Jens Juel Jensen - Niels Bohr Institute, Copenhagen

Jens Juel Jensen
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Name
Jens Juel Jensen
Affiliation
Niels Bohr Institute, Copenhagen
City
Copenhagen
Country
Denmark

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Statistics - Theory (2)
 
Physics - Soft Condensed Matter (2)
 
Mathematics - Statistics (2)
 
Physics - Biological Physics (2)
 
Mathematics - Probability (2)
 
Quantum Physics (2)
 
Physics - Mesoscopic Systems and Quantum Hall Effect (1)
 
Physics - Materials Science (1)
 
Physics - Statistical Mechanics (1)
 
Physics - Other (1)
 
Physics - Strongly Correlated Electrons (1)
 
Physics - Instrumentation and Detectors (1)
 
Physics - Physics and Society (1)
 
Physics - Disordered Systems and Neural Networks (1)
 
Instrumentation and Methods for Astrophysics (1)
 
Nuclear Experiment (1)
 
Physics - Chemical Physics (1)

Publications Authored By Jens Juel Jensen

2016Oct
Authors: Demitri Muna, Michael Alexander, Alice Allen, Richard Ashley, Daniel Asmus, Ruyman Azzollini, Michele Bannister, Rachael Beaton, Andrew Benson, G. Bruce Berriman, Maciej Bilicki, Peter Boyce, Joanna Bridge, Jan Cami, Eryn Cangi, Xian Chen, Nicholas Christiny, Christopher Clark, Michelle Collins, Johan Comparat, Neil Cook, Darren Croton, Isak Delberth Davids, Éric Depagne, John Donor, Leonardo A. dos Santos, Stephanie Douglas, Alan Du, Meredith Durbin, Dawn Erb, Daniel Faes, J. G. Fernández-Trincado, Anthony Foley, Sotiria Fotopoulou, Søren Frimann, Peter Frinchaboy, Rafael Garcia-Dias, Artur Gawryszczak, Elizabeth George, Sebastian Gonzalez, Karl Gordon, Nicholas Gorgone, Catherine Gosmeyer, Katie Grasha, Perry Greenfield, Rebekka Grellmann, James Guillochon, Mark Gurwell, Marcel Haas, Alex Hagen, Daryl Haggard, Tim Haines, Patrick Hall, Wojciech Hellwing, Edmund Christian Herenz, Samuel Hinton, Renee Hlozek, John Hoffman, Derek Holman, Benne Willem Holwerda, Anthony Horton, Cameron Hummels, Daniel Jacobs, Jens Juel Jensen, David Jones, Arna Karick, Luke Kelley, Matthew Kenworthy, Ben Kitchener, Dominik Klaes, Saul Kohn, Piotr Konorski, Coleman Krawczyk, Kyler Kuehn, Teet Kuutma, Michael T. Lam, Richard Lane, Jochen Liske, Diego Lopez-Camara, Katherine Mack, Sam Mangham, Qingqing Mao, David J. E. Marsh, Cecilia Mateu, Loïc Maurin, James McCormac, Ivelina Momcheva, Hektor Monteiro, Michael Mueller, Roberto Munoz, Rohan Naidu, Nicholas Nelson, Christian Nitschelm, Chris North, Juan Nunez-Iglesias, Sara Ogaz, Russell Owen, John Parejko, Vera Patrício, Joshua Pepper, Marshall Perrin, Timothy Pickering, Jennifer Piscionere, Richard Pogge, Radek Poleski, Alkistis Pourtsidou, Adrian M. Price-Whelan, Meredith L. Rawls, Shaun Read, Glen Rees, Hanno Rein, Thomas Rice, Signe Riemer-Sørensen, Naum Rusomarov, Sebastian F. Sanchez, Miguel Santander-García, Gal Sarid, William Schoenell, Aleks Scholz, Robert L. Schuhmann, William Schuster, Peter Scicluna, Marja Seidel, Lijing Shao, Pranav Sharma, Aleksandar Shulevski, David Shupe, Cristóbal Sifón, Brooke Simmons, Manodeep Sinha, Ian Skillen, Bjoern Soergel, Thomas Spriggs, Sundar Srinivasan, Abigail Stevens, Ole Streicher, Eric Suchyta, Joshua Tan, O. Grace Telford, Romain Thomas, Chiara Tonini, Grant Tremblay, Sarah Tuttle, Tanya Urrutia, Sam Vaughan, Miguel Verdugo, Alexander Wagner, Josh Walawender, Andrew Wetzel, Kyle Willett, Peter K. G. Williams, Guang Yang, Guangtun Zhu, Andrea Zonca

The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Read More

Let $(X_1, \dots, X_n)$ be multivariate normal, with mean vector $\boldsymbol{\mu}$ and covariance matrix $\boldsymbol{\Sigma}$, and $S_n=\mathrm{e}^{X_1}+\cdots+\mathrm{e}^{X_n}$. The Laplace transform ${\cal L}(\theta)=\mathbb{E}\mathrm{e}^{-\theta S_n} \propto \int \exp\{-h_\theta(\boldsymbol{x})\} \,\mathrm{d} \boldsymbol{x}$ is represented as $\tilde{\cal L}(\theta)I(\theta)$, where $\tilde{\cal L}(\theta)$ is given in closed-form and $I(\theta)$ is the error factor ($\approx 1$). We obtain $\tilde{\cal L}(\theta)$ by replacing $h_\theta(\boldsymbol{x})$ with a second order Taylor expansion around its minimiser $\boldsymbol{x}^*$. Read More

For high-dimensional classification Fishers rule performs poorly due to noise from estimation of the covariance matrix. Fan, Feng and Tong (2012) introduced the ROAD classifier that puts an $L_1$-constraint on the classification vector. In their Theorem 1 Fan, Feng and Tong (2012) show that the ROAD classifier asymptotically has the same misclassification rate as the corresponding oracle based classifier. Read More

Let $X$ be lognormal$(\mu,\sigma^2)$ with density $f(x)$, let $\theta>0$ and define ${L}(\theta)=E e^{-\theta X}$. We study properties of the exponentially tilted density (Esscher transform) $f_\theta(x) =e^{-\theta x}f(x)/{L}(\theta)$, in particular its moments, its asymptotic form as $\theta\to\infty$ and asymptotics for the Cram\'er function; the asymptotic formulas involve the Lambert W function. This is used to provide two different numerical methods for evaluating the left tail probability of lognormal sum $S_n=X_1+\cdots+X_n$: a saddlepoint approximation and an exponential twisting importance sampling estimator. Read More

Molecular diffusion measurements are widely used to probe microstructure in materials and living organisms noninvasively. The precise relation of diffusion metrics to microstructure remains a major challenge: In complex samples, it is often unclear which structural features are most relevant and can be quantified. Here we classify the structural complexity in terms of the long time tail exponent in the molecular velocity autocorrelation function. Read More

The present supply of 3He can no longer meet the detector demands of the upcoming ESS facility and continued detector upgrades at current neutron sources. Therefore viable alternative technologies are required to support the development of cutting-edge instrumentation for neutron scattering science. In this context, 10B-based detectors are being developed by collaboration between the ESS, ILL, and Link\"{o}ping University. Read More

The Minimum Ignition Energy (MIE) of an initially Gaussian temperature profile is found both by Direct Numerical Simulations (DNS) and from a new novel model. The model is based on solving the heat diffusion equation in zero dimensions for a Gaussian velocity distribution. The chemistry is taken into account through the ignition delay time, which is required as input to the model. Read More

The inelastic scattering of neutrons from magnetic excitations in the antiferromagnetic phase of the langasite compound Ba3NbFe3Si2O14 is analyzed theoretically. In the calculations presented, the strongly coupled spin-5/2 Fe triangles are accounted for as trimerized units. The weaker interactions between the trimers are included within the mean-field/random-phase approximation. Read More

Restrictions to molecular motion by barriers (membranes) are ubiquitous in biological tissues, porous media and composite materials. A major challenge is to characterize the microstructure of a material or an organism nondestructively using a bulk transport measurement. Here we demonstrate how the long-range structural correlations introduced by permeable membranes give rise to distinct features of transport. Read More

On some problems in the article "Efficient Likelihood Estimation in State Space Models" by Cheng-Der Fuh [Ann. Statist. 34 (2006) 2026--2068] [arXiv:math/0611376] Read More

2009Jul
Affiliations: 1Niels Bohr Institute, Copenhagen, 2Niels Bohr Institute, Copenhagen

The dimerized S=1/2 spins of the Cu ions in TlCuCl3 are ordered antiferromagnetically in the presence of a field larger than about 54 kOe in the zero-temperature limit. Within the mean-field approximation all thermal effects are frozen out below 6 K. Nevertheless, experiments show significant changes of the critical field and the magnetization below this temperature, which reflect the presence of low-energetic dimer-spin excitations. Read More

A mean-field model explaining most of the details in the magnetic phase diagram of ErNi_2B_2C is presented. The low-temperature magnetic properties are found to be dominated by the appearance of long-period commensurate structures. The stable structure at low temperatures and zero field is found to have a period of 40 layers along the a direction, and upon cooling it undergoes a first-order transition at T_C = 2. Read More

2000Jun
Affiliations: 1Royal Holloway, University of London, 2Royal Holloway, University of London
Category: Quantum Physics

We describe an algorithm for converting one bipartite quantum state into another using only local operations and classical communication, which is much simpler than the original algorithm given by Nielsen [Phys. Rev. Lett. Read More

2000Mar
Affiliations: 1Royal Holloway, University of London, 2Royal Holloway, University of London
Category: Quantum Physics

Starting from Barnum's recent proposal to use entanglement and catalysis for quantum secure identification [quant-ph/9910072], we describe a protocol for quantum authentication and authenticated quantum key distribution. We argue that our scheme is secure even in the presence of an eavesdropper who has complete control over both classical and quantum channels. Read More

The tendency for the period of the helically ordered moments in holmium to lock into values which are commensurable with the lattice is studied theoretically as a function of temperature and magnetic field. The commensurable effects are derived in the mean-field approximation from numerical calculations of the free energy of various commensurable structures, and the results are compared with the extensive experimental evidence collected during the last ten years on the magnetic structures in holmium. In general the stability of the different commensurable structures is found to be in accord with the experiments, except for the tau=5/18 structure observed a few degrees below T_N in a b-axis field. Read More

The equations of motion of dipolar-coupled spins of I=1/2 placed on a rigid lattice are solved approximately in the high-temperature and high-field limit. The NMR-spectra predicted by this theory are in close agreement with both the theoretical spectral moments, up to the eighth in the case of a simple cubic lattice, and the extremely accurate experimental results which have been obtained in the case of CaF_2. The theory is compared with the recent NMR-experiment of Lefmann et al. Read More

The two main characteristics of the holmium ions in HoF_3 are that their local electronic properties are dominated by two singlet states lying well below the remaining 4f-levels, and that the classical dipole-coupling is an order of magnitude larger than any other two-ion interactions between the Ho-moments. This combination makes the system particularly suitable for testing refinements of the mean-field theory. There are four Ho-ions per unit cell and the hyperfine coupled electronic and nuclear moments on the Ho-ions order in a ferrimagnetic structure at T_C=0. Read More