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Overview

Davis Group research concentrates upon the fundamental physics of electronic, magnetic, atomic and space-time quantum matter. A specialty is development of innovative instrumentation to allow direct visualization (or perception) of characteristic quantum many-body phenomena at atomic scale

Davis Group operates three suites of ultra-low vibration laboratories, one in  at  (US), the second at  at  (IE) and a third is at  at  (UK). The overall objective is to exploit the distinct capabilities and facilities at all laboratories to maximize scientific efficiency.

Ours is as single research group conducting scientifically harmonized studies with complementary scientific instruments at all three locations. Other key components of our program are at the in Dresden.

Research Focus

Cooper-Pair Condensates      

Research Status:        

We recently introduced nanometer resolution Scanned Josephson Tunneling Microscopy (SJTM), a technique allowing imaging of Cooper-pair tunneling from a superconducting STM tip to the Cooper-pair condensate of a superconductor. The SJTM operates at millikelvin temperatures and sequentially forms an array of 65,500 nanoscale Josephson junctions, whose Josephson critical current Ic is then measured to form the condensate image (). For the first time in superconductivity research, one can visualize the Cooper-pair condensate itself.              

Research Plans:         

SJTM is a very promising new approach to research into all kinds of heterogeneous superconductivity. Projects of immediate research interest include:

1. The Cooper-pair density wave (PDW) state occurs when the density of Cooper-pairs modulates periodically in space at wavevector Q_P . Only one instance has ever been detected (). Now we plan a search for new PDW states in several classes of materials. Transition metal dichalcogenides appear ideal, because they often host both superconductivity (SC) and charge density waves (CDW); Ginzburg-Landau theory predicts that a PDW state must be induced by the interactions between the SC and CDW states. Heavy-fermion superconductors e.g. CeCoIn5 at high fields are also reported to host PDW states. Copper-based high temperature superconductors (CuHTS) materials, e.g. YBa₂Cu₃O₇ and La₂BaCuO₄, are widely predicted to host a strong-coupling PDW state.
2. In CuHTS, an exceptional new electronic phase appears at highest magnetic fields. It supports unexplained quantum oscillations and an unidentified density wave (DW) state. Although generally referred to as a CDW, theory indicates that this could actually be a PDW state. Because this field-induced DW state is accessible in the “halo” surrounding quantized vortex cores (), we now plan to image this “halo” DW using SJTM to determine directly if it is a PDW.

Magnetic Monopole Fluids

Status:            

Magnetic monopoles are hypothetical elementary particles exhibiting quantized magnetic charge m₀ and quantized magnetic flux. A classic proposal for detecting such magnetic charges is to measure the quantized jump in magnetic flux threading the loop of a superconducting quantum interference device (SQUID) when a monopole passes through it. Naturally, with the theoretical discovery that a fluid of emergent magnetic charges should exist in several lanthanide-pyrochlore magnetic insulators including Dy₂Ti₂O₇, this SQUID technique was proposed for their direct detection (Castelnovo et al ). Experimentally, this has proven extremely challenging because of the high number density, and generation-recombination (GR) fluctuations, of the monopole plasma. Recently, however, theoretical advances by Prof. S. Blundell of Oxford University have allowed the­ spectral density of spin-noise due to GR fluctuations of magnetic charge pairs to be determined.

In 2018 we developed a high-sensitivity, SQUID based spin-noise spectrometer, and measured the frequency and temperature dependence of spin-noise spectral density for Dy₂Ti₂O₇ samples. Virtually all the elements predicted for a magnetic monopole fluid, including the existence of intense magnetization noise and its characteristic frequency and temperature dependence, are detected.

Research Plans:         

High precision measurement of the spin-noise spectrum is an innovative approach to magnetic quantum fluids. It opens a wide variety of new research avenues including the following projects of immediate interest:

1. Ho₂Ti₂O₇ is a pyrochlore magnetic insulator with many similar characteristics to Dy₂Ti₂O₇ and it is widely believing to also contain a fluid of emergent magnetic monopoles. We plan to use our spin-noise spectroscopy (SNS) technique to search for the flux noise signature of magnetic monopole fluid in Ho₂Ti₂O₇.
2. Based on our measurements, we estimate that the flux jump of individual magnetic monopoles in Dy₂Ti₂O₇ and Ho₂Ti₂O₇ should be detectable in sub-micron scale samples and at mK temperatures.  We plan to develop a millikelvin SNS instrument for this purpose, and to search for individual magnetic monopoles in these and other compounds.
3. Eventually, out SNS approach will be generalized into a visualization technique in the form of a Scanned Spin-Noise Microscope (SSNM). We plan development of this new instrument as part of the suite of new quantum microscopes at our laboratories.

Magnetic Topological Insulators

Status:

Surface states of topological insulators (TIs) are expected to exhibit many valuable new electronic phenomena when a 'mass gap' is opened in their Dirac spectrum by ferromagnetism (FM). Such ferromagnetic topological insulators (FMTI) should exhibit phenomena including the Quantum Anomalous Hall Effect (QAHE), the Jackiw-Rebbi Solitons (JRS), and Emergent Axionic Electrodynamics. The QAHE has indeed been observed but, mysteriously, iti is only detected at mK temperatures.

To explore the intriguing physics of FMTI, we recently developed the first visualization technique for the Dirac mass of FMTI surface states. We found that the Dirac mass m(r) is extremely disordered and correlates with the local density of the magnetic dopant atoms generating FM state. This chaotic Dirac-mass landscape m(r) poses far more questions on FMTI than it answers.

Research Plans:    

1. In general, ferromagnets exhibit both FM domains and magnetic hysteresis, and FMTI are no different. But these phenomena should, in theory, have a profound influence on JRS and QAHE. We plan to measure the atomic-scale electronic structure throughout the hysteresis loops of Cr(BiSb)Te₃ and Va(BiSb)Te₃ and thus to visualize the evolution of FM domains and the network of JR states that should exist between regions of opposite magnetization.
2. The QAHE only stabilizes at temperatures T<<1K. This likely means that nanoscale disorder (Fig.  2C) somehow shorts out the chiral edge currents, allowing them to pass through the centre of the sample so that the conductance is not quantized. Precisely how this happens is unknown. We plan to image topological surface states of FMTI approaching QAHE with falling temperature, to visualize how the bulk currents are destroyed and the QAHE edge current stabilized.
3. The interplay of electric field E and magnetic field B at the surface of FMTI should be analogous to that predicted theoretically for axions. We plan to pursue proposals for how to observe this effect by generating axionic phenomena with an STM tip and observing the nanoscale B-field response.

Topological Kondo Insulators                                                                   

Status:           

In a crystal with a sub-lattice of localized f-electron states, the Kondo effect generates a heavy-fermion band structure. At high temperatures, a conventional (light) electronic band coexists with localized f-electron states on each magnetic atom. At lower temperatures, hybridization between this light band and the f-electron states results in opening a hybridization gap, and its splitting into two new very flat bands with greatly enhanced density-of-electronic-states N(E) within just a few meV of E_F. We developed a dilution-refrigerator-based mK SISTM instrument for mapping simultaneously the r-space and k-space electronic structure of heavy-fermion systems at temperatures down to 20 mK.  Demonstration of the feasibility of this approach for visualizing heavy-fermion formation, and measuring heavy-fermion band-structures, launched the field of STM studies of heavy fermions ().

Research Plans:    The capability to image heavy fermions opens exciting new avenues for research into strongly entangled electronic quantum matter.

1. The theory of topological Kondo insulators (TKI) postulates a strongly anisotropic hybridization gap that inverts the parity of bulk heavy-fermion states. The resulting prediction is for heavy-fermion topological surface states to appear at three points of the surface BZ. To explore these phenomena, we plan to apply high-resolution heavy-fermion visualization technique at millikelvin temperatures to measure the k-space structure of the hybridization gap of the TKI SmB₆.
2. Such mK SISTM techniques also represent an exciting opportunity to achieve direct visualization of electronic quantum criticality. When quantum fluctuations become sufficiently strong, heavy-fermion systems often undergo a quantum phase transition to a new ground state. Indeed, understanding this type of quantum critical electronic matter is one of the key challenges of condensed matter physics.  YbRh₂Si₂ is a heavy-fermion system with a QCP near B=0.66 Tesla (and no superconductivity). We plan to apply mK visualization techniques in magnetic field, to determine the heavy-fermion band structure, and to characterize the quasiparticles in the quantum critical regime surrounding the antiferromagnetic QCP of YbRh₂Si₂.

Cu/Fe HT Superconductors

Status:            

Novel ‘electronic liquid crystal’ phases have long been predicted for correlated electronic materials, especially those where the intense correlations generate the highest temperature superconductivity. By using direct atomic-scale visualization we have discovered several of these phases including the smectic (DW) state in CuHTS (; ; ); the nematic phase in CuHTS (; ); the famous nematic phase of FeHTS (; the Cooper-Pair Density Wave (PDW) state in CuHTS ()).

Research Plans:         

Having established the existence of these broken-symmetry electronic liquid crystal states, the challenge now is to understand their relationship to the HTS.   

1. Recently the effects of quenched disorder on such a two-dimensional DW state have been discovered. While long range order of a unidirectional incommensurate DW cannot exist in the presence of quenched disorder, its short-range remnant survives up to a certain critical disorder strength but in the form of a Q=0 broken rotational-symmetry state. This state was dubbed a vestigial nematic (VN). We plan to search for the VN state by determining if energy scale of nematic state is the same as that of the DW state throughout phase diagram.
2. Intense theoretical interest has emerged in whether a PDW state is actually the competing phase to superconductivity in CuHTS. Thus, we plan to test if the reported charge modulation phenomenology is actually a secondary effect of a fundamental PDW state. We will image conventional density-of-states N(r,E) of charge modulations, simultaneously with imaging of Josephson I_c(r) to visualize the PDW. Comparison between the first ever such pairs of N(r,E):I_c(r) images will be highly revealing as to which state is fundamental.

Viscous Electron Fluids                                               

Status:           

There is now widespread interest in whether some electron fluids exhibit viscosity. Key evidence for this phenomenon comes from studied of ultra-pure dellafossite crystals (A.P. Mackenzie Rep. Prog. Phys. 80 032501 (2017 )).

Research Plans:  

A profound challenge for this field is to detect turbulence of an electronic fluid. No phenomena have ever been observed for any electron fluid. Thus, exploratory studies to visualize viscous phenomena in an electron fluid are of great interest

1. We plan to attempt visualize the impurity scattering interference in Co-dellafossite crystals whose Fermi surface si already very well understood. Subsequently, a large electric current generating (electron fluid flow) will be applied and its effects visualized directly at atomic scale (in a conventional electron gas no detectable effects would be expected) .
2. If effects of electron fluid flow are observable, then the Reynolds number for an atomic scale perturbation will be used to predict the current density necessary to cause turbulence, for which we will then search.

Publications

Selected Publications

1. Quantum oscillations between two weakly coupled reservoirs of superfluid ³He S.V. Pereverzev, A. Loshak, S. Backhaus, J.C. Davis and R.E. Packard, .
2. Direct measurement of the current-phase relationship of a superfluid 3He weak link, Backhaus S., Pereverzev S.V., Davis J.C., and Packard R.E., .
3. Discovery of a metastable  -state in superfluid ³He weak link, S. Backhaus, R. Simmonds, S. Pereverzev, A. Loshak, J.C. Davis R.E. Packard
4. Observation of Third Sound in Superfluid ³He A.M. R Schechter, R.W. Simmonds, R.E. Packard, and J.C. Davis, .
5. Josephson effect and a p-state in superfluid 3He, S. Backhaus, R. W. Simmonds, A. Loshak, J. C. Davis R. E. Packard,
6. Atomic-scale Quasi-Particle Scattering Resonances in Bi₂Sr₂CaCu₂O_8+d, E.W. Hudson, S. H. Pan, A. K. Gupta, K-W Ng, and J.C. Davis, .
7. Imaging the Effects of Individual Zinc Impurity Atoms on Superconductivity in Bi₂Sr₂CaCu₂O_8+d, S.H. Pan, E.W. Hudson, K.M. Lang, H. Eisaki, S. Uchida, and J.C. Davis, .
8. Interplay of magnetism and high-Tc superconductivity at individual magnetic impurity atoms in Bi₂Sr₂CaCu₂O_8+        Hudson, E.W., Lang. K, Madhavan, V., Pan, S.H., Eisaki, H., Uchida, S. Davis, J.C. .
9. Quantum Interference of Superfluid ³He, R. W. Simmonds, A. Marchenkov, J. C. Davis and R.E. Packard, .
10. Microscopic electronic inhomogeneity in the high-temperature superconductor Bi₂Sr₂CaCu₂O_8+d S. H. Pan, J. O’Neil, R.L. Badzey, H. Ding, J. R. Englebrecht, Z. Wang, H. Esiaki, S. Uchida, A. Gupta. K-W Ng, E. W. Hudson K.M. Lang and J. C. Davis, .
11.  Imaging the granular structure of high-T_c superconductivity in underdoped Bi₂Sr₂CaCu₂O_8+d,  K. M. Lang, V. Madhavan, J. Hoffman, E.W. Hudson, H. Eisaki, S. Uchida, and J.C. Davis, .
12.  A four unit cell periodic pattern of quasiparticle states surrounding vortex cores in Bi₂Sr₂CaCu₂O_8+d J. E. Hoffman, E.W. Hudson, K. Lang, V. Madhavan, H. Eisaki, S. Uchida, and J.C. Davis, .
13.  Imaging Quasiparticle Interference in Bi₂Sr₂CaCu₂O_8+d” J. Hoffman, K. McElroy, D-H Lee, K.M. Lang, H Eisaki, S. Uchida, and J. C. Davis, .
14. Relating atomic scale electronic phenomena to wave-like quasiparticle states in superconducting Bi₂Sr₂CaCu₂O_8+d K. McElroy, R. W. Simmonds, J. E. Hoffman, D.-H. Lee, J. Orenstein, H. Eisaki, S. Uchida J.C. Davis^., .
15. A ‘checkerboard’ electronic crystal state in Lightly Hole-Doped Ca_2-xNa_xCuO₂Cl_­2  T. Hanaguri, C. Lupien, Y. Kohsaka, D.-H. Lee,M. Takano, H. Takagi, J. C. Davis. .
16.  Atomic-scale Sources and Mechanism of Nanoscale Electronic Disorder in Bi₂Sr₂CaCu₂O_8+.  K. McElroy, Jinho Lee, J. Slezak, D.-H. Lee, H. Eisaki, S. Uchida, J.C. Davis. .
17. Interplay of electron-lattice interactions and superconductivity in Bi₂Sr₂CaCu₂O_8+,   Jinho Lee, K. Fujita, K. McElroy, J.A. Slezak, M. Wang, Y. Aiura, H. Bando, M. Ishikado,T. Masui, J. -X. Zhu, A. V. Balatsky, H. Eisaki, S. Uchida,andJ. C. Davis, .
18. The Ground State of Pseudogap in Cuprates: La_1.875Ba_0.125CuO₄, T. Valla, A. V. Fedorov, J. C. Davis , Jinho Lee, and G. D. Gu, .
19. An intrinsic bond-centered electronic glass with disperse unidirectional domains in underdoped cuprates, Y. Kohsaka, C. Taylor, A. Schmidt, K. Fujita, C.  Lupien, T. Hanguri, H. Eisaki, S. Uchida, H. Takagi and J. C. Davis, .
20. How Cooper pairs vanish approaching the Mott insulator in Bi₂Sr₂CaCu₂O_8+dY. Kohsaka, C. Taylor, P. Wahl, A. Schmidt, Jhinhwan Lee, K. Fujita, J. Alldredge, Jinho Lee, K. McElroy, H. Eisaki, S. Uchida, D.-H. Lee, J.C. Davis, .
21. Evidence for a ‘Superglass’ State in Solid ⁴He, B. Hunt, E. Pratt, V. Gadagkar, M. Yamashita, A. V. Balatsky J.C. Davis, .
22. Spectroscopic Fingerprint of Phase Incoherent d-Wave Superconductivity in the Cuprate Pseudogap State, Jhinhwan Lee, K. Fujita, C.K. Kim, A. Schmidt, H. Eisaki, S. Uchida, J.C. Davis, .
23. Nematic Electronic Structure in the ‘Parent’ State of Iron-based Superconductor Ca(Fe_1-xCo_x)₂As₂, T.-M. Chuang, M.P.  Allan, J.Lee, Ni Ni, S. Bud’ko, G. Boebinger, P.C. Canfield J.C. Davis, .
24. Imaging the Fano Lattice to Hidden Order transition in URu₂Si₂, A.R. Schmidt, Mohammad H. Hamidian, P. Wahl, F. Meier, A.V. Balatsky, T.J. Williams, G.M. Luke and J.C. Davis, .
25. Intra-unit-cell Electronic Nematicity of the High-Tc Cuprate Pseudogap States, M. J. Lawler, K. Fujita, Jhinhwan Lee, A.R. Schmidt, Y. Kohsaka, Chung Koo Kim, H. Eisaki, S. Uchida,  J.C. Davis, J.P. Sethna, and Eun-Ah Kim, .
26. Interplay of Rotational, Relaxational, and Shear Dynamics in Solid ⁴He, E.J. Pratt, B. Hunt, V. Gadagkar, M. Yamashita, M. J. Graf, A. V. Balatsky and J.C. Davis, .
27. Topological Defects Coupling Smectic Modulation to Intra-Unit–Cell Nematicity in Cuprates A. Mesaros, K. Fujita, H. Eisaki, S.I. Uchida, J.C. Seamus Davis, Subir Sachdev, Jan Zaanen, M.J. Lawler and Eun-Ah Kim, .
28. Anisotropic Energy-Gaps of Iron-based Superconductivity from Intra-band Quasiparticle Interference in LiFeAs M. P. Allan, A. W. Rost, A. P. Mackenzie, Yang Xie, J. C. Davis, K. Kihou, H. Eisaki, and T.-M. Chuang, .
29. Simultaneous Transitions in Cuprate Momentum-Space Topology and Electronic Symmetry Breaking.  K. Fujita, C.K. Kim, Inhee Lee, Jinho Lee, M. H. Hamidian, I. Firmo, H. Eisaki, S. Uchida, M.J. Lawler, E.-A. Kim, and J.C. Davis. .
30. Detection of a Cooper-Pair Density Wave in Bi₂Sr₂CaCu₂O_8+x, M. Hamidian et al,
31. Discovery of Orbital Selective Cooper pairing in FeSe, P.O. Sprau et al,  .
     
32. Magnetic-field Induced Pair Density Wave State in the Cuprate Vortex Halo. S.D. Edkins, A. Kostin, K. Fujita, A. P. Mackenzie, H. Eisaki, S. Uchida, M. J. Lawler, E-A. Kim, S. A. Kivelson, J.C. Séamus Davis,  and M. H. Hamidian,
     
33. Machine Learning in  Electronic Quantum Matter Visualization Experiments, Yi Zhang, A. Mesaros, K. Fujita, S.D. Edkins, M.H. Hamidian, K. Ch'ng, J.C. Séamus Davis, E. Khatami and Eun-Ah Kim,
     
34. Magnetic Monopole Noise, Ritika Dusad, Franziska K.K. Kirschner, Jesse C. Hoke, Benjamin Roberts, Anna Eyal, Felix Flicker, Graeme M. Luke, Stephen J. Blundell and J.C. Séamus Davis,
     
35. Imaging the energy gap modulations of the cuprate pair-density-wave state Zengyi Du, Hui Li, Sang Hyun Joo, Elizabeth P. Donoway, Jinho Lee, J. C. Séamus Davis, Genda Gu, Peter D. Johnson & Kazuhiro Fujita

In the news

• Breakthrough identifies new state of topological quantum matter
• Prestigious Buckley Prize awarded to physicist J.C. Séamus Davis
• Seven faculty members elected AAAS fellows
• Electron-pair discovery advances field of quantum materials
• Physicist receives prize for ‘pioneering research’
• New awards to enable ‘quantum’ leaps in research
• Machine learning unlocks mysteries of quantum physics
• Keck-funded group proposes new topological superconductor
• Jeevak Parpia wins low-temperature physics prize