (6 pages, 0 figures). Total length 17 pages, 6 figures

Recent developments in the PySCF program package

Authors: Qiming Sun, Xing Zhang, Samragni Banerjee, Peng Bao, Marc Barbry, Nick S. Blunt, Nikolay A. Bogdanov, George H. Booth, Jia Chen, Zhi-Hao Cui, Janus Juul Eriksen, Yang Gao, Sheng Guo, Jan Hermann, Matthew R. Hermes, Kevin Koh, Peter Koval, Susi Lehtola, Zhendong Li, Junzi Liu, Narbe Mardirossian, James D. McClain, Mario Motta, Bastien Mussard, Hung Q. Pham , et al. (24 additional authors not shown)

Abstract: PYSCF is a Python-based general-purpose electronic structure platform that both supports first-principles simulations of molecules and solids, as well as accelerates the development of new methodology and complex computational workflows. The present paper explains the design and philosophy behind PYSCF that enables it to meet these twin objectives. With several case studies, we show how users can… ▽ More PYSCF is a Python-based general-purpose electronic structure platform that both supports first-principles simulations of molecules and solids, as well as accelerates the development of new methodology and complex computational workflows. The present paper explains the design and philosophy behind PYSCF that enables it to meet these twin objectives. With several case studies, we show how users can easily implement their own methods using PYSCF as a development environment. We then summarize the capabilities of PYSCF for molecular and solid-state simulations. Finally, we describe the growing ecosystem of projects that use PYSCF across the domains of quantum chemistry, materials science, machine learning and quantum information science. △ Less

Submitted 10 July, 2020; v1 submitted 27 February, 2020; originally announced February 2020.

Journal ref: J. Chem. Phys. 153, 024109 (2020)

Serpentine optical phased arrays for scalable integrated photonic LIDAR beam steering

Authors: Nathan Dostart, Bohan Zhang, Anatol Khilo, Michael Brand, Kenaish Al Qubaisi, Deniz Onural, Daniel Feldkhun, Kelvin H. Wagner, Miloš A. Popović

Abstract: Optical phased arrays (OPAs) implemented in integrated photonic circuits could enable a variety of 3D sensing, imaging, illumination, and ranging applications, and their convergence in new LIDAR technology. However, current integrated OPA approaches do not scale - in control complexity, power consumption, and optical efficiency - to the large aperture sizes needed to support medium to long range L… ▽ More Optical phased arrays (OPAs) implemented in integrated photonic circuits could enable a variety of 3D sensing, imaging, illumination, and ranging applications, and their convergence in new LIDAR technology. However, current integrated OPA approaches do not scale - in control complexity, power consumption, and optical efficiency - to the large aperture sizes needed to support medium to long range LIDAR. We present the serpentine optical phased array (SOPA), a new OPA concept that addresses these fundamental challenges and enables architectures that scale up to large apertures. The SOPA is based on a serially interconnected array of low-loss grating waveguides and supports fully passive, two-dimensional (2D) wavelength-controlled beam steering. A fundamentally space-efficient design that folds the feed network into the aperture also enables scalable tiling of SOPAs into large apertures with a high fill-factor. We experimentally demonstrate the first SOPA, using a 1450 - 1650 nm wavelength sweep to produce 16,500 addressable spots in a 27x610 array. We also demonstrate, for the first time, far-field interference of beams from two separate OPAs on a single silicon photonic chip, as an initial step towards long-range computational imaging LIDAR based on novel active aperture synthesis schemes. △ Less

Submitted 17 February, 2020; originally announced February 2020.

Comments: Nathan Dostart and Bohan Zhang contributed equally to this work. Main text and supplementary material in the same document. 11 pages and 11 figures total

Journal ref: Optica 7 (2020) 726-733

A light weight regularization for wave function parameter gradients in quantum Monte Carlo

Authors: Shivesh Pathak, Lucas K. Wagner

Abstract: The parameter derivative of the expectation value of the energy, $\partial E/\partial p$, is a key ingredient in variational quantum Monte Carlo (VMC) wave function optimization methods. In some cases, a naïve Monte Carlo estimate of this derivative suffers from an infinite variance which inhibits the efficiency of optimization methods that rely on a stable estimate of the derivative. In this work… ▽ More The parameter derivative of the expectation value of the energy, $\partial E/\partial p$, is a key ingredient in variational quantum Monte Carlo (VMC) wave function optimization methods. In some cases, a naïve Monte Carlo estimate of this derivative suffers from an infinite variance which inhibits the efficiency of optimization methods that rely on a stable estimate of the derivative. In this work, we derive a simple regularization of the naïve estimator which is trivial to implement in existing VMC codes, has finite variance, and a negligible bias which can be extrapolated to zero bias with no extra cost. We use this estimator to construct an unbiased, finite variance estimation of $\partial E/\partial p$ for a multi-Slater-Jastrow trial wave function on the LiH molecule. This regularized estimator is a simple and efficient estimator of $\partial E/\partial p$ for VMC optimization techniques. △ Less

Submitted 22 February, 2020; v1 submitted 4 February, 2020; originally announced February 2020.

Zero-field propagation of spin waves in waveguides prepared by focused ion beam direct writing

Authors: Lukáš Flajšman, Kai Wagner, Marek Vaňatka, Jonáš Gloss, Viola Křižáková, Michael Schmid, Helmut Schultheiss, Michal Urbánek

Abstract: Metastable face-centered-cubic Fe78Ni22 thin films grown on Cu(001) substrates are excellent candidates for focused ion beam direct writing of magnonic structures due to their favorable magnetic properties after ion-beam-induced transformation. The focused ion beam transforms the originally nonmagnetic fcc phase into the ferromagnetic bcc phase with additional control over the direction of uniaxia… ▽ More Metastable face-centered-cubic Fe78Ni22 thin films grown on Cu(001) substrates are excellent candidates for focused ion beam direct writing of magnonic structures due to their favorable magnetic properties after ion-beam-induced transformation. The focused ion beam transforms the originally nonmagnetic fcc phase into the ferromagnetic bcc phase with additional control over the direction of uniaxial magnetic in-plane anisotropy. The magnetocrystalline anisotropy in transformed areas is strong enough to stabilize the magnetization in transverse direction to the long axis of narrow waveguides. Therefore, it is possible to propagate spin waves in these waveguides without the presence of an external magnetic field in the favorable Demon-Eshbach geometry. Phase-resolved micro-focused Brillouin light scattering yields the dispersion relation of these waveguides in zero as well as in nonzero external magnetic fields. △ Less

Submitted 18 July, 2019; v1 submitted 28 June, 2019; originally announced June 2019.

Journal ref: Phys. Rev. B 101, 014436 (2020)

Non-orthogonal determinants in multi-Slater-Jastrow trial wave functions for fixed-node diffusion Monte Carlo

Authors: Shivesh Pathak, Lucas K. Wagner

Abstract: The accuracy and efficiency of ab-initio quantum Monte Carlo (QMC) algorithms benefits greatly from compact variational trial wave functions that accurately reproduce ground state properties of a system. We investigate the possibility of using multi-Slater-Jastrow trial wave functions with non-orthogonal determinants by optimizing identical single particle orbitals independently in separate determ… ▽ More The accuracy and efficiency of ab-initio quantum Monte Carlo (QMC) algorithms benefits greatly from compact variational trial wave functions that accurately reproduce ground state properties of a system. We investigate the possibility of using multi-Slater-Jastrow trial wave functions with non-orthogonal determinants by optimizing identical single particle orbitals independently in separate determinants. As a test case, we compute variational and fixed-node diffusion Monte Carlo (FN-DMC) energies of a C$_2$ molecule. For a given multi-determinant expansion, we find that this non-orthogonal orbital optimization results in a consistent improvement in the variational energy and the FN-DMC energy on the order of a few tenths of an eV. Our calculations indicate that trial wave functions with non-orthogonal determinants can improve computed energies in a QMC calculation when compared to their orthogonal counterparts. △ Less

Submitted 20 August, 2018; originally announced August 2018.

Wave-front shaping in nonlinear multimode fibers

Authors: Omer Tzang, Antonio M. Caravaca-Aguirre, Kelvin Wagner, Rafael Piestun

Abstract: Recent remarkable progress in wave-front shaping has enabled control of light propagation inside linear media to focus and image through scattering objects. In particular, light propagation in multimode fibers comprises complex intermodal interactions and rich spatiotemporal dynamics. Control of physical phenomena in multimode fibers and its applications is in its infancy, opening opportunities to… ▽ More Recent remarkable progress in wave-front shaping has enabled control of light propagation inside linear media to focus and image through scattering objects. In particular, light propagation in multimode fibers comprises complex intermodal interactions and rich spatiotemporal dynamics. Control of physical phenomena in multimode fibers and its applications is in its infancy, opening opportunities to take advantage of complex mode interactions. In this work, we demonstrate a wave-front shaping approach for controlling nonlinear phenomena in multimode fibers. Using a spatial light modulator at the fiber input and a genetic algorithm optimization, we control a highly nonlinear stimulated Raman scattering cascade and its interplay with four wave mixing via a flexible implicit control on the superposition of modes that are coupled into the fiber. We show for the first time versatile spectrum manipulations including shifts, suppression, and enhancement of Stokes and anti-Stokes peaks. These demonstrations illustrate the power of wave-front shaping to control and optimize nonlinear wave propagation. △ Less

Submitted 14 July, 2017; v1 submitted 18 January, 2017; originally announced January 2017.

Comments: 11 pages 5 figures

Accurate barrier heights using diffusion Monte Carlo

Authors: Kittithat Krongchon, Brian Busemeyer, Lucas K. Wagner

Abstract: Fixed node diffusion Monte Carlo (DMC) has been performed on a test set of forward and reverse barrier heights for 19 non-hydrogen-transfer reactions, and the nodal error has been assessed. The DMC results are robust to changes in the nodal surface, as assessed by using different mean-field techniques to generate single determinant wave functions. Using these single determinant nodal surfaces, DMC… ▽ More Fixed node diffusion Monte Carlo (DMC) has been performed on a test set of forward and reverse barrier heights for 19 non-hydrogen-transfer reactions, and the nodal error has been assessed. The DMC results are robust to changes in the nodal surface, as assessed by using different mean-field techniques to generate single determinant wave functions. Using these single determinant nodal surfaces, DMC results in errors of 1.5(5) kcal/mol on barrier heights. Using the large data set of DMC energies, we attempted to find good descriptors of the fixed node error. It does not correlate with a number of descriptors including change in density, but does correlate with the gap between the highest occupied and lowest unoccupied orbital energies in the mean-field calculation. △ Less

Submitted 23 November, 2016; originally announced November 2016.

Comments: 6 pages, 4 figures, 1 table

Continuous wave approach for simulating Ferromagnetic Resonance in nanosized elements

Authors: K. Wagner, S. Stienen, M. Farle

Abstract: We present a numerical approach to simulate the Ferromagnetic Resonance (FMR) of micron and nanosized magnetic elements by a micromagnetic finite difference method. In addition to a static magnetic field a linearly polarized oscillating magnetic field is utilized to excite and analyze the spin wave excitations observed by Ferromagnetic Resonance in the space- and time-domain. Our continuous wave a… ▽ More We present a numerical approach to simulate the Ferromagnetic Resonance (FMR) of micron and nanosized magnetic elements by a micromagnetic finite difference method. In addition to a static magnetic field a linearly polarized oscillating magnetic field is utilized to excite and analyze the spin wave excitations observed by Ferromagnetic Resonance in the space- and time-domain. Our continuous wave approach (CW) provides an alternative to the common simulation method, which uses a pulsed excitation of the magnetic system. It directly models conventional FMR-experiments and permits the determination of the real and imaginary part of the complex dynamic susceptibility without the need of post-processing. Furthermore not only the resonance fields, but also linewidths, ellipticity, phase relations and relative intensities of the excited spin wave modes in a spectrum can be determined and compared to experimental data. The magnetic responses can be plotted as a function of spatial dimensions yielding a detailed visualization of the spin wave modes and their localization as a function of external magnetic field and frequency. This is illustrated for the case of a magnetic micron sized stripe. △ Less

Submitted 17 June, 2015; originally announced June 2015.

Using Local Operator Fluctuations to Identify Wave Function Improvements

Authors: Kiel T. Williams, Lucas K. Wagner

Abstract: A method is developed that allows analysis of quantum Monte Carlo simulations to identify errors in trial wave functions. The purpose of this method is to allow for the systematic improvement of variational wave functions by identifying degrees of freedom that are not well-described by an initial trial state. We provide proof of concept implementations of this method by identifying the need for a… ▽ More A method is developed that allows analysis of quantum Monte Carlo simulations to identify errors in trial wave functions. The purpose of this method is to allow for the systematic improvement of variational wave functions by identifying degrees of freedom that are not well-described by an initial trial state. We provide proof of concept implementations of this method by identifying the need for a Jastrow correlation factor, and implementing a selected multi-determinant wave function algorithm for small dimers that systematically decreases the variational energy. Selection of the two-particle excitations is done using quantum Monte Carlo within the presence of a Jastrow correlation factor, and without the need to explicitly construct the determinants. We also show how this technique can be used to design compact wave functions for transition metal systems. This method may provide a route to analyze and systematically improve descriptions of complex quantum systems in a scalable way. △ Less

Submitted 21 January, 2016; v1 submitted 5 June, 2015; originally announced June 2015.

Comments: 8 pages, 7 figures

Journal ref: Phys. Rev. E 94, 013303 (2016)

Quantum Monte Carlo for minimum energy structures

Authors: Lucas K. Wagner, Jeffrey C. Grossman

Abstract: We present an efficient method to find minimum energy structures using energy estimates from accurate quantum Monte Carlo calculations. This method involves a stochastic process formed from the stochastic energy estimates from Monte Carlo that can be averaged to find precise structural minima while using inexpensive calculations with moderate statistical uncertainty. We demonstrate the applicabili… ▽ More We present an efficient method to find minimum energy structures using energy estimates from accurate quantum Monte Carlo calculations. This method involves a stochastic process formed from the stochastic energy estimates from Monte Carlo that can be averaged to find precise structural minima while using inexpensive calculations with moderate statistical uncertainty. We demonstrate the applicability of the algorithm by minimizing the energy of the H2O-OH- complex and showing that the structural minima from quantum Monte Carlo calculations affect the qualitative behavior of the potential energy surface substantially. △ Less

Submitted 3 May, 2010; originally announced May 2010.

Comments: 7 pages, 4 figures

Experimental verification of the "rainbow" trapping effect in plasmonic graded gratings

Authors: Qiaoqiang Gan, Yongkang Gao, Kyle Wagner, Dmitri V. Vezenov, Yujie J. Ding, Filbert J. Bartoli

Abstract: We report the first experimental observation of trapped rainbow1 in graded metallic gratings2-4, designed to validate theoretical predictions for this new class of plasmonic structures. One-dimensional tapered gratings were fabricated and their surface dispersion properties tailored by varying the grating period and depth, whose dimensions were confirmed by atomic force microscopy. Reduced group v… ▽ More We report the first experimental observation of trapped rainbow1 in graded metallic gratings2-4, designed to validate theoretical predictions for this new class of plasmonic structures. One-dimensional tapered gratings were fabricated and their surface dispersion properties tailored by varying the grating period and depth, whose dimensions were confirmed by atomic force microscopy. Reduced group velocities and the plasmonic bandgap were observed. Direct measurements on graded grating structures show that light of different wavelengths in the 500-700nm region is "trapped" at different positions along the grating, consistent with computer simulations, thus verifying the "rainbow" trapping effect. The trapped rainbow effect offers exciting pathways for optical information storage and optical delays in photonic circuits at ambient temperature. △ Less

Submitted 22 March, 2010; originally announced March 2010.

PANIC: the new panoramic NIR camera for Calar Alto

Authors: H. Baumeister, M. Alter, M. C. Cardenas Vazquez, M. Fernandez, J. Fried, J. Helmling, A. Huber, J. Ibanez Mengual, J. F. Rodriguez Gomez, W. Laun, R. Lenzen, U. Mall, V. Naranjo, J. Ramos, R. Rohloff, A. Garcia Segura, C. Storz, M. Ubierna, K. Wagner

Abstract: PANIC is a wide-field NIR camera, which is currently under development for the Calar Alto observatory (CAHA) in Spain. It uses a mosaic of four Hawaii-2RG detectors and covers the spectral range from 0.8-2.5 micron(z to K-band). The field-of-view is 30x30 arcmin. This instrument can be used at the 2.2m telescope (0.45arcsec/pixel, 0.5x0.5 degree FOV) and at the 3.5m telescope (0.23arcsec/pixel,… ▽ More PANIC is a wide-field NIR camera, which is currently under development for the Calar Alto observatory (CAHA) in Spain. It uses a mosaic of four Hawaii-2RG detectors and covers the spectral range from 0.8-2.5 micron(z to K-band). The field-of-view is 30x30 arcmin. This instrument can be used at the 2.2m telescope (0.45arcsec/pixel, 0.5x0.5 degree FOV) and at the 3.5m telescope (0.23arcsec/pixel, 0.25x0.25 degree FOV). The operating temperature is about 77K, achieved by liquid Nitrogen cooling. The cryogenic optics has three flat folding mirrors with diameters up to 282 mm and nine lenses with diameters between 130 mm and 255 mm. A compact filter unit can carry up to 19 filters distributed over four filter wheels. Narrow band (1%) filters can be used. The instrument has a diameter of 1.1 m and it is about 1 m long. The weight limit of 400 kg at the 2.2m telescope requires a light-weight cryostat design. The aluminium vacuum vessel and radiation shield have wall thicknesses of only 6 mm and 3 mm respectively. △ Less

Submitted 25 July, 2008; originally announced July 2008.

Comments: This paper has been presented in the SPIE of Astronomical Telescopes and Instrumentation 2008 in Marseille (France)

Journal ref: In Ground-based and Airborne Instrumentation for Astronomy II, edited by Ian S. McLean, Mark M. Casali, Proceedings of SPIE Vol. 7014 (SPIE, Bellingham, WA 2008) 70142R

QWalk: A Quantum Monte Carlo Program for Electronic Structure

Authors: Lucas K. Wagner, Michal Bajdich, Lubos Mitas

Abstract: We describe QWalk, a new computational package capable of performing Quantum Monte Carlo electronic structure calculations for molecules and solids with many electrons. We describe the structure of the program and its implementation of Quantum Monte Carlo methods. It is open-source, licensed under the GPL, and available at the web site http://www.qwalk.org We describe QWalk, a new computational package capable of performing Quantum Monte Carlo electronic structure calculations for molecules and solids with many electrons. We describe the structure of the program and its implementation of Quantum Monte Carlo methods. It is open-source, licensed under the GPL, and available at the web site http://www.qwalk.org △ Less

Submitted 23 October, 2007; originally announced October 2007.

Comments: 12 pages, 3 figures, describes code available at http://www.qwalk.org