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Stephen Hill

Publications and source records attributed to Stephen Hill.

At least 19 recordsLinked to original sources
arXiv2025

OpenPBR: Novel Features and Implementation Details

OpenPBR is a physically based, standardized uber-shader developed for interoperable material authoring and rendering across VFX, animation, and design visualization workflows. This document serves as a companion to the official specification, offering deeper insight into the model's development and more detailed implementation guidance, including code examples and mathematical derivations. We begin with a description of the model's formal structure and theoretical foundations - covering slab-based layering, statistical mixing, and microfacet theory - before turning to its physical components. These include metallic, dielectric, subsurface, and glossy-diffuse base substrates, followed by thin-film iridescence, coat, and fuzz layers. A special-case mode for rendering thin-walled objects is also described. Additional sections explore technical topics in greater depth, such as the decoupling of specular reflectivity from transmission, the choice of parameterization for subsurface scattering, and the detailed physics of coat darkening and thin-film interference. We also discuss planned extensions, including hazy specular reflection and retroreflection.

cs.GR↗
arXiv2024

EON: A practical energy-preserving rough diffuse BRDF

We introduce the "Energy-preserving Oren--Nayar" (EON) model for reflection from rough surfaces. Unlike the popular qualitative Oren--Nayar model (QON) and its variants, our model is energy-preserving via analytical energy compensation. We include self-contained GLSL source code for efficient evaluation of the new model and importance sampling based on a novel technique we term "Clipped Linearly Transformed Cosine" (CLTC) sampling.

cs.GR↗
arXiv2021

Demonstration of electron-nuclear decoupling at a spin clock transition

The ability to design quantum systems that decouple from environmental noise sources is highly desirable for development of quantum technologies with optimal coherence. The chemical tunability of electronic states in magnetic molecules combined with advanced electron spin resonance techniques provides excellent opportunities to address this problem. Indeed, so-called clock transitions (CTs) have been shown to protect molecular spin qubits from magnetic noise, giving rise to significantly enhanced coherence. Here we conduct a spectroscopic and computational investigation of this physics, focusing on the role of the nuclear bath. Away from the CT, linear coupling to the nuclear degrees of freedom causes a modulation and decay of electronic coherence, as quantified via electron spin echo signals generated experimentally and $\textit{in silico}$. Meanwhile, the effective hyperfine interaction vanishes at the CT, resulting in electron-nuclear decoupling and an absence of quantum information leakage to the nuclear bath, providing opportunities to characterize other decoherence sources.

quant-ph↗
arXiv2019

Decoherence in Molecular Electron Spin Qubits: Insights from Quantum Many-Body Simulations

Quantum states are described by wave functions whose phases cannot be directly measured, but which play a vital role in quantum effects such as interference and entanglement. The loss of the relative phase information, termed decoherence, arises from the interactions between a quantum system and its environment. Decoherence is perhaps the biggest obstacle on the path to reliable quantum computing. Here we show that decoherence occurs even in an isolated molecule although not all phase information is lost via a theoretical study of a central electron spin qubit interacting with nearby nuclear spins in prototypical magnetic molecules. The residual coherence, which is molecule-dependent, provides a microscopic rationalization for the nuclear spin diffusion barrier proposed to explain experiments. The contribution of nearby molecules to the decoherence has a non-trivial dependence on separation, peaking at intermediate distances. Molecules that are far away only affect the long-time behavior. Because the residual coherence is simple to calculate and correlates well with the coherence time, it can be used as a descriptor for coherence in magnetic molecules. This work will help establish design principles for enhancing coherence in molecular spin qubits and serve to motivate further theoretical work.

physics.chem-ph↗
arXiv2019

Optimal coupling of HoW$_{10}$ molecular magnets to superconducting circuits near spin clock transitions

A central goal in quantum technologies is to maximize $G$T$_{2}$, where $G$ stands for the coupling of a qubit to control and readout signals and T$_{2}$ is the qubit's coherence time. This is challenging, as increasing $G$ (e.g. by coupling the qubit more strongly to external stimuli) often leads to deleterious effects on T$_{2}$. Here, we study the coupling of pure and magnetically diluted crystals of HoW$_{10}$ magnetic clusters to microwave superconducting coplanar waveguides. Absorption lines give a broadband picture of the magnetic energy level scheme and, in particular, confirm the existence of level anticrossings at equidistant magnetic fields determined by the combination of crystal field and hyperfine interactions. Such 'spin clock transitions' are known to shield the electronic spins against magnetic field fluctuations. The analysis of the microwave transmission shows that the spin-photon coupling becomes also maximum at these transitions. The results show that engineering spin-clock states of molecular systems offers a promising strategy to combine sizeable spin-photon interactions with a sufficient isolation from unwanted magnetic noise sources.

quant-ph↗
arXiv2019

MADHAT: Model-Agnostic Dark Halo Analysis Tool

We present the Model-Agnostic Dark Halo Analysis Tool (MADHAT), a numerical tool which implements a Fermi-LAT data-driven, model-independent analysis of gamma-ray emission from dwarf satellite galaxies and dwarf galaxy candidates due to dark matter annihilation, dark matter decay, or other nonstandard or unknown astrophysics. This tool efficiently provides statistical upper bounds on the number of observed photons in excess of the number expected, based on empirical determinations of foregrounds and backgrounds, using a stacked analysis of any selected set of dwarf targets. It also calculates the resulting bounds on the properties of dark matter under any assumptions the user makes regarding dark sector particle physics or astrophysics. As an application, we determine new bounds on Sommerfeld-enhanced dark matter annihilation in a set of eight dwarfs. MADHAT v1.0 includes 58 dwarfs and dwarf candidate targets, and we discuss future planned developments. MADHAT is available and will be maintained at https://github.com/MADHATdm

hep-ph↗
arXiv2018

Magic angle effects in a trigonal Mn(III)3 cluster: deconstruction of a single-molecule magnet

We present angle-dependent high-frequency EPR studies on a single-crystal of a trigonal Mn3 cluster with an unusual structure in which the local magnetic easy-axes of the constituent Mn(III) ions are tilted significantly away from the molecular C3 axis towards the magic-angle of 54.7 degrees, resulting in an almost complete cancelation of the 2nd-order axial magnetic anisotropy associated with the ferromagnetically coupled total spin ST = 6 ground state. This contrasts the situation in many related Mn3 single-molecule magnets (SMMs) that have been studied intensively in the past, for which the local MnIII anisotropy tensors are reasonably parallel, resulting in substantial barriers to magnetization relaxation (Ueff = 30 to 35 cm 1) and magnetization blocking below about 2.5 K. The suppression of the 2nd-order anisotropy in the present case results in a situation in which the zero-field splitting (ZFS) of the ST = 6 ground state is dominated by 4th- and higher-order interactions. This provides a unique opportunity to study in depth how molecular geometry influences these interactions that are responsible for quantum tunneling of magnetization in high-symmetry SMMs. Angle-dependent EPR measurements provide a full mapping of the molecular magneto-anisotropy. Meanwhile, irreducible tensor operator (ITO) methods are employed in order to obtain analytic expressions that directly relate molecular anisotropy to the microscopic physics, i.e., the ZFS tensors associated with the individual MnIII ions, their orientations, and the exchange coupling between the three spins. We find that the magic-angle tilting leads to a massive compression of the ST = 6 ground state energy level diagram and strong mixing between spin projection states. Although these characteristics are antagonistic to SMM behavior, they provide important insights into the physics of polynuclear molecular nanomagnets.

cond-mat.mtrl-sci↗
arXiv2018

The silver route to cuprate analogs

The parent compound of high-Tc superconducting cuprates is a unique Mott state consisting of layers of spin-1/2 ions arranged on a square lattice and with a record high antiferromagnetic coupling within the layers. Compounds with similar characteristics have long been searched for. Nickelates and iridates had been proposed as cuprate analogs but so far have not reached a satisfactory similarity. Here we use a combination of experimental and theoretical tools to show that the commercial compound AgF2 is an excellent cuprate analog with remarkably similar electronic parameters to La2CuO4 but larger buckling of planes. Two-magnon Raman scattering reveals a superexchange constant which reaches 70% of that of a typical cuprate. We argue that structures that reduce or eliminate the buckling of the AgF2 planes could have an antiferromagnetic coupling that matches or surpasses the cuprates.

cond-mat.str-el↗
arXiv2017

Effects of uniaxial pressure on the quantum tunneling of magnetization in a high-symmetry Mn12 single-molecule magnet

The symmetry of single-molecule magnets dictates their spin quantum dynamics, influencing how such systems relax via quantum tunneling of magnetization (QTM). By reducing a system's symmetry, through the application of a magnetic field or uniaxial pressure, these dynamics can be modified. We report measurements of the magnetization dynamics of a crystalline sample of the high-symmetry [Mn12O12(O2CMe)16(MeOH)4]MeOH single-molecule magnet as a function of uniaxial pressure applied either parallel or perpendicular to the sample's "easy" magnetization axis. At temperatures between 1.8 and 3.3 K, magnetic hysteresis loops exhibit the characteristic steplike features that signal the occurrence of QTM. After applying uniaxial pressure to the sample in situ, both the magnitude and field position of the QTM steps changed. The step magnitudes were observed to grow as a function of pressure in both arrangements of pressure, while pressure applied along (perpendicular to) the sample's easy axis caused the resonant-tunneling fields to increase (decrease). These observations were compared with simulations in which the system's Hamiltonian parameters were changed. From these comparisons, we determined that parallel pressure induces changes to the second-order axial anisotropy parameter as well as either the fourth-order axial or fourth-order transverse parameter, or to both. In addition, we find that pressure applied perpendicular to the easy axis induces a rhombic anisotropy E ~ D/2000 per kbar that can be understood as deriving from a symmetry-breaking distortion of the molecule.

cond-mat.mes-hall↗
arXiv2013

A Microscopic and Spectroscopic View of Quantum Tunneling of Magnetization

This chapter takes a microscopic view of quantum tunneling of magnetization (QTM) in single-molecule magnets (SMMs), focusing on the interplay between exchange and anisotropy. Careful consideration is given to the relationship between molecular symmetry and the symmetry of the spin Hamiltonian that dictates QTM selection rules. Higher order interactions that can modify the usual selection rules are shown to be very sensitive to the exchange strength. In the strong coupling limit, the spin Hamiltonian possess rigorous $D_{2h}$ symmetry (or $C_{\infty}$ in high-symmetry cases). In the case of weaker exchange, additional symmetries may emerge through mixing of excited spin states into the ground state. Group theoretic arguments are introduced to support these ideas, as are extensive results of magnetization hysteresis and electron paramagnetic resonance measurements.

cond-mat.mes-hall↗
arXiv2010

Magnetic Quantum Tunneling: Insights from Simple Molecule-Based Magnets

This article takes a broad view of the understanding of magnetic bistability and magnetic quantum tunneling in single-molecule magnets (SMMs), focusing on three families of relatively simple, low-nuclearity transition metal clusters: spin S = 4 Ni4, Mn(III)3 (S = 2 and 6) and Mn(III)6 (S = 4 and 12). The Mn(III) complexes are related by the fact that they contain triangular Mn3 units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the Mn(III) centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low- and high-spin molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn3 and Ni4). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc..) of the molecules highlighted in this study proves to be of crucial importance.

cond-mat.mes-hall↗
arXiv2009

On the anisotropy barrier reduction in fast relaxing Mn12 single-molecule magnets

A novel angle-swept high-frequency EPR (HFEPR) technique is described that enables in-situ alignment of single-crystal samples containing low-symmetry magnetic species such as single-molecule magnets (SMMs). This cavity-based method involves recording spectra at fixed frequency and field, while sweeping the field orientation. The method is applied to the study of a low-symmetry Jahn-Teller variant of the spin S = 10 Mn12 SMMs (e.g. Mn12-acetate). The low-symmetry complex is also an SMM, but with a significantly reduced barrier to magnetization reversal (Ueff ~ 43 K) and, hence, faster relaxation at low temperature in comparison with the high-symmetry species. Mn12 complexes that crystallize in lower symmetry structures exhibit a tendency for one or more of the Mn(III) Jahn-Teller axes to be abnormally oriented, which is believed to be the cause of the faster relaxation. An extensive HFEPR study of [Mn12O12(O2CCH2But)16(H2O)4].CH2Cl2.MeNO2 is presented in order to examine the influence of the abnormally oriented Jahn-Teller axis on the effective barrier. The reduction in the axial anisotropy, D, is found to be insufficient to account for the nearly 40% reduction in Ueff. However, the reduced symmetry of the Mn12 core gives rise to a very significant 2nd order transverse anisotropy, E ~ D/6. This, in turn, causes a significant mixing of spin projection states well below the top of the classical barrier. Thus, magnetic quantum tunneling is the dominant factor contributing to the barrier reduction in fast relaxing Mn12 SMMs.

cond-mat.mes-hall↗
arXiv2008

Reply to Wernsdorfer's post: "Correspondence on: Quantum interference of tunnel trajectories between states of different spin lenght in a dimeric molecular nanomagnet"

We present here an exact version of our response (dated April 27) to Wernsdorfer's correspondence submitted to Nature Physics on March 31, 2008. After consultation with a referee, Nature Physics chose not publish any part of this exchange. We would therefore like to point out that our original study has now been considered favorably by four separate referees chosen by Nature Physics. Unfortunately, Wernsdorfer subsequently posted two further variations of his correspondence on this archive (arXiv:0804.1246v1 and arXiv:0804.1246v2). We note that aspects of the most recent posting (dated after submission of our response) contradict the version submitted to Nature Physics. However, none of the revisions add weight to Wernsdorfer's original correspondence.

cond-mat.mes-hall↗
arXiv2008

Strongly Correlated Electrons in the $[Ni(hmp)(ROH)X]_4$ Single Molecule Magnet: A DFT+U Study

The single-molecule magnet $\mathrm{[Ni(hmp)(MeOH)Cl]_4}$ is studied using both density functional theory (DFT) and the DFT+U method, and the results are compared. By incorporating a Hubbard-U like term for both the nickel and oxygen atoms, the experimentally determined ground state is successfully obtained, and the exchange coupling constants derived from the DFT+U calculation agree with experiment very well. The results show that the nickel 3d and oxygen 2p electrons in this molecule are strongly correlated, and thus the inclusion of on-site Coulomb energies is crucial to obtaining the correct results.

cond-mat.str-el↗
arXiv2008

Fiber-optical analogue of the event horizon

The physics at the event horizon resembles the behavior of waves in moving media. Horizons are formed where the local speed of the medium exceeds the wave velocity. We use ultrashort pulses in microstructured optical fibers to demonstrate the formation of an artificial event horizon in optics. We observed a classical optical effect, the blue-shifting of light at a white-hole horizon. We also show by theoretical calculations that such a system is capable of probing the quantum effects of horizons, in particular Hawking radiation.

gr-qc↗
arXiv2006

Microwave spectroscopy of Q1D and Q2D organic conductors

This chapter reviews recent experimental studies of a novel open-orbit magnetic resonance phenomenon. The technique involves measurement of angle-dependent microwave magneto-conductivity and is, thus, closely related to the cyclotron resonance and angle-dependent magnetoresistance techniques. Data for three contrasting materials are presented: (TMTSF)2ClO4, alpha-(BEDT-TTF)2KHg(SCN)4 and kappa-(BEDT-TTF)2I3. These studies reveal important insights into the Fermiology of these novel materials, as well as providing access to important electronic parameters such as the in-plane Fermi velocity and quasiparticle scattering rate. It is argued that all three compounds exhibit coherent three-dimensional band transport at liquid helium temperatures, and that their low-energy magnetoelectrodynamic properties appear to be well explained on the basis of a conventional semiclassical Boltzmann approach. It is also suggested that this technique could be used to probe quasiparticles in nodal superconductors.

cond-mat.supr-con↗
arXiv2005

A rotating cavity for high-field angle-dependent microwave spectroscopy of low-dimensional conductors and magnets

The cavity perturbation technique is an extremely powerful method for measuring the electrodynamic response of a material in the millimeter- and sub-millimeter spectral range (10 GHz to 1 THz), particularly in the case of high-field/frequency magnetic resonance spectroscopy. However, the application of such techniques within the limited space of a high-field magnet presents significant technical challenges. We describe a 7.62 mm x 7.62 mm (diameter x length) rotating cylindrical cavity which overcomes these problems.

cond-mat.other↗