News & Notes
October 2024: Ilija Receives a Gordon and Betty Moore Grant
Congratulations to Ilija for being awarded a Gordon and Betty Moore Foundation grant! The funding will go towards adding electron spin resonance capabilities to one of our scanning tunneling microscopes. To read more about this, see the BC News article linked here. To see the full list of 2024 Experimental Physics award receipients as well as descriptions of each project, see the Gordon and Betty Moore Foundation article linked here.
August 2024: An end of year celebration dinner for Muxian and Sylvia Passing
Congratulations to Muxian Xu for passing his Research Proposal Exam on "Scanning Tunneling Microscopy/ Spectroscopy on 2D Materials," and to Sylvia Chen for passing the comprehensive exam! The group belatedly celebrated their accomplishments with a hot pot dinner.
June 10, 2024: Dr. Hong Li successfully defends his Ph.D. thesis defense
Congratulations to Dr. Hong Li on completing his Ph.D. dissertation titled "Revealing band topology and electronic correlation in Kagome crystals with STM." During his time in Zeljkovic lab, Dr. Li served as an author on an impressive 22 papers, 10 of which he was a first author or co-first author. Hong is headed to Brookhaven National Laboratory next year where he will work for Columbia Professor Abhay Pasupathy. We wish you all the best Hong!
May 25, 2024: Alex and Hong's paper is published in Nature Communications
Alexander LaFleur, Hong Li, Corey E. Frank, Muxian Xu, Siyu Cheng, Ziqiang Wang, Nicholas P. Butch, and Ilija Zeljkovic
Charge, spin and Cooper-pair density waves have now been widely detected in exotic superconductors. Understanding how these density waves emerge — and become suppressed by external parameters — is a key research direction in condensed matter physics. Here we study the temperature and magnetic-field evolution of charge density waves in the rare spin-triplet superconductor candidate UTe2 using scanning tunneling microscopy/spectroscopy. We reveal that charge modulations composed of three different wave vectors gradually weaken in a spatially inhomogeneous manner, while persisting to surprisingly high temperatures of 10–12 K. We also reveal an unexpected decoupling of the three-component charge density wave state. Our observations match closely to the temperature scale potentially related to short-range magnetic correlations, providing a possible connection between density waves observed by surface probes and intrinsic bulk features. Importantly, charge density wave modulations become suppressed with magnetic field both below and above superconducting Tc in a comparable manner. Our work points towards an intimate connection between hidden magnetic correlations and the origin of the unusual charge density waves in UTe2.
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April 22, 2024: Hong and Siyu's paper is published in Nature Physics
Spin Berry curvature-enhanced orbital Zeeman effect in a kagome metal
Hong Li, Siyu Cheng, Ganesh Pokharel, Philipp Eck, Chiara Bigi, Federico Mazzola, Giorgio Sangiovanni, Stephen D. Wilson, Domenico Di Sante, Ziqiang Wang, and Ilija Zeljkovic
Berry phases and the related concept of Berry curvature can give rise to many unconventional phenomena in solids. Here, we discover a colossal orbital Zeeman effect of topological origin in a bilayer kagome metal, TbV6Sn6. Using spectroscopic imaging scanning tunnelling microscopy, we reveal that the magnetic field leads to a splitting of the gapped Dirac dispersion into two branches with enhanced momentum-dependent g factors, resulting in a substantial renormalization of the Dirac band. These measurements provide a direct observation of a magnetic field-controlled orbital Zeeman coupling to the orbital magnetic moments of up to 200 Bohr magnetons near the gapped Dirac points. Our work provides direct insight into the momentum-dependent nature of topological orbital moments and their tunability via the magnetic field, concomitant with the evolution of the spin Berry curvature. These results can be extended to explore large orbital magnetic moments driven by the Berry curvature governed by other quantum numbers beyond spin, such as the valley in certain graphene-based structures.
April 5, 2024: Chris and Hong's paper is published in Phys. Rev. B
Christopher Candelora, Hong Li, Muxian Xu, Brenden R. Ortiz, Andrea Capa Salinas, Siyu Cheng, Alexander LaFleur, Ziqiang Wang, Stephen D. Wilson, and Ilija Zeljkovic
A wide array of unusual phenomena has recently been uncovered in kagome solids. The charge density wave (CDW) state in the kagome superconductor 𝐴V3Sb5, in particular, intrigued the community; the CDW phase appears to break the time-reversal symmetry despite the absence of spin magnetism, which has been tied to exotic orbital loop currents possibly intertwined with magnetic field tunable crystal distortions. To test this connection, precise determination of the lattice response to an applied magnetic field is crucial but can be challenging at the atomic scale. We establish a scanning tunneling microscopy (STM) based method to study the evolution of the 𝐴V3Sb5 atomic structure as a function of magnetic field. The method substantially reduces the errors of typical STM measurements, which are at the order of 1% when measuring an in-plane lattice constant change. We find that the out-of-plane lattice constant of 𝐴V3Sb5 remains unchanged (within 10−6) by the application of both in-plane and out-of-plane magnetic fields. We also reveal that the in-plane lattice response to magnetic field is at most at the order of 0.05%. Our experiments provide further constraints on time-reversal symmetry breaking in kagome metals and establish a tool for higher-resolution extraction of the field-lattice coupling at the nanoscale applicable to other quantum materials.
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January 27, 2024: Siyu, Zheng, and Hong's paper is published in npj quantum materials
Siyu Cheng, Zheng Ren, Hong Li, Jiseop Oh, Hengxin Tan, Ganesh Pokharel, Jonathan M. DeStefano, Elliott Rosenberg, Yucheng Guo, Yichen Zhang, Ziqin Yue, Yongbin Lee, Sergey Gorovikov, Marta Zonno, Makoto Hashimoto, Donghui Lu, Liqin Ke, Federico Mazzola, Junichiro Kono, R. J. Birgeneau, Jiun-Haw Chu, Stephen D. Wilson, Ziqiang Wang, Binghai Yan, Ming Yi and Ilija Zeljkovic
Charge density waves (CDWs) in kagome metals have been tied to many exotic phenomena. Here, using spectroscopic-imaging scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we study the charge order in kagome metal ScV6Sn6. The similarity of electronic band structures of ScV6Sn6 and TbV6Sn6 (where charge ordering is absent) suggests that charge ordering in ScV6Sn6 is unlikely to be primarily driven by Fermi surface nesting of the Van Hove singularities. In contrast to the CDW state of cousin kagome metals, we find no evidence supporting rotation symmetry breaking. Differential conductance dI/dV spectra show a partial gap Δ1CO ≈ 20 meV at the Fermi level. Interestingly, dI/dV maps reveal that charge modulations exhibit an abrupt phase shift as a function of energy at energy much higher than Δ1CO, which we attribute to another spectral gap. Our experiments reveal a distinctive nature of the charge order in ScV6Sn6 with fundamental differences compared to other kagome metals.
December 11, 2023: Yidi and Hong's paper is posted to arXiv
Nanoscale strain manipulation of smectic susceptibility in kagome superconductors
Yidi Wang, Hong Li, Siyu Cheng, He Zhao, Brenden R. Ortiz, Andrea Capa Salinas, Stephen D. Wilson, Ziqiang Wang, and Ilija Zeljkovic
Exotic quantum solids can host electronic states that spontaneously break rotational symmetry of the electronic structure, such as electronic nematic phases and unidirectional charge density waves (CDWs). When electrons couple to the lattice, uniaxial strain can be used to anchor and control this electronic directionality. Here we reveal an unusual impact of strain on unidirectional "smectic" CDW orders in kagome superconductors AV3Sb5 using spectroscopic-imaging scanning tunneling microscopy. We discover local decoupling between the smectic electronic director axis and the direction of anisotropic strain. While the two are generally aligned along the same direction in regions of small CDW gap, the two become misaligned in regions where CDW gap is the largest. This in turn suggests nanoscale variations in smectic susceptibility, which we attribute to a combination of local strain and electron correlation strength. Overall, we observe an unusually high decoupling rate between the smectic electronic director of the 3-state Potts order and anisotropic strain, revealing weak smecto-elastic coupling in the CDW phase of kagome superconductors. This is phenomenologically different from the extensively studied nemato-elastic coupling in the Ising nematic phase of Fe-based superconductors, providing a contrasting picture of how strain can control electronic unidirectionality in different families of quantum materials.
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September 15, 2023: Hong's paper is published in Phys. Rev. X
Hong Li, Dongjin Oh, Mingu Kang, He Zhao, Brenden R. Ortiz, Yuzki Oey, Shiang Fang, Zheng Ren, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Joseph G. Checkelsky, Ziqiang Wang, Stephen D. Wilson, Riccardo Comin, and Ilija Zeljkovic
The kagome superconductor family 𝐴V3Sb5 (𝐴=Cs, K, Rb) emerged as an exciting platform to study exotic Fermi surface instabilities. Here, we use spectroscopic-imaging scanning tunneling microscopy (SI-STM) and angle-resolved photoemission spectroscopy (ARPES) to reveal how the surprising cascade of higher- and lower-dimensional density waves in CsV3Sb5 is intimately tied to a set of small reconstructed Fermi pockets. ARPES measurements visualize the formation of these pockets generated by a 3D charge density wave transition. The pockets are connected by dispersive 𝑞* wave vectors observed in Fourier transforms of STM differential conductance maps. As the additional 1D charge order emerges at a lower temperature, 𝑞* wave vectors become substantially renormalized, signaling further reconstruction of the Fermi pockets. Remarkably, in the superconducting state, the superconducting gap modulations give rise to an in-plane Cooper pair density wave at the same 𝑞* wave vectors. Our work demonstrates the intrinsic origin of the charge stripes and the pair density wave in CsV3Sb5 and their relationship to the Fermi pockets. These experiments uncover a unique scenario of how Fermi pockets generated by a parent charge density wave state can provide a favorable platform for the emergence of additional density waves.
August 17, 2023: Hong and Siyu's paper is published in Nature Physics
Electronic nematicity without charge density waves in titanium-based kagome metal
Hong Li, Siyu Cheng, Brenden R. Ortiz, Hengxin Tan, Dominik Werhahn, Keyu Zeng, Dirk Johrendt, Binghai Yan, Ziqiang Wang, Stephen D. Wilson, and Ilija Zeljkovic
Layered crystalline materials that consist of transition metal atoms on a kagome network have emerged as a versatile platform for the study of unusual electronic phenomena. For example, in the vanadium-based kagome superconductors AV3Sb5 (where A can stand for K, Cs or Rb), there is a parent charge density wave phase that appears to simultaneously break both the translational and rotational symmetries of the lattice. Here we show a contrasting situation, where electronic nematic order—the breaking of rotational symmetry without the breaking of translational symmetry—can occur without a corresponding charge density wave. We use spectroscopic-imaging scanning tunnelling microscopy to study the kagome metal CsTi3Bi5 that is isostructural to AV3Sb5 but with a titanium atom kagome network. CsTi3Bi5 does not exhibit any detectable charge density wave state, but a comparison to density functional theory calculations reveals substantial electronic correlation effects at low energies. In comparing the amplitudes of scattering wave vectors along different directions, we discover an electronic anisotropy that breaks the sixfold symmetry of the lattice, arising from both in-plane and out-of-plane titanium-derived d orbitals. Our work uncovers the role of electronic orbitals in CsTi3Bi5, suggestive of a hexagonal analogue of the nematic bond order in Fe-based superconductors.
July 5, 2023: Shrinkhala and Hong's paper is published in Phys. Rev. Materials
Shrinkhala Sharma, Hong Li, Zheng Ren, Wilber Alfaro Castro, and Ilija Zeljkovic
Antiferromagnetic order, being a ground state of a number of exotic quantum materials, is of immense interest both from the fundamental physics perspective and for driving potential technological applications. For a complete understanding of antiferromagnetism in materials, nanoscale visualization of antiferromagnetic domains, domain walls, and their robustness to external perturbations is highly desirable. Here, we synthesize antiferromagnetic FeTe thin films using molecular-beam epitaxy. We visualize local antiferromagnetic ordering and domain formation using spin-polarized scanning tunneling microscopy. From the atomically resolved scanning tunneling microscopy topographs, we calculate local structural distortions to find a high correlation with the distribution of the antiferromagnetic order. This is consistent with the monoclinic structure in the antiferromagnetic state. Interestingly, we observe a substantial domain-wall change by small temperature variations, unexpected for the low-temperature changes used compared to the much higher antiferromagnetic ordering temperature of FeTe. This is in contrast to electronic nematic domains in the cousin FeSe multilayer films, where we find no electronic or structural change within the same temperature range. Our experiments provide the atomic-scale imaging of perturbation-driven magnetic domain evolution simultaneous with the ensuing structural response of the system. The results reveal surprising thermally driven modulations of antiferromagnetic domains in FeTe thin films well below the Néel temperature.
February 9, 2023: Hong and He's paper is published in Nature Physics
Hong Li, He Zhao, Brenden R. Ortiz, Yuzki Oey, Ziqiang Wang, Stephen D. Wilson, and Ilija Zeljkovic
Kagome metals AV3Sb5 (where the A can stand for K, Cs or Rb) display a rich phase diagram of correlated electron states, including superconductivity and density waves. Within this landscape, recent experiments have revealed signs of a transition below approximately 35 K attributed to an electronic nematic phase that spontaneously breaks the rotational symmetry of the lattice8. Here we show that the rotational symmetry breaking initiates universally at a high temperature in these materials, towards the 2 × 2 charge density wave transition temperature. We do this via spectroscopic-imaging scanning tunnelling microscopy and study the atomic-scale signatures of the electronic symmetry breaking across several materials in the AV3Sb5 family: CsV3Sb5, KV3Sb5 and Sn-doped CsV3Sb5. Below a substantially lower temperature of about 30 K, we measure the quantum interference of quasiparticles, a key signature for the formation of a coherent electronic state. These quasiparticles display a pronounced unidirectional feature in reciprocal space that strengthens as the superconducting state is approached. Our experiments reveal that high-temperature rotation symmetry breaking and the charge ordering states are separated from the superconducting ground state by an intermediate-temperature regime with coherent unidirectional quasiparticles. This picture is phenomenologically different compared to that in high-temperature superconductors, shedding light on the complex nature of rotation symmetry breaking in AV3Sb5 kagome superconductors.