News

[Sept, 2021] A new anatomy for topology: Our recent PRL on the topological insulator SmB₆ presented a result so strange that it needed double, triple, and quadruple checking (read the lengthy supplement!): the material remains an insulator in spite of charge carrier doping, and keeps its topological surface state even it's heavily alloyed with topology-disrupting magnetic elements.

This new paper unravels some of the underlying mysteries, in a continuing collaboration with GIST, NSLS-II, and the ALS. Yishuai's measurements show that the topological `phase' extends even further than previously seen - out to at least 50% Ce alloying. This robustness is at odds with the small insulating gap, which should be rendered decoherent at much lower doping. However, it all turns out to make sense because... the topological surface state spectral feature appears to come from a different band pair than the insulating gap!

This attribution helps explain longstanding mysteries, such as the high emergence temperature of the surface state, and the unusual surface state spectral function which lacks a visible Dirac point. It is also a new and definition-stretching paradigm for topological physics:<\p>

Mapping out the emergence of topological features in the highly alloyed topological Kondo insulators Sm_1-xM_xB₆ (M=Eu, Ce), Yishuai Xu, Erica C. Kotta, M. S. Song, B. Y. Kang, J. W. Lee, B. K. Cho, Shouzheng Liu, Turgut Yilmaz, Elio Vescovo, Jonathan D. Denlinger, Lin Miao, and L. Andrew Wray, Physical Review B 104, 115118 (2021) - Editors' Suggestion.

[Sept, 2021] The peril of uneven vaccination: If a state's average vaccination rate is 70%, and the standard deviation across zip codes is 20%, how will an epidemic play out? (Choose all that apply)

A: As if 70% of people are vaccinated.
B: As if fewer than 70% are vaccinated.
C: As if more than 70% are vaccinated.
D: Onset of the epidemic will be slower, understating its eventual severity.
E: Onset of the epidemic will be faster.

Bear in mind that epidemiological outcomes are highly nonlinear in the vaccination rate, and total case numbers can be strongly influenced by outlier subpopulations.

Click here to see some quick simulations addressing this question in a multi-population SIR model. (the answer from this model is: B and D)

[Jan, 2020] Ingredients for the superconductivity of tomorrow: The dream for superconductors used to be simple. We would find a material that could superconduct at room temperature, and use it to revolutionize the design of electrical grids. This would push power generation out of cities, and might even smooth out some of the regional fluctuations associated with renewable power sources like wind farms. Other technologies aided by superconductivity would also become much more commonplace, from levitating trains to MRI machines.

Room temperature superconductivity seems much farther off now than it did in 1990, and the new dream is different. Some kinds of "topological" superconductor can manifest exotic quasiparticles called 'Majorana fermions' that may provide a stable medium for storing quantum information. If it all works out, this will circumvent some of the biggest obstacles to developing quantum computing technologies with truly widespread application.

Our recent work investigates UTe₂ a uniquely exciting superconductor from this angle. UTe₂ seems to be the first material to unambiguously be called a "triplet" superconductor in the absence of any other many-body symmetry breaking, where triplet superconductivity refers to a class of states that are roughly synonymous with topological superconductivity. I've even heard proponents refer to it as a candidate to be the "silicon of quantum computing". In any case, our work has consisted of mapping out the low energy states that electrons can occupy, as a basis for understanding how superconductivity came to manifest in such a unique way. The research was conducted in close collaboration with scientists at the University of Maryland, Rutgers, and the Lawrence Berkeley Lab:

Low energy band structure and symmetries of UTe₂ from angle resolved photoemission spectroscopy, Lin Miao, Shouzheng Liu, Yishuai Xu, Erica C. Kotta, Chang-Jong Kang, Sheng Ran, Johnpierre Paglione, Gabriel Kotliar, Nicholas P. Butch, Jonathan D. Denlinger, and L. Andrew Wray, Physical Review Letters 124, 076401 (2020). Other press: Berkeley Lab ALS News

[Jan, 2020] Meeting the demographics inside our samples: Electrons at the surface of a topological insulator are strange - they move 'masslessly', refuse to be trapped by physical barriers, and respond bizarrely to a wide range perturbations. Now, if only we could precisely manipulate their environment to explore all of this!

Enter spectromicroscopy, a technique in which the beam used for spectroscopic measurement is focused down to a tiny micron- or nanometer-scale focal point. In this study, a ~10 micron beam spot at the MAESTRO beamline (Berkeley Lab ALS) was used to map the surface of a mildly disordered topological insulator, looking at the behavior of surface electrons in 961 independent surface regions. Applying big data analysis techniques to this measurement enables us to see each surface region as a distinct 'natural experiment' encoding the response of the surface electrons to a unique setting. It isn't control per se, but very similar with a large enough data set. The result is an unprecedented view of the variability and quantum hybridization of electronic states at a topological insulator surface.

Samples were developed by the Checkelsky group (MIT), and the investigation is supported by scanning tunneling microscopy zoom-ins (Wu group, Rutgers) that resolve atomic-scale disorder:

Spectromicroscopic measurement of surface and bulk band structure interplay in a disordered topological insulator, Erica Kotta, Lin Miao, Yishuai Xu, S. Alexander Breitweiser, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Wenhan Zhang, Weida Wu, Takehito Suzuki, Joseph Checkelsky, and L. Andrew Wray, Nature Physics DOI:10.1038/s41567-019-0759-2 (2020). Selected news highlights: Science Daily, Phys.org

[July, 2019] Hybridizing the unicorn: Topological quantum states tend to be found in otherwise innocuous semiconductors or semimetals, and represent a fork from the historical focus on strongly correlated electron systems (such as rare earth and transition metal oxides) in the quest for next generation electronics. A striking exception - a 'unicorn outlier', if you will - has been found in the compound SmB₆, which seems to be both strongly correlated and a topological insulator.

In this work, we've shown that unconventional electronic topology can persist even when SmB6 is heavily alloyed with magnetic elements, as Sm_0.7Ce_0.3B₆ or Sm_0.8Eu_0.2B₆. These systems behave very differently from the undoped compound, and Sm_0.8Eu_0.2B₆ is even magnetic. Charge carriers from the doped atoms cause the samarium sublattice to be transformed to a greater degree than one would expect. Not only does this demonstrate the emergence of topological surface physics in new physical regimes, but the phenomenology observed within these modified systems is suggestive of a unifying mechanism for surface state emergence in strongly correlated systems:

Robust coherence phenomena and surface states in magnetically alloyed SmB₆, Lin Miao, Chul-Hee Min, Yishuai Xu, Erica C. Kotta, Rourav Basak, M. S. Song, B. Y. Kang, B. K. Cho, K. Kißner, F. Reinert, Turgut Yilmaz, Elio Vescovo, Yi-De Chuang, Jonathan D. Denlinger, and L. Andrew Wray, accepted for publication in Physical Review Letters (2021).

[July, 2019] Surprises in the deep: Our collaborator Pegor Aynajian at Binghamton university has led a fascinating investigation of low temperature coherence phenomena in USb₂, which shows several unusual phenomena, including what appears to be an "orbital selective" Kondo lattice that helps Kondo physics persist in a magnetic environment. Our paper from earlier this year speculated that the "moderate Hundness" and singlet-based magnetism USb₂ would lead to surprises at low temperature, possibly including Kondo-like coherence phenomena. These measurements show that the low temperature regime truly does not disappoint!

Orbital-selective Kondo-lattice and enigmatic f-electrons emerging from inside the antiferromagnetic phase of a heavy fermion, Ioannis Giannakis, Justin Leshen, Mariam Kavai, Sheng Ran, Chang-Jong Kang, Shanta R. Saha, Y. Zhao, Z. Xu, J. W. Lynn, Lin Miao, L. Andrew Wray, Gabriel Kotliar, Nicholas P. Butch, and Pegor Aynajian, Science Advances 5, eaaw9061 (2019).

[May, 2019] When in doubt, take a Fourier transform: Spectroscopists often take second derivatives of experimental data, as a trick to sharpen poorly resolved features. It's an easy approach to justify, but there's no reason to think that it's the best way to clean up data. In this paper, we show that the second derivative approach makes sense on a whole new level - and becomes easy to improve upon - whey you look at its action in Fourier space. When it works well, the "2nd derivative" is really just a band pass filter selecting for desired Fourier space regions. This perspective extends beautifully to the higher dimensional data sets we get with ARPES, to improve the resolution of features like the Dirac cone of graphene (thanks to NSLS-II ESM for spectromicroscopy measurements!):

Second derivative analysis and alternative data filters for multi-dimensional spectroscopies: a Fourier-space perspective, R.-J. Li, X.-N. Zhang, L. Miao, L. Stewart, E. Kotta, D. Qian, K. Kaznatcheev, J. T. Sadowski, E. Vescovo, A. Alharbi, T. Wu, T. Taniguchi, K. Watanabe, D. Shahrjerdi, and L. Andrew Wray, Journal of Electron Spectroscopy and Related Phenomena DOI:10.1016/j.elspec.2019.05.001 (2019).

[May, 2019] Dissecting Hamiltonians: Spin orbit coupling has been increasingly in the limelight for cuprates. Spin-ARPES measurements last year found a baffling result that the spin orientation of cuprate superconductor electrons can be strongly locked to momentum. However, spin orbit coupling is usually thought of as a perturbative correction to local energetics, and one usually can't see the coupling constant directly within a measurement. Here, we teamed up with a team based at Elettra (Trieste, Italy) to show that resonant inelastic X-ray scattering (RIXS) at the copper M-edge includes prominent features that are very sensitive to 3d spin orbit coupling, and even vanish when it's turned off:

Direct observation of spin-orbit-induced hybridization via resonant inelastic extreme ultraviolet scattering on an edge-sharing cuprate, Marco Malvestuto, Antonio Caretta, Barbara Casarin, Roberta Ciprian, Martina Dell'Angela, Simone Laterza, Yi-De Chuang, Wilfried Wurth, Alexandre Revcolevschi, L. Andrew Wray, Fulvio Parmigiani, Physical Review B 99, 115120 (2019).

[Jan, 2019] A different way to grok magnetism: If you zoom in to look at a typical magnet on the nanometer scale, you'll find that it contains a host of tiny magnetic moments that are locked into alignment with their neighbors. Heat it up until the magnetism goes away, and these little moments will still be there - they'll just be pointing in more-or-less random directions.

However, magnets, like people, tried a lot of new things in the 60's and 70's. One unconventional idea dating to this period is termed a 'singlet ground state magnet'. In this case, the hot non-magnetic phase can look inert, with no (or little) magnetic moment on most of the crystallographic sites. Larger magnetic moments only appear via thermally-induced excitations ("spin excitons"), and a lower temperature magnetic phase can only come about if these large-moment excitations achieve stability by clumping together (in something like a "spin exciton condensate"). Some strong candidates for this sort of physics are known, but they're rather fragile, with magnetic phase transition temperatures on the order of 10 Kelvin or less.

We've recently teamed up with scientists at a number of institutions to show that uranium compounds (and maybe heavy transition metals) provide a much more robust route to this sort of magnetic phenomenon. (the team includes people at LBNL, NIST, U. of Maryland, Rutgers, BNL, Binghamton U., and LLNL! It was particularly nice having Gabi Kotliar here for frequent in-person discussions during his sabbatical.) Our measurements show that a host of bizarre properties recently attributed to the compound USb₂ can be understood by attributing it as the new king of singlet ground state magnets - with a transition temperature of 200 Kelvin! It's more normal sibling compound UBi₂ is also a magnet, but lacks a critical ingredient needed for the singlet ground state (a property called "Hundness"):

High temperature singlet-based magnetism from Hund's rule correlations, Lin Miao, Rourav Basak, Sheng Ran, Yishuai Xu, Erica Kotta, Haowei He, Jonathan D. Denlinger, Yi-De Chuang, Y. Zhao, Z. Xu, J. W. Lynn, J. R. Jeffries, S. R. Saha, Ioannis Giannakis, Pegor Aynajian, Chang-Jong Kang, Yilin Wang, Gabriel Kotliar, Nicholas P. Butch, and L. Andrew Wray, Nature Communications 10, 644 (2019). Selected news highlights: Science Daily, IEEE Spectrum, New Scientist; Also featured as a university research highlight on the DOE Office of Science homepage (see Feb. 12 archive)

[Dec, 2018] You say you want a revo-lution: Mobile quantum mechanical particles are often described as 'wavepackets'. A wavepacket is a small moving probability cloud that responds to forces like a classical particle (see the Ehrenfest theorem), but has a finite size due to the quantum mechanical uncertainty relations.

Here's where things get weird. Certain classes of wavepackets patterned on Bessel functions have been said to revolve around a central axis (implying acceleration) in the absence of external forces, something that simply isn't supposed to happen. Also, if you put them in a quantum well, these Bessel waves can exhibit center of mass motion that runs in the opposite direction to what you would expect from their angular momentum.

Well, we're here to tell you that everything is OK. This collaborative paper led by the Grier Lab (NYU CSMR) shows that the free particles only superficially look like they're running in circles. If you integrate over an entire wavepacket, it's actually undergoing uniform translation. Meanwhile, we find that the center of mass motion of trapped particles is governed by a force from their confining potential, just as one would expect. As to the backwards trajectories - they're bizarre and interesting, and possibly even significant for light-matter interactions in the QED limit - but they don't break any physical rules:

Classically accelerating solenoidal wave packets in two dimensions, Argha Mondal, Yishuai Xu, L. Andrew Wray, and David G. Grier, Physical Review A 98, 060101(R) (2018), Rapid Communication.

[May, 2018] New particles from quantum topology! Topologically ordered quantum materials exhibit a property known as “bulk boundary correspondence”, in which new quantum states appear at locations where the material structure is strongly disrupted. These two new papers represent our ongoing projects to discover new quasiparticle species that emerge from the interplay between this effect and different kinds of nanoscale structure at a material surface.

Paper #1: A collaboration with groups at Rutgers, MIT, Purdue and LBNL follows on our theory and STM work from last year to provide the first experimental images of how light-like topological surface states are reshaped in the presence of strong disorder. Buried in these slightly blurry data is believed to be the first known electronic state that moves like a particle, but emerges from a quantum basis that (almost) completely lacks translational symmetry:

Observation of a Topological Insulator Dirac Cone Reshaped by Non-magnetic Impurity Resonance, Lin Miao, Yishuai Xu, Wenhan Zhang, Daniel Older, S. Alexander Breitweiser, Erica Kotta, Haowei He, Takehito Suzuki, Jonathan D. Denlinger, Rudro R. Biswas, Joseph Checkelsky, Weida Wu, and L. Andrew Wray, Nature Partner Journals: Quantum Materials 3, 29 (2018).

Paper #2: It’s been noticed in the last few years that 1D perturbations to topological insulator surfaces tend to create 1D ‘edge mode’ electronic states that connect to the topological insulator surface Dirac point. In this close collaboration with the group of Rudro Biswas at Purdue, we explore the phenomenology of these modes, and show that their connection topology shares the same symmetry protections as the topological surface states themselves:

Connection topology of step edge state bands at the surface of a three dimensional topological insulator, Yishuai Xu, Guodong Jiang, Janet Chiu, Lin Miao, Erica Kotta, Yutan Zhang, Rudro R Biswas, and L. Andrew Wray, New Journal of Physics 20, 073014 (2018)

[Mar, 2018] Bringing a +1 to the party: Prussian blue analogue-based batteries are a leading electrode material class for grid-scale electrical storage, in part because they work well with sodium ions, which don’t suffer the same resource limitations as lithium. Unfortunately, widespread adoption has been held back by the relative weakness of partnering anodes. In this work, we explore the physics and chemistry of an exciting new PBA-based anode developed by Natron Energy, which achieves a winning combination of improved redox potential, high rate performance, and low synthesis cost. Our RIXS and XAS analysis shows that the large redox potential comes from a rare 1+ valence state of Mn that had been speculated to occur in these materials for nearly a century, but was not definitively observed in prior work. The Mn 1+ valence state also drives some unusual correlated electron physics that seems to be a large factor driving the high rate performance. (more on this later!)

A summary of this work was recently presented in testimony to the United States House of Representatives Committee on Science, Space, and Technology, on the subject of “NATIONAL LABORATORIES: WORLD-LEADING INNOVATION IN SCIENCE” (see pages 22-23).

Monovalent manganese based anodes and co-solvent electrolyte for stable low-cost high-rate sodium-ion batteries, Ali Firouzi, Ruimin Qiao, Shahrokh Motallebi, Christian Valencia, Hannah Israel, Mai Fujimoto, L. Andrew Wray, Yi-De Chuang, Wanli Yang, and Colin Wessells, Nature Communications 9, 861 (2018). Selected news highlights: Science Daily, Berkeley Lab News, PRWeb

[Nov, 2017]  Un-mixing water and electricity: Cutting edge Prussian Blue analogue (PBA)-based batteries offer a promising route to achieving robust and cost-effective energy storage inside electrical grids. The PBA electrodes are generally conceptualized in terms of their crystallographic formula unit, but also pack in a large density of interstitial water left over from aqueous synthesis. In this work, scientists from Stanford, LBNL, Novasis, NYU, Tsinghua, PKU, and Shenyang NEU, have teamed up to ask the question: what happens when these water molecules are removed from the electrodes?

One answer is that you can get a better battery. Our resonant X-ray analysis shows that the microscopic details of charge storage are dramatically altered. In particular, different transition metal cation lattice sites begin to discharge simultaneously, in the same flat voltage plateau, instead of discharging in a series of staged voltage drops. The reasons for this beneficial effect have to do with an interplay of water molecules with the crystal field, and with electronic itinerancy:

Modification of Transition-Metal Redox by Interstitial Water in Hexacyanometallate Electrodes for Sodium-Ion Batteries, Jinpeng Wu, Jie Song, Kehua Dai, Zengqing Zhuo, L. Andrew Wray, Gao Liu, Zhi-xun Shen, Rong Zeng, Yuhao Lu, Wanli Yang, J. Am. Chem. Soc. 139, 18358-18364 (2017).

[Oct, 2017]  Resonant photons as an atom-sized tool set: Several years ago, we explored the resonant inelastic X-ray scattering (RIXS) spectrum of a large spin antiferromagnet (CoO) with unprecedented energy resolution, and discovered a new phenomenon called "pseudo anti-Stokes" scattering. Here, in a collaboration of scientists at NYU, Elettra, the ALS, and DESY, we've had a chance to revisit the same types of excitation within a material that has a simpler cubic structure (KCoF3). These measurements provide a wonderful illustration of ways in which, with good enough energy resolution, resonant excitations can be used as atom-sized tools for interacting with the nanometer-scale structure of atoms and electron spins. For example, excitations that orient the charge cloud around an atom to point towards neighboring atoms are seen to excite strong lattice vibrations that ripple out into the crystal:

High resolution resonant inelastic EUV scattering from orbital and spin excitations in a Heisenberg antiferromagnet, Antonio Caretta, Martina Dell’Angela, Yi-De Chuang, Roman V. Pisarev, Alexandra M. Kalashnikova, Davide Bossini, Wilfried Wurth, Fulvio Parmigiani, Surge Wexler, L. Andrew Wray, and Marco Malvestuto, Physical Review B 96, 184420 (2017).

[July, 2017]  Dazzling scattering from filled fullerenes: Gadolinium is a large-spin rare earth element that can show up everywhere from iron arsenide superconductors and topologically ordered materials, to MRI contrast agents and imitation diamonds. Faux jewelry aside, the real gem for an X-ray spectroscopist is the N_4/5 (~140eV) 4d-to-4f resonance. Scattering X-rays inelastically at N_4/5 reveals intense and well-differentiated ff electronic transitions that present a detailed fingerprint of the atomic-scale environment for electrons. In this international collaboration, we worked closely with scientists at Tamkang University (Taiwan) and the Advanced Light Source to decipher the information hidden in this resonance, and better understand why gadolinium embedded in C₈₀ fullerenes can result in highly effective MRI contrast agents:

The key energy scales of Gd-based metallofullerene determined by resonant inelastic x-ray scattering spectroscopy, Yu-Cheng Shao, L. Andrew Wray, Shih-Wen Huang, Yi-Sheng Liu, Wang Song, Shangfeng Yang, Yi-De Chuang, Jinghua Guo, and Way-Faung Pong, Scientific Reports 7, 8125 (2017).

[July, 2017]  Where's the charge? Ask the light brigade! This collaboration with Wanli Yang (ALS) and scientists at DuPont uses X-ray resonance to find the atomic-scale underpinnings of a trade-off between energy density and longevity in the next generation electrode candidate LiNi_0.5Mn_0.3Co_0.2O₂ (LNMCO). Stable lower-voltage cycling is found to involve discrete transitions from Ni(II)↔Ni(III)↔Ni(IV). Higher voltages drive a continuous shift in the chemical potential of a more itinerant oxygen/Co(III/IV) electron system:

Transition-Metal Redox Evolution in LiNi_0.5Mn_0.3Co_0.2O₂ Electrodes at High Potentials, Ruimin Qiao, Jun Liu, Kostantinos D. Kourtakis, Mark G. Roelofs, Darin L. Peterson, James P. Duff, Dean T. Deibler, L. Andrew Wray, and Wanli Yang, Journal of Power Sources 360, 294-300 (2017).

[Apr, 2017]  The birth of the Center for Quantum Phenomena: We will host the Frontiers in Emergent Quantum Phenomena symposium from June 28-30, featuring more than 25 very exciting speakers. The EQP symposium will provide a forum to explore what the future holds for the realization, measurement and manipulation of new phenomena derived from quantum matter. The symposium is made possible by generous support from the Gordon and Betty Moore Foundation, IBM, the NYU College of Arts and Sciences, and Vinci Technologies. Be sure to register early, and if you are a student or postdoc, don't forget to submit the brief application form to request reimbursement for lodging fees!

The EQP symposium is an inaugural event for the Center for Quantum Phenomena, and coincides with the opening of the newly renovated CQP facilities. It will herald several exciting developments for the Center, such as the installation of a powerful and unique multi-technique system for materials synthesis and characterization, with major components from Vinci Technologies, Scienta-Omicron, and Kurt Lesker. We would also like to acknowledge M. Zahid Hasan (Princeton), Andy Millis (Columbia), and Subir Sachdev (Harvard), who helped to set the tone for CQP by giving excellent and inspiring presentations at the Frontiers of Quantum Matter provost's retreat event one year ago.

[Apr, 2017]  Spins proliferate, atop a knotted state: SmB₆ is a topological insulator in which electrons passing through the samarium (Sm) sites interact very strongly with one another. This is unusual and turns out to be significant, because many of the exciting device applications and emergent physical phenomena proposed for topological insulators require that topological order be combined with material phases that are caused by electron-electron interactions, like superconductivity and magnetism. In fact, recent work has suggested that the surface of SmB₆ can be intrinsically magnetic, even though the bulk is not. In this NYU-led collaboration with partners at the ALS, GIST, and RCCM, we used resonant X-ray spectroscopies (XAS and RIXS) to establish new methods for separately monitoring the physics of samarium atoms with large and small magnetic moments. These approaches reveal that a previously identified aging effect causes large magnetic moments to proliferate over time on cleaved SmB₆ surfaces. Even in ultra-high vacuum, the data show that this effect can handily explain a transition to magnetism. Exposure to air takes the aging process even further, giving a surface that is almost fully saturated with large magnetic moments:

Irreversible proliferation of magnetic moments at cleaved surfaces of the topological Kondo insulator SmB₆, Haowei He, Lin Miao, Edwin Augustin, Janet Chiu, Surge Wexler, S. Alexander Breitweiser, Boyoun Kang, B. K. Cho, Chul-Hee Min, Friedrich Reinert, Yi-De Chuang, Jonathan Denlinger, and L. Andrew Wray, Physical Review B 95, 195126 (2017).

[Feb, 2017]  A spectrograph for the masses: A new resonant inelastic X-ray scattering (RIXS) spectrograph developed at the Lawrence Berkeley National Lab has a modular design, making it possible to selectively configure for high throughput or high energy resolution applications. High throughput is great for time-limited measurements on reactive battery electrodes, while high resolution can be a prerequisite for cleanly measuring collective excitations like magnons. These instruments are suitable for more than one beamline, greatly pushing down the effective cost per unit. In fact, several have already been built and installed at multi-spectrograph endstations that our group is closely involved with (iRIXS and qRIXS at the Advanced Light Source):

Modular soft X-ray spectrometer for applications in energy sciences and quantum materials, Yi-De Chuang, YuCheng Shao, Alejandro Cruz, Kelly Hanzel, Adam Brown, Alex Frano, Ruimin Qiao, Brian Smith, Edward Domning, Shih-Wen Huang, L. Andrew Wray, Wei-Sheng Lee, Zhi-Xun Shen, Thomas Devereaux, Jaw-Wern Chiou, Way-Faung Pong, Valeriy Yashchuk, Eric Gullikson, Ruben Reininger, Wanli Yang, Jinghua Guo, Robert Duarte, and Zahid Hussain, Rev. Sci. Instrum. 88, 013110 (2017).

[Dec, 2016]  Go (Ultra)Violets! Being a spectroscopist means constantly questioning the physical meaning behind scattering events. Our group has a particular interest in how light-matter interactions evolve in the “vacuum ultraviolet” (VUV) crossover region between the ultraviolet and X-ray regimes. New light source technologies make this an up-and-coming frontier for spectroscopy. There are two conflicting characterizations of photon resonance in the VUV, relative to X-rays: (1) that VUV photons are a much gentler probe, because atomic VUV dipole transitions don’t directly change the atomic radial charge density distributions very much; and (2) that VUV photons are a more violent probe, because of the large multipolar interactions that shallow core holes have with valence electrons. These exploratory investigations suggest that either answer can be largely correct, depending on the precise measurement performed, and what one wants to learn from it:

Charge transfer excitations in VUV and soft X-ray resonant scattering spectroscopies, Edwin Augustin, Haowei He, Lin Miao, Yi-De Chuang, Zahid Hussain, and L. Andrew Wray, Journal of Electron Spectroscopy and Related Phenomena, DOI: 10.1016/j.elspec.2016.12.004 (2016).

[Oct, 2016]  Embracing the unstoppable: A 2200 year old Chinese proverb tells the story of a hyperbolic salesman who claims to have an unbreakable shield and an unstoppable spear, but can’t explain what will happen if you strike one with the other. As it turns out, electrons found at the surface of topological insulator (TI) materials are also unstoppable - they’re immune to localization by static, non-magnetic disorder (Anderson localization). Even so, pronounced resonance states are known to form around lattice defects at a TI surface, and superficially resemble the highly localized impurity states that form around conventional semiconductor dopant atoms. So what actually happens when you liberally dose such a surface with defects? Our research suggests that the localization-resistant TI impurity states can collectively give rise to a new conducting electron gas, representing an exciting (and very unusual) mechanism for band structure engineering. Strangely enough, the Chinese paradox can also be resolved if you allow the shield to undergo a topological phase transition. This project was led by the Wray group together with Rudro Biswas at Purdue University, and makes use of high quality STM data from the Kapitulnik group (Stanford):

Disorder enabled band structure engineering of a topological insulator surface, Yishuai Xu, Janet Chiu, Lin Miao, Haowei He, Zhanybek Alpichshev, A. Kapitulnik, Rudro R. Biswas, and L. Andrew Wray, Nature Communications 8, 14081 (2017).

[Oct, 2016]  A new spectroscopic window into battery science: We've contributed to a review of X-ray absorption spectroscopy as a probe of transition metal electronic states inside of battery electrodes. This approach can be applied at any point on a charge cycling curve, and provides a very powerful way to identify electronic symmetries both in the electrode bulk, and in the fascinating solid-electrolyte interphase (SEI) region:

Quantitative probe of the transition metal redox in battery electrodes through soft x-ray absorption spectroscopy, Qinghao Li, Ruimin Qiao, L. Andrew Wray, Jun Chen, Zengqing Zhuo, Yan-Xue Chen, Shi-shen Yan, Feng Pan, Zahid Hussain, and Wanli Yang, J. Phys. D 49, 413003 (2016), Review Article.

[July, 2016]  The magnetic middle-man: Multiferroics are materials with strongly coupled degrees of freedom in which electric fields (or lattice strain) can influence magnetic properties, and vice versa. This makes them great candidates for a wide range of applications in spintronics, sensor technology and digital information storage. A collaboration involving scientists at LBL, MAX IV and National Chiao Tung University and NYU has carried out a very nice detailed study comparing resonant elastic scattering on ligand oxygen (O K-edge) with the better known scattering signal from large-moment rare earth and transition metal sites in multiferroic DyMnO₃ and TbMnO₃. Understanding the temperature dependence of electronic states on oxygen provides an important piece of information for disentangling how multiple magnetic interaction channels collectively give rise to the rich phase diagrams of these materials:

Prominent role of oxygen in the multiferroicity of DyMnO₃ and TbMnO₃: A resonant soft x-ray scattering spectroscopy study, S. W. Huang, J. M. Lee, Horng-Tay Jeng, YuCheng Shao, L. Andrew Wray, J. M. Chen, R. Qiao, W. L. Yang, Y. Cao, J.-Y Lin, R. W. Schoenlein, and Y.-D. Chuang, Physical Review B 94, 035145 (2016).

[March, 2016]  Snapping links in an entangled insulator: Vanadium dioxide undergoes a dramatic transition from metallic to insulating conductive behavior close to room temperature. The transition is accompanied by symmetry breaking within the crystal lattice, but this may in part be a red herring, as studies have suggested that non-symmetry-breaking physical changes (growing Mott correlations and Kondo-like singlet coherence) are also fundamental to the loss of metallicity. In a broad collaboration led by the Wray group and Gray group (Temple University), we have now used the RIXS technique to perform the first direct measurement of entangled spin states that are the principle microscopic constituent for these classes of non-symmetry-breaking physics:

Measurement of collective excitations in VO₂ by resonant inelastic X-ray scattering, Haowei He, A. X. Gray, P. Granitzka, J. W. Jeong, N. P. Aetukuri, R. Kukreja, Lin Miao, S. Alexander Breitweiser, J. Wu, Y. B. Huang, P. Olalde-Velasco, J. Pelliciari, W. F. Schlotter, E. Arenholz, T. Schmitt, M. G. Samant, S. S. P. Parkin, H. A. Dürr, and L. Andrew Wray, Physical Review B 94, 161119(R) (2016), Rapid Communication.

[Jan, 2016]  Jammin’ the Prussian blues: The last year has seem some exciting progress towards potential low cost and eco-friendly battery chemistries based on Prussian blue analogue (PBA) electrodes. Our role has been to identify how electronic states in and around transition metal atoms evolve as the electrodes are cycled. For example, one thing seen in the JMCA paper listed below is that cobalt atoms can flip from low spin to high spin during operation, causing the atom to balloon in size and push on its neighbors. More to come:

Manganese-Cobalt Hexacyanoferrate Cathodes for Sodium-ion Batteries, Mauro Pasta, Richard Y. Wang, Riccardo Ruffo, Ruimin Qiao, Hyun-Wook Lee, Badri Shyam, Minghua Guo, Yayu Wang, L. Andrew Wray, Wanli Yang, Michael F. Toney, and Yi Cui, Journal of Materials Chemistry A 4, 4211 (2016).

[Nov, 2015]  Powerful redox, one electron at a time: The cathode compound LiNi_0.5Mn_1.5O₄ allows high operating voltages (4.7V!) and is a promising candidate for application in next generation Li-ion batteries. However, the microscopic picture of charge cycling has been incomplete. It’s been clear that the cycling endpoints involve a valence change of 2 (4+ to 2+) centered on the nickel site, but do these two electrons come all at once? In this collaboration with the the Berkeley Lab (LBNL) and General Motors, we find that cycling half way does indeed populate an intermediate 3+ valence state of the oxygen coordinated Ni complex. The measurements also reveal an interesting contrast between inert Ni and Mn atoms on the electrode surface and electrochemically active sites in the bulk. The study is another exciting development in the application of resonant X-ray spectroscopies to measure functional electronic states inside battery electrodes:

Direct Experimental Probe of the Ni(II)/Ni(III)/Ni(IV) Redox Evolution in LiNi_0.5Mn_1.5O₄ Electrodes, Ruimin Qiao, L. Andrew Wray, Jung-Hyun Kim, Nicholas Pieczonka, Stephen Harris, and Wanli Yang, J. of Phys. Chem. C 119, 27228-27233 (2015).

[Oct, 2015]  Ferromagnetism inside a d-wave superconductor: Many of the most interesting quantum materials known today lie at the threshold between magnetic and superconducting ground states. It was recently discovered that the cuprate superconductor YBa₂Cu₃O_7-x (YBCO) grown on top of ferromagnetic La_0.7Ca_0.3MnO₃ (LCMO) can lead to a magnetic exchange field overlapping with superconductivity, a scenario that may give rise to new physics such as the Fulde-Ferrel-Larkin-Ovchinnikov state (FFLO, predicted in 1964). However, while overlap at the interface provides a fascinating quantum environment, it is unclear if it represents superimposed proximity effects, or a true many-body ground state characterized by the interplay of magnetism and d-wave superconductivity. In this study, resonant X-ray scattering from the bulk of a 15nm thick YBCO layer on LCMO is analyzed to show that ferromagnetism from the LCMO substrate does indeed appear to penetrate into the superconducting YBCO bulk:

Selective interlayer ferromagnetic coupling between the Cu spins in YBa₂Cu₃O_7-x grown on top of La_0.7Ca_0.3MnO₃, Shih-Wen Huang, L. Andrew Wray, Horng-Tay Jeng, V. T. Tra, J. M. Lee, M. C. Langner, J. M. Chen, S. Roy, Y. H. Chu, R. W. Schoenlein, Yi-De Chuang, and J.-Y. Lin, Scientific Reports 5, 16690 (2015). (Nature Publishing Group)

[May, 2015]  The atomic basis of hidden order: The low temperature “hidden order” state of URu₂Si₂ has been a subject of intense speculation since the 1980s, and is thought to represent an as yet undetermined many-body quantum state not realized by other known materials. A central challenge for efforts to pinpoint the nature of this hidden state has been that basic properties of the atomic scale wavefunction on uranium are very difficult to identify. Recent measurements by the Wray Lab have zeroed in on this problem by establishing a strong connection between the excitation spectrum of uranium electrons in URu₂Si₂ and the spectrum expected for a strongly correlated 5f² basis that is modified by electronic itinerancy:

Spectroscopic determination of the atomic f-electron symmetry underlying hidden order in URu₂Si₂, L. Andrew Wray, Jonathan Denlinger, Shih-Wen Huang, Haowei He, Nicholas P. Butch, M. Brian Maple, Zahid Hussain, and Yi-De Chuang, Phys. Rev. Lett. 114, 236401 (2015).

[May, 2015]  Light-matter interactions: Performing resonant inelastic scattering (RIXS) with shallow core holes accessed in the extreme ultraviolet (EUV) can provide record breaking energy resolution, but often yields qualitatively different spectra than similar measurements performed with higher energy photons. In this paper, experimental data and multiplet-based numerical simulations for the M-edges of Co-, Ni- and Cu-based Mott insulators are used to review the properties that distinguish EUV RIXS from more commonly performed higher energy measurements:

Extending resonant inelastic X-ray scattering to the extreme ultraviolet, L. Andrew Wray, Shih-Wen Huang, Ignace Jarrige, Kazuhiko Ikeuchi, Kenji Ishii, Jia Li, Zi Qiang Qiu, Zahid Hussain and Yi-De Chuang, Frontiers in Physics 3, 32 (2015).

[Jan, 2015]  Ultrafast dynamics: When light scatters from a material it can undergo changes in both frequency and phase. Standard frequency-resolving measurement techniques are phase-blind, and phase-resolving measurements that work on-resonance are limited to the elastic (constant frequency) component of scattered light (with notable exceptions for non-resonant scattering). In this paper, two classes of interference patterns are identified in resonant inelastic X-ray scattering, and used to partially recover phase information and sub-femtosecond time dynamics that underly the creation of elementary excitations in Mott insulators:

Experimental signatures of phase interference and subfemtosecond time dynamics on the incident energy axis of resonant inelastic x-ray scattering, L. Andrew Wray, Shih-Wen Huang, Yuqi Xia, M. Zahid Hasan, Charles Mathy, Hiroshi Eisaki, Zahid Hussain, and Yi-De Chuang, Phys. Rev. B 91, 035131 (2015).

[Jan, 2015]  Cheaper, longer lived batteries: Sodium ion batteries have the potential to offer high density energy storage at a lower cost than lithium ion batteries, and may ultimately help even out highs and lows in the electrical grid. In this collaboration led by Sharp Laboratories of America, numerical simulations performed by the Wray Lab and X-ray absorption (XAS) measurements by the group of Wanli Yang (LBNL) contributed an atomically resolved picture of how iron redox states cycle on inequivalent chemical sites in a new air-stable sodium ion battery cathode (R-Na1.92Fe[Fe(CN)6]):

Rhombohedral Prussian White as Cathode for Rechargeable Sodium-Ion Batteries, Long Wang, Jie Song, Ruimin Qiao, L. Andrew Wray, Muhammed A. Hossain, Yi-De Chuang, Wanli Yang, Yuhao Lu, David Evans, Jong-Jan Lee, Sean Vail, Xin Zhao, Motoaki Nishijima, Seizoh Kakimoto, and John B. Goodenough, J. Am. Chem. Soc. 137, 2548 (2015).

L. Andrew Wray Lab
Center for Quantum Phenomena
Department of Physics
New York University