Condensed Matter Seminars

Condensed
Matter Seminars are in Room NPB 2205 Contact: Yasu Takano or Dmitrii Maslov 
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January 8 


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January 15 (No seminar  Martin Luther King Jr. Day) 


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January 17, Wednesday, 4:00 pm (Colloquium. Note special date and time) 


Speaker 
Brian Skinner (MIT) 

Title 
Semimetals unlimited 

Abstract 
Modern electronics is built on semiconductors, whose utility comes from their ability to operate on either side of the conductorinsulator dichotomy. For practical applications, however, semiconductors face certain unavoidable limitations imposed by the physics of Anderson localization and by the disorder introduced through doping. In this talk I discuss whether these same limitations apply to nodal semimetals, which are a novel class of threedimensional materials that have a vanishing density of states (like insulators) but no gap to electronhole excitations (like conductors). I show that, surprisingly, in a certain class of nodal semimetals the electronic mobility can far exceed the bounds that constrain doped semiconductors, becoming divergingly large even with a finite concentration of charged impurities. I then discuss the thermoelectric effect in semimetals, and show that their electronhole symmetry allows for a thermopower that grows without bound under the application of a strong magnetic field. This large thermopower apparently enables the development of devices with recordlarge thermoelectric figure of merit. 

Host 
Dmitrii Maslov 
January 22 (Colloquium. Note special date) 


Speaker 
Yuxuan Wang (U. Illinois UrbanaChampaign) 

Title 
Topological superconductivity from electronic interactions 

Abstract 
Topological superconductors exhibit exotic Majorana modes at the boundaries and vortices, and can provide important applications in quantum computing. In addition to usual path by “mimicking” topological superconductors with conventional superconducting heterostructures, we show that intrinsic topological superconductivity can also be naturally realized through electronic interactions. Specifically, we analyze the topological superconducting state that emerges near the onset of an oddparity electronic order. We show that the system has an enhanced U(1)xU(1) symmetry and a rich phase diagram. We address the relevance of our results with recent experiments in Cd2Ce2O7 and halfHeusler superconductors. We discuss other examples in which simple interactions induce exotic topological superconducting phases, and argue that important progress can be made at the intersection of topological superconductivity and unconventional superconductivity. 

Host 
David Tanner 
January 29, 1:55 pm, Rm. 2165 (Colloquium. Note special time and place) 


Speaker 
Chia Wei Hsu (Yale University) 

Title 
New frontiers of electromagnetic phenomena at the nanoscale 

Abstract 
Optics and photonics today enjoy unprecedented freedom. The ability to synthesize arbitrary light fields (through wavefront shaping) and the ability to design material structures at the subwavelength scale (through nanofabrication) enable us to use optics to realize phenomena that could only be imagined in the past. In this talk, I will present several experiments and related theory that demonstrate exciting new phenomena which were previously inaccessible. Conventional textbook wisdom is that waves cannot be perfectly confined within the continuum spectrum of an open structure. In 1929, von Neumann and Wigner proposed an intriguing exception that is now called a “bound state in the continuum” [1], but their proposal cannot be realized in matter waves. I will show that tailoring the band structure of a photonic crystal slab enables nontrivial bound states in the continuum to be realized for the first time [2]. These states manifest themselves as polarization vortices in momentum space, protected by topologically conserved “charges” [3]. Our control over radiation loss enables us to probe another unique phenomenon: nonHermitian “exceptional points,” where two eigenstates of a system coalesce into one [4,5]. The exceptional points generate distinct doubleRiemannsheet topologies in the complexvalued band structures of photonic crystals, and the coalescence of states leads to unique light–matter interactions. Strong disorder in naturally occurring lightscattering media allows us to study mesoscopic physics in a new arena. With wavefront shaping, we can control the propagation of light even in unknown disordered structures, violating the standard diffusion model. I will show that longrange mesoscopic correlations between the multiply scattered waves, which are important for Anderson localization and Universal Conductance Fluctuations of electron waves, also facilitate the control of photon transport at a global scale [6,7]. A quantitative description of these effects in optics is provided by a “filtered” random matrix theory [7]. This correlationenhanced coherent control shows promise for applications in biomedical imaging and optical communications. [1] C. W. Hsu, B. Zhen et al., Nature Reviews Materials 1, 16048 (2016). [2] C. W. Hsu, B. Zhen et al., Nature 499, 188 (2013). [3] B. Zhen, C. W. Hsu et al., Phys. Rev. Lett. 113, 257401 (2014). [4] B. Zhen, C. W. Hsu et al., Nature 525, 354 (2015). [5] H. Zhou et al., Science, eaap9859 (2018). [6] C. W. Hsu et al., Phys. Rev. Lett. 115, 223901 (2015). [7] C. W. Hsu et al., Nature Physics 13, 497 (2017).


Host 
Yoonseok Lee 
January 29 (Colloquium. Note special date) 


Speaker 
Arijeet Pal (U. Oxford) 

Title 
Entanglement in outofequilibrium quantum matter 

Abstract 
Thermodynamics, one of the most universal theories in all of
science, applies to an enormous range of physical systems, from
distant stars in the universe to electrons in materials on earth.
The postulates of thermodynamics are underpinned by an
assumption, known as ergodicity, which states that the
microscopic pieces of a physical system must lose memory of their
initial state over time. In recent years, it was discovered that
manybody localization (MBL) in quantum systems realizes a phase
of matter which violates ergodicity and retains memory for
infinitely long times, showing us that there are exceptions to
the principles of thermodynamics. This phenomenon has no
classical analogue and is intimately tied to quantum entanglement
and its propagation. 

Host 
Chris Stanton 
February 1, Thursday (Colloquium. Note special date) 


Speaker 
Chunhui Du (Harvard University) 

Title 
Control and local measurement of the spin chemical potential in a magnetic insulator 

Abstract 
In recent decades, a large scientific effort has focused on harnessing spin transport for providing insights into novel materials and lowdissipation information processing. We introduce single spin magnetometry based on nitrogenvacancy (NV) centers in diamond as a new and generic platform to locally probe spin chemical potentials which essentially determine the flow of spin currents. We use this platform to investigate magnons in a magnetic insulator yttrium iron garnet (YIG) on a 100 nanometer length scale. We demonstrate that the local magnon chemical potential can be systematically controlled through both ferromagnetic resonance and electrical spin excitation, which agrees well with the theoretical analysis of the underlying multimagnon processes. Our results open up new possibilities for nanoscale imaging and manipulation of spinrelated phenomena in condensedmatter systems. 

Host 
Andrew Rinzler 
February 5, 2:00 pm, Rm. 2165 (Colloquium. Note special time and place) 


Speaker 
Brian Zhou (U. Chicago) 

Title 
Shaping the Quantum Dynamics of Single Spins in Diamond 

Abstract 
Remarkable progress in the control of atomicscale systems has confirmed the quantum foundation of our world, as well as inspired new opportunities for technological innovation. Due to its spinphoton interface, the nitrogenvacancy (NV) center in diamond – a single electron spin at an atomic defect – presents a premier platform for quantum networks, while its longlived quantum coherence and robust deployability across an extended phase diagram in temperature, field, and pressure make it uniquely suitable for highprecision sensing. In essence, these applications rely on the purposeful control of coherent quantum dynamics. In this talk, I will discuss new tools using resonant optical fields that manipulate single NV center spins with high efficiency and spatial resolution. We engineer ‘superadiabatic’ dynamics to overcome conventional speed limits to adiabatic protocols, thereby hybridizing their robustness with speed. Exploiting geometric aspects of quantum evolution, we demonstrate the robustness of adiabatic Berry phases to noise and the implementation of arbitrary quantum gates through highspeed, nonadiabatic holonomies. I will look ahead to the required groundwork and longterm directions for using NV centers as probes of diverse physical systems. 

Host 
Amlan Biswas 
February 5 (Colloquium. Note special date) 


Speaker 
Thomas Scaffidi (UC Berkeley) 

Title 
Electron hydrodynamics in solidstate physics 

Abstract 
Wolfgang Pauli called solidstate physics "the physics of dirt effects", and this name might appear welldeserved at first sight since transport properties are more often than not set by extrinsic properties, like impurities. In this talk, I will present solidstate systems in which electrons behave like a hydrodynamic fluid, and for which transport properties are instead set by intrinsic properties, like the viscosity. This new regime of transport opens the way for a “viscous electronics”, and provides a new angle to study how quantum mechanics can constrain and/or enrich hydrodynamics. 

Host 
Peter Hirschfeld 
February 7, Wednesday (Colloquium. Note special date) 


Speaker 
Pankaj Jha (Caltech) 

Title 
Nanoengineered materials for quantum technologies 

Abstract 
The first quantum revolution, which occurred at the beginning of the 20th century, gave us the principles to understand how nature works. These principles led to several breakthroughs in, both, fundamental science and developing technologies that guided the social and economic growth of our society. Currently, we are in the midst of a second quantum revolution where we want to leverage our capabilities of controlling and manipulating individual quantum systems to develop technologies with quantumenhanced performances. Profiting from the stateoftheart nanofabrication technologies and nanoelectronics, we developed a hybrid nanophotonic quantum interface based on atomic and atomlike quantum optical systems integrated with materials that are structured on the nanoscale. Such nanoengineered materials exhibit extraordinary lightmanipulation capabilities that enable functionalities not possible with conventional and natural materials. We harnessed these functionalities to control the quantum vacuum on demand, enter a new regime of quantum selforganized phase transition and study manybody light localization far away from thermodynamic equilibrium. Our work opens up an exciting opportunity towards an ultracompact, integrated and scalable architecture for quantum technologies in sensing, metrology and communications. 

Host 
Yoonseok Lee 
February 8, Thursday, 4:00 pm (Colloquium. Note special date and time) 


Speaker 
Dima Pesin (U. Utah) 

Title 
Theory of nonlocal transport in metals with nontrivial band geometry 

Abstract 
I will discuss the topological and geometric aspects of optical and transport phenomena in metals with nontrivial band geometry. Motivated by the chiral anomaly and the chiral magnetic effect in Weyl metals, I will outline the full theory of linearinq contribution to the nonlocal conductivity in a disordered metal. Physical applications of the theory include the natural optical activity of metals and the dynamic chiral magnetic effect, as well as the kinetic magnetoelectric effect/the currentinduced magnetization in metallic systems. The theory is similar in spirit to the one of the anomalous Hall effect in metals, and is directly applicable to the analysis of the typical optical and transport measurements (e.g. Faraday rotation, currentinduced magnetization) in the THz frequency range. 

Host 
Dmitrii Maslov 
February 12, 2:00 pm, Rm. 2165 (Colloquium. Note special date, time, and place) 


Speaker 
Jyoti Katoch (Ohio State University) 

Title 
Quantum phenomena in twodimensional materials driven by atomic scale modifications 

Abstract 
The extreme surface sensitivity of twodimensional (2D) materials provides an unprecedented opportunity to engineer the physical properties of these materials via changes to their surroundings, including substrate, adsorbates, defects, etc. In particular, the decoration of the 2D material with adatoms can be utilized to tailor material properties and induce novel quantum phenomena. In this context, first I will discuss the case of 2D semiconducting transition metal dichalcogenides (TMDs), wherein new electronic phenomena such as tunable bandgaps and strongly bound excitons and trions emerge from strong manybody effects, beyond the spin and valley degrees of freedom induced by spin–orbit coupling and by lattice symmetry. Combining singlelayer TMDs with other 2D materials in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these manybody effects, by means of engineered interlayer interactions. We utilized stateoftheart microfocused angle resolved photoemission spectroscopy (microARPES) and insitu surface doping to manipulate the electronic structure of singlelayer tungsten disulfide (WS_{2}) on hexagonal boron nitride (WS_{2}/hBN). Upon surface doping, we observe an unexpected giant renormalization of the spin–orbit splitting of the singlelayer WS_{2} valence band in addition to a bandgap reduction, which is attributed to the formation of trionic quasiparticles. These findings suggest that the electronic, spintronic, and excitonic properties are widely tunable in 2D TMD/hBN heterostructures, as these are intimately linked to the quasiparticle dynamics of the materials. In another example, insitu low temperature transport measurements are performed in ultrahigh vacuum to systematically investigate resonant scattering by hydrogen adatoms on bilayer graphene. Resonant scatterers produce strong momentum scattering, and can generate much sought after spin galvanic effects in graphene. I will present the observation of two distinct resonant scattering peaks in the electric field dependent bilayer graphene sheet resistance, which evolve as a function of atomic hydrogen dosage. Theoretical calculations show that the observed peaks are due to graphene sublatticedependent resonances, and analysis of the gate dependent resistance curves show that hydrogen atoms preferentially adsorb to the nondimer site over the dimer site. Using this newly developed capability for sublatticeresolved transport spectroscopy, we investigate the thermallyinduced diffusion and desorption of hydrogen adatoms on bilayer graphene and find that with increasing temperature, the hydrogen vacates the dimer site before the nondimer site. This is crucial for magnetic ordering of localized moments in graphene, which is predicted to become ferromagnetic if the localized moments lie on the same sublattice. These experiments open up the pathway to generate robust spin currents in bilayer graphene via spindependent charge carrier scattering and provide an important insight for developing long range spin ordering in adatom decorated graphene layers. 

Host 
Andrew Rinzler 
February 12 (Colloquium. Note special date) 


Speaker 
Laura Fanfarillo (SISSA) 

Title 
Modeling complexity: Theoretical approaches to multiorbital correlated systems 

Abstract 
A fundamental feature of complex systems is the presence of emergent behaviors, that are not properties of the single system components but arise from their interactions and relations. In condensed matter, the collective behavior of electrons in solids can give rise to many different unconventional quantum states of matter of extreme interest for technological applications. Interestingly, many of these unconventional quantum phases arise from the cooperation/competition of different energy scales of the interactions: lowenergy electronic excitations, well described by the socalled Fermi liquid theory, and highenergy interactions, coming from shortrange Coulomb repulsion MottHubbard physics. To model such complex systems, it is fundamental to extract the relevant ingredients that a theoretical description needs to take into account. In my talk I will review the fundamental aspects of the Fermi liquid and Mott picture and I will discuss how such lowenergy and highenergy approaches can be used as a tool to extract the relevant degrees of freedom and to make order from complexity. I will discuss as a concrete example the case of the Ironbased unconventional superconductors. The analysis of electronic correlations in this systems is complicated by the multiorbital character of the electronic band structure close to the Fermi level. Despite the complexity, relevant information can be extracted using both lowenergy and highenergy approaches. A clear result is the emergence of the orbital selectivity as the main feature of the system at all scales. Our results suggest a new scenario in which the key ingredient of the superconducting phase itself comes from a new and unconventional cooperative interplay between low and highenergy scale of electronic interactions. [1] L. Fanfarillo and E. Bascones, Phys. Rev. B 92, 075136 (2015). [2] L. Fanfarillo, G. Giovannetti, M. Capone and E. Bascones, Phys. Rev. B 95, 144511 (2017). [3] L. Fanfarillo, A. Cortijo, B. Valenzuela, Phys. Rev. B 91, 214515 (2015). [4]. L. Fanfarillo, J. Mansart, P. Toulemonde, H. Cercellier, P. Le Fevre, F. Bertran, B. Valenzuela, L. Benfatto, and V. Brouet, Phys. Rev. B 94, 155138 (2016). [5] L. Fanfarillo, L. Benfatto, B. Valenzuela, arXiv: 1706.08953. 

Host 
Peter Hirschfeld 
February 15, Thursday (Colloquium. Note special date) 


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(Canceled) 

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February 19 


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February 26 


Speaker 
(Reserved for Astrophysics Colloquium) 

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March 5 (No Seminar  UF Spring Break Week, APS March Meeting in Los Angeles) 


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March 12 


Speaker 
Rudi Hackl (Walter Meissner Institute) 

Title 
Fingerprints of Cooper pairing in Fe based superconductors 

Abstract 
One of the most vexing problems of research into superconductors or even in the field of condensed matter is the limited understanding of the origin of Cooper pairing in unconventional systems. In basically all superconductors with high transition temperature (except for H_{3}S) magnetism and superconductivity are in close proximity. However, there are no fingerprints of how the electrons are glued together which are similarly conclusive as the isotope effect or the signatures of electronphonon coupling in the tunneling spectra in conventional systems. In our joint theoretical and experimental study of Ba_{1x}K_{x}Fe_{2}As_{2} we show that the observation of excitonic δlike (BardasisSchrieffer) modes inside the gap indicates a hierarchy of pairing tendencies and may afford a window into the pairing potential and, consequently, into the origin of Cooper pairing. We analyze the Raman light scattering data in a phenomenological way and compare the derived quantities with results from simulations using both functional renormalization group (fRG) techniques and spinfluctuation theory in the random phase approximation (RPA). The results of fRG scheme which includes all type of interactions from the outset agrees qualitatively with those from RPA simulations focusing on spin fluctuations alone. This agreement and the semiquantitative description of the experimental results is strong support for spin fluctuations as the main pairing instability in the iron pnictides. 

Host 
Peter Hirschfeld 
March 19 


Speaker 
Saw Wai Hla (Ohio University and Argonne National Laboratory) 

Title 
Quantum molecular machines 

Abstract 
One of the goals of nanotechnology is the development of complex molecular machines that can be operated with atomic level control in a solidstate environment. Most biological molecular machines have sizes from tens of nanometers to a few microns — a range where classical machine concepts hold. However, artificially designed molecular machines can be in the size range down to a few nanometers or less, which is in the range of quantum processes. In this talk, we will present various artificial molecular machines such as molecular motors and linear transport devices such as molecular cars operating in the quantum regime on materials surfaces. Fundamental operations of these synthetic molecular machines are investigated one molecular machine at a time in an atomically clean environment using low temperature scanning tunneling microscopy (STM), tunneling spectroscopy, and molecular manipulation schemes. These investigations reveal how charge and energy transfer takes place within single molecular machines as well as among molecular machines in molecular networks. 

Host 
Yasu Takano 
March 26 


Speaker 
Steven May (Drexel University) 

Title 
Structural approaches for controlling magnetism and metalinsulator transitions in oxide heterostructures 

Abstract 
Complex oxide heterostructures continue to generate significant interest both as a platform for exploring new fundamental materials physics and for their potential in new electronic devices. In this talk, I will present our work on using subtle structural distortions, namely octahedral rotations, to control physical properties of perovskite oxide films and superlattices. I will discuss how epitaxial heterostructures can be used to alter the positions of oxygen atoms to stabilize nonbulklike bond angles and lengths. As a recent example, I’ll describe how rotations can be controlled over length scales of less than 1 nm to achieve confined magnetism through structural “deltadoping” in La_{0.5}Ca_{0.5}MnO_{3}based superlattices. I’ll then describe how octahedral rotations enable the metalinsulator transition in CaFeO_{3} and present result spectroscopic results that reveal how its electronic structure is modified across the transition. 

Host 
Amlan Biswas 
April 2 


Speaker 
Khandker Quader (Kent State University) 

Titl 
Lifshitz, soft mode, and other transitions in 122pnictides under pressure 

Abstract 
Ironarsenic pnictides are correlated electron systems in which underlying Coulomb interactions manifest themselves in striking properties. Sizable amount of theory work dealing with such interactions are rooted in density functional methods, while another large body employs complementary manyparticle techniques on renormalized shorterrange interactions. This talk, displaying almost no equations, is on our work using the first approach. Our results for energy band dispersions, lattice parameters, enthalpy, magnetism, and elastic constants over a wide range of hydrostatic pressure provide a coherent understanding of multiple pressuredriven transitions in A122 pnictides (A= Ca, Sr, Ba): enthalpic transition at pressure P_{H} from the striped AFM orthorhombic (OR) to tetragonal (T) or collapsed tetragonal (cT) phase; a transition at P_{M} > P_{H} from the metastable AFM OR to a T or cT phase; a Lifshitz transition at P_{L}, arising from nontrivial changes in Fermi surface topology. The TcT transition and anomalies in lattice parameters and elastic properties are interpreted as arising from proximity to T=0 Lifshitz transitions. The transition at P_{M} occurs through a loss of elastic stability caused by softening of a shear mode, reminiscent of martensitictype transition, albeit, with pressure. Simultaneously, magnetism and orthorhombicity approach limiting values with an approximately squareroot singularity. 

Host 
David Tanner 
April 3, Tuesday (Colloquium. Note special date) 


Speaker 
Yingjie Zhang (U. Illinois UrbanaChampaign) 

Title 
Electronic transport in artificial solids: percolation, miniband conduction, and beyond 

Abstract 
Nanoscience offers a unique opportunity to design electronic phases from the bottom up, via controlled assembly of nanoscale building blocks. By tuning the onsite energy, hopping integral, and charging energy of the nano units, we can navigate through the phase space of Anderson localization, delocalized minibands and Mott insulation in the assembled artificial solids. The first system I will discuss is quantum dot arrays. We experimentally imaged, for the first time, percolation conduction phenomena which are typical for electronic systems in the regime of strong localization. We further utilized this effect to fabricate a visibleNIR photodetector achieving the world’s highest detectivity. The other experimental system is twodimensional superlattices consisting of graphene on top of arrays of dielectric nanospheres. This hybrid material shows emergent superlattice minibands that can be tuned by controlling the nanoscale deformation of graphene. Carrier confinement modulations further lead to pronounced quantum oscillations which are characteristic of electron correlation and artificial Mott insulating states. Going beyond the above artificial solids, I will discuss our recent efforts in engineering magnetotransport of topological spintronics systems using a “modular design” approach, which enables us to achieve a large magnetoresistance. 

Host 
Sergey Klimenko 
April 5, Thursday (Colloquium. Note special date) 


Speaker 
Yasuo Yoshida (ISSP, U. Tokyo) 

Title 
Atomicscale investigations of stronglycorrelated materials using spin and orbitalresolved scanning tunneling microscopy 

Abstract 
Since the invention of scanning tunneling microscopy (STM), scanning probe microscopy techniques have been developed in many different directions in the last 40 years. Spin and orbitalresolved STM measurements are one of the cutting edges and highly appreciated in condensed matter researches. In this talk, I will present experiments that demonstrate such capabilities of STM and new phenomena which were previously inaccessible. Spinresolved STM visualized not only spin structures of chiral magnets but also spinselective orbital shapes of Co atoms on chiral magnetic templates. Orbitalselective STM revealed the emergence of surfaceassisted orbital ordering in a heavilyinvestigated heavy fermion superconductor. Prospects for future opportunities of spin and orbitalresolved STM and their possible implications for condensed matter physics will be discussed at the end of the talk. 

Host 
Greg Stewart 
April 9 


Speaker 
Jian Kang (NHMFL, Tallahassee) 

Title 
Interplay between nematicity and superconductivity in ironbased superconductors 

Abstract 
The ironbased highTc superconductors exhibit several remarkable features, including the multiorbital character and the ubiquity of the nematic phase. One consequence of the multiorbital Fermi surface is that the spinfluctuation mediated pairing interactions are sensitive to the orbital spectral weight at the Fermi surface, leading to several different possible gap structures, such as nodeless s±, nodal s±, and dwave. Focused on the orbital order induced in the nematic phase, I will discuss how the nematic order can manipulate the properties of SC. Our calculation shows that not only T_{c} is enhanced, but more importantly, the gap structure becomes a mixture of nearly degenerate s and dwave states by increasing the external strain. This mixture of s and d wave pairing channels has been recently found in the superconducting phase of the bulk FeSe, when SC occurs deeply insider the nematic phase. [1] Jian Kang, Alexander F. Kemper, and Rafael M. Fernandes, Phys. Rev. Lett. 113, 217001 (2014). [2] Jian Kang, R. M. Fernandes, and A. V. Chubukov, arXiv:1802.01048. 

Host 
Peter Hirschfeld 
April 16 


Speaker 
Sasha Chernyshev (UC Irvine) 

Title 
Quantum orderbydisorder and excitations in kagomelattice magnets 

Abstract 
I will discuss quantum orderbydisorder effect and will present an evidence that the nonlinear terms in the anisotropic kagomelattice antiferromagnets can yield a rare example of the ground state that is different from the one favored by thermal fluctuations. The corresponding order selection will be shown to be generated by the topologically nontrivial tunneling processes, yielding a new energy scale in the system. I will also discuss the effect of the nonlinear terms in the spectra of the kagomelattice systems and will provide an analysis of the spectral properties of realistic kagomelattice antiferromagnets such as Fejarosite, for which a remarkable wipeout effect for a significant portion of the spectrum should exist due to a resonantlike decay processes involving two flat modes. Recent result concerning the spectrum of the kagomelattice ferromagnets will also be presented. 

Host 
Yasu Takano 
April 17, Tuesday (Colloquium. Note special date) 


Speaker 
Dominique Laroche (TU Delft) 

Title 
Probing the building blocks of topological qubits in superconducting InAs nanowires 

Abstract 
Utilizing the properties of nonAbelian quasiparticles, topological qubits offer an approach towards quantum computing where the information is stored nonlocally, making this an architecture resilient against most decoherence sources. Majorana bound states (MBS) in proximityinduced superconducting nanowires are the prime candidate for the implementation of such topological quantum bits. Thus far, MBS signatures chiefly relied on single electron tunneling measurements [1,2], which lead to decoherence of the quantum information stored in the MBS. Here, I present a novel experimental platform where nanowirebased devices are coupled onchip to microwave detectors and spectrometers [3,4], allowing for measurement of the building blocks of topological qubits in a parity conserving manner. Utilizing this platform, we directly measured a transition from a 2π to a 4πperiodic Josephson radiation in InAs nanowire based Josephson junctions [5], as a function of both magnetic field and chemical potential. This transition is a clear signature of the onset of MBS in the nanowire. We also performed microwave spectroscopy of the fundamental unit of a prospective topological qubit, the Cooper pair transistor, allowing us to directly measure the population of the odd and of the even parity sector in the ground state of this system. [1] Mourik, V. et al., Science 336, 1003 (2012). [2] Zhang, H. et al., Nature 556, 74 (2018). [3] van Woerkom, D. J. et al., Nat. Phys. 13, 876 (2017). [4] van Woerkom, D. J. et al., Phys. Rev. B 96, 094508 (2017). [5] Laroche, D. et al., arXiv:1712.08459 (2017). 

Host 
Neil Sullivan 
April 19, Thursday (Colloquium. Note special date) 


Speaker 
Franziska Weickert (Los Alamos National Laboratory and FSU) 

Title 
BoseEinstein condensation in quantum magnets 

Abstract 
In 1925, Einstein predicted based on Bose’s work on photons that at low temperatures particles with integer spin condense in a coherent state – the BoseEinstein condensate, opening the door to the experimental study of quantum phenomena on a large scale. Quantum magnets are insulating paramagnets exhibiting lowlying energy levels of integer spins separated by a few meV tunable with the application of an external field. As demonstrated 60 years ago, integer spin states can be described in an elegant way as a gas of interacting bosons with hardcore repulsion. Here, the boson concentration is controlled by the applied field, which acts as chemical potential. Uniaxial symmetry of the spin environment is a precondition for the gas of bosons to condense in a phase coherent state (BEC) equivalent to fieldinduced XYantiferromagnetism in spin language. In my talk, I will discuss two examples of quantum magnets with fieldinduced magnetic order. The first one is NiCl_{2}SC(NH_{2})_{2}, also known as DTN, where Ni^{2+} single ion anisotropy D = 8.9 K opens an energy gap between the S_{z} = 0 ground state and the S_{z} = ±1 first exited states. XYantiferromagnetism is induced between H_{c1 }= 2 T and H_{c2 }= 10.5 T. Our experimental investigations by magnetization, specific heat and thermal expansion measurements down to 50 mK probe scaling laws in the vicinity of the quantum critical point at H_{c1}, where we find critical exponents consistent with 3dimensional BEC. The second example AgVOAsO_{4} is a low dimensional spin system based on V^{4+}, S = 1/2 spins as indicated by NMR experiments. The underlying spin structure in this material is unknown, but theoretical modeling points to a complicated cross pattern of the alternating spin chains with significant frustration. Measurements of the specific heat up to 28 T reveal a double phase transition developing above 10 T, where the spin gap closes. The double transition promotes AgVOAsO_{4} as a promising candidate for multiQ BEC with Q being the wave vector of the singleparticle ground state in boson language. MultiQ BECs can host topological spin textures such as magnetic vortex crystals, equivalent to skyrmions in metallic systems, but were never observed before. 

Host 
Yasu Takano 
April 23 


Speaker 
Rena Zieve (UC Davis) 

Title 
Waves on single vortices in superfluid helium 

Abstract 
Superfluid helium supports welldefined vortices with quantized circulation. Despite the quantization, vortex behavior obeys the same rules governing classical vortices  indeed with simplifications, since quantization forbids many of the motions allowed in the classical case. Our measurement technique allows tracking of single superfluid vortices. I will describe some of the characteristic behaviors we observe, which follow simple principles of hydrodynamics. I will particularly emphasize our observations of the vortex oscillations first predicted by Lord Kelvin. 

Host 
Yoonseok Lee 