Jacob W. Smith

Observation of a discrete time crystal

Spontaneous symmetry breaking is a fundamental concept in many areas of physics, including cosmology, particle physics and condensed matter. An example is the breaking of spatial translational symmetry, which underlies the formation of crystals and the phase transition from liquid to solid. Using the analogy of crystals in space, the breaking of translational symmetry in time and the emergence of a ‘time crystal’ was recently proposed, but was later shown to be forbidden in thermal equilibrium. However, non-equilibrium Floquet systems, which are subject to a periodic drive, can exhibit persistent time correlations at an emergent subharmonic frequency. This new phase of matter has been dubbed a ‘discrete time crystal’. Here we present the experimental observation of a discrete time crystal, in an interacting spin chain of trapped atomic ions. We apply a periodic Hamiltonian to the system under many-body localization conditions, and observe a subharmonic temporal response that is robust to external perturbations. The observation of such a time crystal opens the door to the study of systems with long-range spatio-temporal correlations and novel phases of matter that emerge under intrinsically non-equilibrium conditions.

Cation-cation contact pairing in water: Guanidinium

The formation of like-charge guanidinium-guanidinium contact ion pairs in water is evidenced and characterized by X-ray absorption spectroscopy and first-principles spectral simulations based on molecular dynamics sampling. Observed concentration-induced nitrogen K-edge resonance shifts result from π* state mixing and the release of water molecules from each first solvation sphere as two solvated guanidinium ions associate into a stacked pair configuration. Possible biological implications of this counterintuitive cation-cation pairing are discussed.

Dynamic Monte Carlo simulation of spin-lattice relaxation of quadrupolar nuclei in solids. Oxygen-17 in yttria-doped ceria

Although measurement of spin‐lattice relation time (T1) can provide valuable information about atomic motion in solids, T1 data for quadrupolar nuclei are often difficult to interpret because the relevant physical interactions cannot be expressed analytically. In order to address this problem, we have developed an extension to the dynamic Monte Carlo method for simulating spin‐lattice relaxation of quadrupolar nuclei in solids. In this paper we develop the simulation method generally, and then apply the method to understanding published T1 measurements of oxygen‐17 in yttria‐doped ceria. We show that even for simple geometries of motion, multiple time scales for electric field gradient fluctuations are possible, resulting in complex relaxation behavior including multiple T1 minima. The method can be used to explain and deconvolute data in which these effects are present. In the case of yttria‐doped ceria, we show that two experimentally observed T1 minima result from simultaneous movement of oxygen vacanc...

Effects of long-range forces on oxygen transport in yttria-doped ceria: simulation and theory

Oxygen transport in many ionically conducting oxides occurs by diffusion or migration of oxygen-ion vacancies. An important issue in our understanding of these materials is the mechanism by which vacancies become trapped or ordered, and how this process affects transport. In this paper we reinvestigate the mechanism of vacancy trapping in yttria-doped ceria, and show that long-range forces play an important role in the transport properties of this material. We demonstrate, using dynamic Monte Carlo (MC) simulation, that these long-range forces cause strong deviations from the behaviour predicted by a simple point-defect model, deviations which are evident in previously published measurements of the conductivity. Interpretation of the physics observed in the simulation leads naturally to a Debye–Huckel modification of the point-defect model, which, when combined with kinetic parameters extracted independently from NMR spectroscopy, is capable of predicting measured macroscopic transport with no adjustable parameters. We find that in the case of yttria-doped ceria, the vacancy–defect association enthalpy is well approximated by considering only Coulombic interactions.

Experimental Realization of a Quantum Integer-Spin Chain with Controllable Interactions

Ions with multiple quantum states are useful test beds for quantum magnetism and memory. Researchers use trapped

Communication: Hydrogen bonding interactions in water-alcohol mixtures from X-ray absorption spectroscopy

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Properties of aqueous nitrate and nitrite from x-ray absorption spectroscopy

Yb ions to control interactions among ions with three quantum states.

Soft X-Ray Second Harmonic Generation as an Interfacial Probe

While methanol and ethanol are macroscopically miscible with water, their mixtures exhibit negative excess entropies of mixing. Despite considerable effort in both experiment and theory, there remains significant disagreement regarding the origin of this effect. Different models for the liquid mixture structure have been proposed to address this behavior, including the enhancement of the water hydrogen bonding network around the alcohol hydrophobic groups and microscopic immiscibility or clustering. We have investigated mixtures of methanol, ethanol, and isopropanol with water by liquid microjet X-ray absorption spectroscopy on the oxygen K-edge, an atom-specific probe providing details of both inter- and intra-molecular structure. The measured spectra evidence a significant enhancement of hydrogen bonding originating from the methanol and ethanol hydroxyl groups upon the addition of water. These additional hydrogen bonding interactions would strengthen the liquid-liquid interactions, resulting in additional ordering in the liquid structures and leading to a reduction in entropy and a negative enthalpy of mixing, consistent with existing thermodynamic data. In contrast, the spectra of the isopropanol-water mixtures exhibit an increase in the number of broken alcohol hydrogen bonds for mixtures containing up to 0.5 water mole fraction, an observation consistent with existing enthalpy of mixing data, suggesting that the measured negative excess entropy is a result of clustering or micro-immiscibility.

Realization of a Quantum Integer-Spin Chain with Controllable Interactions

Nitrate and nitrite ions are of considerable interest, both for their widespread use in commercial and research contexts and because of their central role in the global nitrogen cycle. The chemistry of atmospheric aerosols, wherein nitrate is abundant, has been found to depend on the interfacial behavior of ionic species. The interfacial behavior of ions is determined largely by their hydration properties; consequently, the study of the hydration and interfacial behavior of nitrate and nitrite comprises a significant field of study. In this work, we describe the study of aqueous solutions of sodium nitrate and nitrite via X-ray absorption spectroscopy (XAS), interpreted in light of first-principles density functional theory electronic structure calculations. Experimental and calculated spectra of the nitrogen K-edge XA spectra of bulk solutions exhibit a large 3.7 eV shift between the XA spectra of nitrate and nitrite resulting from greater stabilization of the nitrogen 1s energy level in nitrate. A similar shift is not observed in the oxygen K-edge XA spectra of NO3 (-) and NO2 (-). The hydration properties of nitrate and nitrite are found to be similar, with both anions exhibiting a similar propensity towards ion pairing.

Soft X-ray Absorption Spectroscopy of Liquids and Solutions

Nonlinear optical processes at soft x-ray wavelengths have remained largely unexplored due to the lack of available light sources with the requisite intensity and coherence. Here we report the observation of soft x-ray second harmonic generation near the carbon K edge (∼284  eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems.