Wenxin's work on the origin of the sensitive magnetism in Fe intercalated NbS2 is now accepted in Physical Review Letters, also highlighted as Editors' Suggestion!

In highly tunable magnetic metals, minute changes in atomic composition can dramatically alter macroscopic properties. We investigated the layered metal iron-intercalated niobium disulfide, a material that undergoes an abrupt transformation in its magnetic structure when the iron concentration crosses a critical threshold. To uncover the driving force behind this extreme sensitivity, we utilized angle-resolved photoemission spectroscopy to map the energetic behaviors and interactions of the material's electrons. Our measurements revealed that below the threshold, localized electrons from the iron atoms actively mix with the mobile electrons of the host niobium disulfide lattice. However, above the critical concentration threshold, this mixture disappears. This abrupt loss of electronic connectivity alters the indirect magnetic exchange interactions between the iron atoms, forcing the material's overall magnetic arrangement to reorganize completely. This work pinpoints the origin of magnetic sensitivity in this intercalated metal to out-of-plane orbital hybridizations, and establishes an electronic spectroscopy approach to evaluate magnetic interactions in complex metals.

Jinming's work with Siqi has now been published on PNAS after a 5-year-long heroic effort. The scope substantially expanded and consolidated over the grind of time, but what emerges is a wonderful revelation.

For decades, it has been a puzzle why praseodymium - unlike other rare earth substitutions - so effectively suppresses superconductivity in YBa2Cu3O7−δ. Prevailing theories attributed this to a 4f–2p hybridization effect depleting holes on the CuO2 plane. Our work challenges this view: the hypothesized low-energy f-electron mechanism is absent up to 28% substitution with 50% Tc suppression. Through angle-resolved photoemission spectroscopy and first-principles calculations, we reveal remarkably efficient hole reduction of 1.5e−/Pr for surface and 0.5e−/Pr for bulk to the superconducting plane. Meanwhile, Pr substitution substantially diminishes the CuO2 plane–plane interaction, which implies the existence of additional charge release channels beyond simple 4f electron ionization. Such bilayer coupling has been long considered to play an important role behind high-Tc superconductivity in cuprates, and this work is one of the few direct experimental evidence.

Zhibo's work using momentum-resolved phonon spectroscopy (IXS) to diagnose putative excitonic condensate is now accepted at Physical Review Materials as a Letter, and selected as Editors' Suggestion!

A long-running question in quantum materials is how to diagnose a bulk “excitonic insulator”, a state in which electrons and the positively charged “holes” lock together and move collectively. In this work, instead of speculating only from qualitative static behaviors at low temperatures, we used high-resolution inelastic x-ray scattering to perform a more direct test: if an exciton condensate is present, its phase-twisting mode should mix with the crystal’s atomic vibrations and leave a clear fingerprint in the vibrational spectrum. We also show that Ta2NiSe5 lies in a rare ultra-strong-coupling regime, where the coupling energy dwarfs the vibration energy itself by almost a decade.

We introduce an on-chip design using nanomagnetic metamaterial that generates sub-tesla fields directly beneath ARPES samples without distorting electron trajectories. Using monolayer graphene, we demonstrate band structure mapping under 30 mT field with non-degrading momentum and energy resolution — opening new ways to watch quantum states evolve under magnetic fields in real time. A general protocol is also presented for reaching field up to 1 T.

Sub-tesla on-chip nanomagnetic metamaterial platform for angle-resolved photoemission spectroscopy

Wenxin Li, Wisha Wanichwecharungruang, Mingyang Guo, Ioan-Augustin Chioar, Nileena Nandakumaran, Justin Ramberger, Senlei Li, Zhibo Kang, Jinming Yang, Donghui Lu, Makoto Hashimoto, Chunhui Du, Chris Leighton, Peter Schiffer, Qiong Ma, Ming Yi, Yu He

Journal of Physical Chemistry Letters 16, 12947 (2025)

Our collaboration with our theoretician friends at the Wang and Liu groups (Emory university) is now published on the first issue of Newton and featured as the front cover. This study employs machine-learning models to parse single-shot photoemission spectra and accurately pinpoint long-range superconducting phase ordering. The model recognizes subtle spectral cues invisible to traditional energy-gap analysis, unveiling the authentic transition even under heavy fluctuations. This approach enables rapid, non-contact detection of emergent phases in quantum materials, fueling fundamental exploration of strongly correlated systems and expediting the discovery of next-generation superconductors. Additionally, the domain-adversarial framework that merges large-scale simulation data with minimal experimental data provides a robust solution to the persistent "small data" issue in scientific machine learning.

Explainable Machine Learning Identification of Superconductivity from Single-Particle Spectral Functions

X Chen, Y Sun, E Hruska, V Dixit, J Yang, Y He, Y Wang, F Liu

Newton 1, 100066 (2025)

Congratulations to Maria and Joshua for their NSF Graduate Research Fellowship awards, especially amidst this very challenging time! Go get'em the high temperature superconductors!

Manipulator is back with expedited service from Fermi Instrument - big thanks! Getting to 2e-11 torr vacuum with a 3-week hard bakeout despite an unplanned building power outage - scary but we pulled through nearly unscathed, thanks to good vacuum circuit designs by Rongting and all. No better reason for a group toast!

Bam! Cold head of the cryo-manipulator busted with a clogged LN2 line in an early morning experiment... some down time ahead for a major repair.  Seeing water condensation and dripping mark inside an UHV chamber is not an everyday sight, and everyone rallying to the occasion is definitely a silver lining. Everybody chin up - bad times make good times better!

Our collaboration with Sohrab Ismail-Beigi, Charles Ahn and Fred Walker is now published as a perspective on how to use photoemission as a wavefunction mapping tool, instead of a boring band structure mapping tool. We show that by conceptually marrying the methodology from x-ray diffraction with multi-Brillouin zone photoemission matrix element, one may extend the framework of molecular orbital tomography to crystalline solids, and reconstruct the Wannier orbital after applying the sum rule on the spectral function. We propose three major future application scenarios: mapping density wave ordering, mapping surface catalysis processes, and probing the depth profile of surface electronic states.

Probing the Wannier function with multi-Brillouin-zone angle-resolved photoemission spectroscopy

Yu He, Frederick J. Walker, Charles H. Ahn, Sohrab Ismail-Beigi

Advanced Materials Interfaces 12, 2400427 (2024)

Farewell, Maria and David. Good luck to your next journey in quantum materials and energy sciences!

We finally found the LEEK and the Leak back to back! Hail to 2x10-11 torr vacuum after a year's struggle! Key takeaway: mumetal-SST welding joint is total bonkers.

Cheng and Weicheng's work on the band gap tuned excitonic system Ta2Ni(Se,S)5 system is now out online. In testing a half-century-old proposition by Nevill Mott (later expanded by Maurice Rice, Walter Kohn and Bertrand Halperin) on how a semimetal would turn into a semiconductor in the presence of electron-hole Coulomb interaction as the electronic bandgap is tuned from negative to positive, we discovered how strong interaction can fundamentally alter the behavior of such an evolution.

Anomalous excitonic phase diagram in band-gap-tuned Ta2Ni(Se,S)5

Cheng Chen, Weichen Tang, Xiang Chen, Zhibo Kang, Shuhan Ding, Kirsty Scott, Siqi Wang, Zhenglu Li, Jacob PC Ruff, Makoto Hashimoto, Dong-Hui Lu, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Eduardo H Neto, Robert J Birgeneau, Yulin Chen, Steven G Louie, Yao Wang, Yu He

Nature Communications 14, 7512 (2023)

Cheng and Xiang's work on lattice fluctuation induced pseudogap and the role of electron-phonon interaction in quasi-1-dimensional metal-to-insulator transition is published on Physical Review Research. This 18-page work pedagogically lay out the importance of lattice fluctuations and nonadiabatic phonons at low dimensions. It also reveals a negative electronic compressibility where the symmetry is on the brink of being broken. 

More at Yale News.

Role of electron-phonon coupling in excitonic insulator candidate Ta2NiSe5

Cheng Chen, Xiang Chen, Weichen Tang, Zhenglu Li, Siqi Wang, Shuhan Ding, Zhibo Kang, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Makoto Hashimoto, Donghui Lu, Jacob PC Ruff, Steven G Louie, Robert Birgeneau, Yulin Chen, Yao Wang, Yu He

Physical Review Research 5, 043089 (2023)

Congratulations to Wenxin for the BIOXFEL award, for his work on low dimensional antiferromagnet FexNbS2 presented at the 2023 SSRL/LCLS user meeting. 

Check out our recent collaboration with the Yi group at Rice University on how to introduce magnetic field into an in situ ARPES sample environment:

Angle-resolved photoemission spectroscopy with an in situ tunable magnetic field 

J Huang, Z Yue, A Baydin, H Zhu, H Nojiri, J Kono, Y He, M Yi 

Rev. Sci. Instrum. 94, 093902 (2023) 

Summer BBQ and boating at Yale OEC, planned by Zhibo and co. Siqi showed his sous vide skills, Rongting showed his boating arms, Tyler, Peter and Maria used boggle to help Jinming and Wenxin pick up new words.

That's a wrap! Another good summer with the Pathways to Science program and Yale Office of New Haven Affairs. Kudos to Maria, Jeremy, Brendan, and Fedora for being great TAs during the "magic of levitation" physics enrichment sessions!

Welcome, our new summer interns - Brendan, Fedora, and Jeremy! They will be working on hightrahigh vacuum bakeout tent design, micromagnetic field simulation, and mutual inductance investigations of superconductivity.

Check out our N&V on high-Tc superconductivity in the heavily hole-doped regime.

Ingrid, Nick, Peter, Eric, and Eddy join hands to pull together an exciting summer enrichment program with Yale Office of New Haven Affairs. Themed "the magic of levitation", live demonstrations of aerodynamic, diamagnetic, electromagnetic, acoustic, and superconducting levitations are shown to students from the greater New Haven area.

Congratulations to Eddy on winning the American Society for Metals (ASM) Materials Science Award as well as Terra Fairs Honors at this year’s Terra Science & Engineering Fairs for his work on microscopic breakdowns of Ohm’s Law using a mechanical Drude model.

Our multi-modal spectroscopy investigation of an electron-phonon coupling induced pseudogap in 1-D semimetal, entitled "Lattice fluctuation induced pseudogap in quasi-one-dimensional Ta2NiSe5", is posted on arXiv. Congrats to the hardwork by Cheng, Xiang, and the multi-continent collaboration!

Zhibo and Yu went to Cornell High Energy Synchrotron Source (CHESS) to find out the structural transition evolution in quasi-1D semimetal-semiconductor systems. We collaborate with Dr. Jacob Ruff and Dr. Xiang Chen on these high energy x-ray diffraction projects. By end of the 12 day beamtime, we have acquired > 50 TB data, for which both traditional data analysis and machine learning methods will be carried out on the high performance cluster with Yale Center for Research Computing (YCRC).

In metallic magnets, the magnetism can couple strongly to the conduction electrons. This offers many new possibilities to engineer "bespoke" magnets by tuning their low-energy electronic structures. For the opening experiment of 2022, we collaborate with Dr. Xiang Chen (UC Berkeley) and continued our adventure at the SLAC national lab (BL5-2@SSRL). Sprinkle some alkaline metal atoms on the surface of these 2D magnets (evaporation from the red-hot filament), and it shudders and emerges as a stronger magnet!

In collaboration with Han Wu, Dr. Ming Yi (Rice U) and Dr. Alexei Federov, with Tyler, Jinming and Zhibo connecting remotely, we jointly examine the electron spin distributions in low dimensional magnets at BL10 of Advanced Light Source (ALS). Ramming a 0.5 T magnet towards the sample surface in a chamber magnetically shielded to < 0.00000005 T is criminally fun (and nerve wrecking). The timeless view of the bay bridge, the golden gate and the city skyline from ALS gives a fitting end to the entire synchrotron run of 2021!

In collaboration with Dr. Ahmet Alatas and Dr. Ayman Said on the remote inelastic x-ray scattering beamtime at the Advanced Photon Source (APS, Argonne National Lab), we control the massive x-ray spectrometer to record the phonon excitations in the momentum space of quasi-1D metal-to-insulator transition systems. As the sample cools from water-boiling temperature, the rattling of the atoms counterintuitively grows near room temperature before calming down again at ice-cold temperatures. During this process, the electrons of this system receives the message from the lattice, and turn the material from a metal (conductor) into an insulator.

Jinming, Siqi and Rongting joined the in-person beamtime at the Stanford Synchrotron Radiation Lightsource (SLAC National Lab), with Tyler in remote standby, in collaboration with Dr. Donghui Lu, Dr. Makoto Hashimoto and Dr. Cheng Chen.  They worked in shifts over the full 72 hours to hunt down some of the most elusive electronic structures and energy gaps in magnets, thin film materials, and high-Tc superconductors.

Fluctuating superconductivity rarely coexists with good metals, because densely packed electrons render fluctuations energetically prohibitive. However, superconducting fluctuations are seen at least 40% above the transition temperature in the resistive state of a highly metallic high-Tc superconductor. A deeper look reveals the long overlooked role of an anisotropic flat band and the heavy electrons therein. This clarifies decades of dispute on the origin of the "pseudogap" in overdoped cuprates, with direct implications in novel superconductors such as moire flat band systems. Our work on persistent fluctuating superconductivity is now online at Phys. Rev. X 11, 031068 (2021).

The stars have finally aligned for the team at the summer's end. Welcome Siqi, buh-bye Ida and Henry! A group BBQ seems like the only right thing to do on a Friday afternoon. Guess who's the advisor in the pic?

Our lab building got christened by the torrential downpour from Hurricane Ida. Power went out at midnight. The backup generator was soaked under 4ft of water. The HiCube in the lab was shock vented. Luckily BCT326 had no substantial damage. Hopefully our UPS unit will arrive before the next Hurricane season. Yu and Rongting's grand midnight escape from the pitch-dark underground tunnel - along with the burbling sound from the ephemeral stairwell waterfalls - will likely be remembered for many years.

Congratulations to Ian on winning the Neville Smith poster award at the annual ALS user meeting! And who says it's too early for first-years to make original scientific contributions?

Our recent 72-page  ARPES review on Rev. Mod. Phys. – in collaboration with Dr. Jonathan Sobota and Dr. Zhi-Xun Shen – is featured on Yale School of Engineering and Applied Science news! This is a second installment to the 2003 review, which mostly focused on the basic formulation of photoemission principles and the high temperature cuprate superconductors. This new review aims to capture the vibrant diversification and modernization of angle-resolved photoemission spectroscopy over the past two decades. Give it a read!

Artwork: Jonathan A. Sobota/SLAC

After some 4000 miles of driving, sprinkled with lovely covid restrictions, midnight gunshot scare, thunderstorm escape, and hurricane-toppled light pole on I-95, the blue sky above Becton center is absolutely a sight of relief. To our neighbors just across the street, aka Dr. Lars Onsager and Dr. Josiah Willard Gibbs, hello! Here's to a great new school year!