Buhin selection 2021 ~ Top Ten Condensed Matter Physics Papers of the Year ~

It's the end of 2021.
One year goes by fast. It goes by so fast.
This year has been full of challenges.

However, research in the world has not stopped.
Many interesting papers have been published this year.
In this blog, I have covered about 2,100 Cond-mat papers as a hobby.
So, I selected 10 papers (themes) from them that I liked in the past year.

【1, Realization of topological superconductivity in 45°-twisted bilayer copper oxides.】
 Twisttronics", which is the emergence and control of physical properties by twisting and stacking van der Waals materials, is in vogue. Starting with magic angle graphene, various phenomena such as superconductivity, Chern insulators, and Wigner crystals have been observed in transition metal dichalcogenides and various heterojunctions.
 One of them is Bi2Sr2CaCu2O8+x (Bi2212), which is stacked by van der Waals forces. When Bi2212 is stacked with 45° twisting, the Cooper pair tunneling between the layers is suppressed due to the d-wave superconducting symmetry, but it has been predicted that a high-temperature topological superconducting state would be realized due to the secondary tunneling effect. Since the topological superconducting state has Majorana excitations that can be applied to quantum computers, we will be happy if it is actually realized at the transition temperature of cuprate superconductors (Tc~100K).
 In this year, a study that experimentally realizes this bold prediction [1] was finally reported. They developed a reproducible method to stack Bi2212 with 45° twisting, which is difficult to maintain the superconducting state due to moisture damage and oxygen migration in a single layer. Using this method, they succeeded in observing half-integer Shapiro steps and Fraunhofer patterns of dV/dI, which are signs of topological superconductivity. Theoretical explanations for this phenomenon [2, 3] and follow-up tests [4] have also been reported.
 The time has come for copper oxides after all!

【2, Discovery of the magnetic field-induced superconducting phase in CeRh2As2】
    Although many unconventional superconductors have been discovered, such as copper oxides and URu2Si2, only UPt3 has been reported as a material whose superconducting state changes with magnetic field and temperature. (According to [5]. What about, say, FFLO phase in organic superconducting materials?)
    In this year, a second superconducting phase material was reported: CeRh2As2[5], a heavy fermion material with broken local inversion symmetry, where a new superconducting phase was observed to appear when a magnetic field of 5T or higher was applied in the c-axis direction at Tc~0.3K. This is exciting.
 It seems that spin-orbit interaction, multipole order, and antiferromagnetic order below the superconducting transition temperature affect this phenomenon, but there are still many unanswered questions[6-10]. I look forward to future research.

【3, Grand Research Festival on the Topological Kagome Superconductor RV3Sb5】
  The geometrical structure of a crystal is an important factor in determining the properties of solid-state properties, as well as electron correlation. In such a situation, RV3Sb5 (R=K, Cs, Rb) with a Kagome lattice structure was discovered. This material is a superconductor with Tc~0.9K, has a transition associated with some kind of ordered state at T=80K, shows anomalous Hall effect without magnetic ordering, and has a band structure with Z2 topological invariants. 
 This year was the year of the topological Kagome material RV3Sb5, and so many papers were submitted to Cond-mat on a daily basis. Starting with the observation of topological charge ordering [15], which appeared on New Year's Day, many discoveries were made, including the discovery (or no discovery) of a hidden chiral flux phase  [12, 13], and the discovery of a two-dome superconducting phase by pressure application or element substitution [14-16].
 The question "What is the material of the year?" It would not be an exaggeration to say that RV3Sb5 is the material of the year.

【4. The Great Controversy of C-S-H High-Pressure Room Temperature Superconductors】
 A criticism paper on the C-S-H high-pressure superconductor, the first superconductor above room temperature in human history, which was discovered in 2020 and reported in Nature, has appeared, asking "Is it really superconducting?  Science has also been stirring things up (*2).
 Dr. J. Hirsh, who is famous for inventing the h-index, criticized the paper on high-pressure superconductivity in the C-S-H system, saying, "They wouldn't disclose the raw data when I asked for it," "The experimental results look just like the old results," and "Isn't it a fraud? (17). On the other hand, the authors of the paper on high-pressure superconductivity in the C-S-H system also countered by saying, "We can't give data to trolls who don't understand BCS theory," "We can't get accurate values from the figures in the paper, which are not vector images," and "Our research has progressed, and the pressure that used to require 200 GPa is now about 20 GPa, and we are preparing a paper!" Dr. Hirsh's critical paper was once published in Physica C, but was withdrawn for including a private letter in the paper without permission, while a paper on Eu high-pressure superconductivity, in which the author of the C-S-H paper was involved in the past, was also withdrawn. It has been a pleasure to watch from outside the ring.
 In the end, I think the key will be to identify the structure of the material that realizes superconductivity and the follow-up tests by multiple groups, but this is a theme that we will continue to look forward to next year and beyond.

【5,Discovery of a string-like 3D skyrmion structure】
 Skyrmions, which are topological spin excitations, have been observed in chiral magnets and magnetic multilayers, and have attracted attention from both fundamental and applied aspects. Actually, The Young Scientist Award of the Physical Society of Japan 2021 was given to researchers who study Skyrmions. However, the skyrmions observed so far are in two-dimensional states.
 This year, several groups have reported the observation of skyrmion tubes in bulk magnetic materials, where the skyrmions have a three-dimensional string-like shape [19-22], visualized by X-ray and electron tomography.

【6, Application of quantum geometry to condensed matter physics】
 It has been known that the quantum Hall effect and the anomalous Hall effect can be explained by bringing a geometrical viewpoint to the description of quantum states. On the other hand, it has been considered difficult to apply quantum geometry to the description of optical transition processes. This is because quantum geometry is defined for a single quantum state, while optical transitions are phenomena that occur between multiple quantum states.
 In this paper [23], they propose a theory that can describe optical transitions in a unified manner from the viewpoint of quantum geometry by introducing Riemannian geometry. Based on this theory, they find that the third-order photovoltaic Hall effect can be linked to the Riemann curvature tensor. Furthermore, by extending the theory, they have shown that various static electromagnetic field responses can be connected to Riemannian geometry.
 To be honest, I don't fully understand anything about it, but given that topological condensed matter theory is all the rage, I wonder if quantum geometry with Riemannian geometry will be the next big thing. This is why I selected it as a field to watch.

【7, Development of Band Structure Visualization Method】
  in condensed matter, it is a band structure that determines its various electronic properties. Angle Resolved Photo Emission Spectroscopy (ARPES) is one of the most famous band structure visualization methods to understand electronic states and make cool graphs.
    Among the yearly evolution of ARPES, three interesting methods have been introduced: 1) FeSuMa (Fermi Surface Mapper) [29], a method to visualize 3D Fermi surfaces, 2) pump-probe method to visualize excited-state band structures [30], and spin-angle-resolved "inverse" photoemission [31]. All of these methods have achieved sophisticated measurements that were previously considered extremely difficult, if not impossible, and I expect that they will reveal new phenomena that have not been observed so far. 
    After all, ARPES is the most powerful!

【8, Is topological Kondo insulator SmB6 actually a metal?】
 The origin of Kondo insulator state in SmB6 has attracted much attention because the electrical resistance of SmB6 saturates at a constant value at low temperatures despite being an insulator, and quantum oscillations suggesting the existence of the Fermi surface are observed only in the magnetization. In particular, it has been a matter of debate whether these properties originate from the topological surface state or from the mysterious Fermi surface in the bulk.
 This year, two interesting results were reported on this phenomenon.
 In the first paper [32], the authors asked the question, "If we can see quantum oscillations in magnetization, shouldn't we also see them in specific heat, the same bulk thermodynamic quantity?" they reported a positive answer to this question by measuring the specific heat in magnetic fields down to 0.1 K and up to 32 T.
 The second paper [33] reports a positive answer to the question "Does the Fermi surface really exist in SmB6? By using X-ray Compton scattering, which is sensitive to the electron momentum distribution, the authors reported that the remnants of the Fermi surface exist in the bulk SmB6. This suggests that SmB6 is neither a perfect metal nor a perfect insulator, but an unconventional insulator.
 The possibility of a new material phase that has not been observed so far is exciting, and I look forward to follow-up tests and further developments.

【9, Realization of Zero-field Superconducting Diode Effect】
 As the recent news on the shortage of semiconductors indicate, semiconductors have important properties in terms of both fundamentals and applications. One of the most important is the so-called diode effect, which is a rectifying effect in which current flows only in one direction. It has been reported that the analogy of this diode effect in superconductors, i.e., the Josephson diode effect, in which superconducting current flows only in one direction, can be realized by successfully combining magnetic fields and magnetic anisotropy. However, it has difficulties to use a magnetic field because it destroys the superconductivity.
 This year, a study was reported in which the Josephson diode effect was realized in a zero magnetic field [34]. They reported that Josephson diodes consisting of NbSe2/Nb3Br8/NbSe2 heterostructures, which break the inversion symmetry, have an amplification factor of 10^4 and a repetition tolerance of 10^4 times. In addition, many other superconducting diodes using other materials, such as magic angle graphene and EuS, and related theories have been reported [35-42].
    I have the impression that superconducting diode research is quietly booming. Recently, a race to develop a quantum computer using superconducting qubits has been raging around the world, and I was selected in the hope that these new superconducting devices will accelerate such a race.

【10, The discovery of Z2 bosonic metallic state of iron-based superconductors.】
 The characteristics of superconductors are zero resistance and the Meissner effect, which is repulsive to magnetic fields. These phenomena should be observed in superconductors. However, there are always exceptions to this rule.
 In this paper [43], they report that in Ba1-xKxFe2As2, one of the iron-based superconductors, fermionic pairing occurs above the superconducting transition temperature only around x=0.8, resulting in a bosonic metallic state with broken time-reversal symmetry and superconductivity due to four-electron correlation. The existence of the bosonic metallic state has been verified by various measurement methods such as specific heat, Nernst effect, Seebeck effect, electrical resistivity, MuSR, and ultrasound measurements. I was impressed by their ability to capture a phenomenon that exists only in a very limited range through precise experiments. How did they find such subtle changes? 
 The bosonic metallic state of iron-based superconductivity has also been reported in a study of nanofabricated FeSe thin films [44], and I look forward to further research on this new material phase.

【 Extra papers 】
【11. Controversy over whether the thermal Hall effect is quantized in the field-induced quantum spin liquid phase of RuCl3. 】
 Since it was reported that the thermal Hall effect in the magnetic field-induced quantum spin liquid phase of RuCl3 is quantized (*1, *2), various groups have been conducting follow-up tests.
 One of the major developments is a follow-up study by the German group [45]. Using the same sample as one of the original Japanese group, they succeeded in observing the quantized thermal Hall effect at low temperatures. They have also provided new insights into the mechanism of the quantization. On the other hand, the American and Canadian groups also conducted follow-up experiments. The American group did not observe quantization of the thermal Hall effect, but instead reported quantum oscillations of Thermal conductivity [46]. Although the quantum oscillations indicate the existence of mysterious neutral excitations in the quantum spin liquid phase, the thermal Hall effect is interpreted as caused by bosonic excitations and not half-integer fermion excitations (Seminar video). A follow-up study by the Canadian group also questioned the quantized thermal Hall effect due to Majorana excitations [47], as no quantization was observed, but rather the phonon contribution was dominant.
 In response to this confusion, Prof. P.A. Lee has published a comment [48] in the Journal Club for Condensed Matter summarizing the recent situation. In this comment, in the form of a private letter from the German group, he stated that "quantization or not depends on whether the sample is made by the FZ method or the CVT method."
 I will keep my eyes on this theme next year.
【12, Tardigrade Quantum Entanglement. 】
 A paper on the realization of quantum entanglement by embedding tardigrades in superconducting qubit circuits [49]. To be honest, this is the number one paper of the year from a pictorial point of view. It seems that the tardigrades were still alive after being exposed to ultra-low temperatures, and I take my hat off to them for their vitality.
【13, Catalytic effect using moiré structure. 】
 Many papers related to moiré structure were published this year. Most of them were reports of emergent phenomena such as superconductivity and Chern insulators. Theoretical calculations using NbS2 as a representative example show that its efficiency for hydrogen evolution is higher than that of platinum, the highest efficiency available.
【14, Discovery of a new p-wave superconductor, A2Cr3As3. 】
 After the p-wave superconductivity of Sr2RuO4 was sadly rejected, a new p-wave superconductor, A2Cr3AS3 (A is an alkali metal), has been discovered.
【15, Observation of Cooper pair formation in a cooled atomic system.】
 Cooper pairs formed by pairs of electrons are the basis of superconductivity, but it is difficult to directly observe Cooper pairs in a solid. However, it is difficult to directly observe Cooper pairs in solids. In this paper [53], they report the observation of the Cooper pair formation process using cold atoms. 
[16, Microsoft's Majorana paper retraction issue. 】
 The Majorana excitation in superconductor/semiconductor heterostructure  reported by Microsoft and Delft group was retracted from the Nature due to inappropriate presentation of the data. A subsequent elaborate theoretical paper [54] reported that the observed Majorana excitations were not real, but that they were watching a different phenomenon of disorder origin. This was a case where I can learn the importance of a clean sample and careful consideration before making strong claims.

【17, Observation of a mysterious neutral particle in YbB12 and new phenomena in other Yb compounds. 】
 YbB12 is a Kondo insulator that exhibits quantum oscillations in both electrical resistance and magnetization, similar to SmB6. Research on this material has progressed this year, and a theory that it may be a coexistence of an ordinary Fermi particle and a mysterious neutral particle has emerged and, is still being discussed [55-59].
 In the field of Yb-based compounds, a finite thermal conduction originating from a mysterious neutral particle was observed in the insulating phase of YbIr3Si7 from thermal conduction measurements [60], and an ultralow-temperature superconductivity of Tc~6mK was discovered in YbRh2Si2 [61].
【18, Interesting research using Bose-Einstein condensation. 】
 Bose-Einstein condensation is an interesting phenomenon in itself, but this year there were many interesting researches that utilized it. Gravitational wave detection [62], quantum gas microscope with lattice resolution [63], observation of Hubble decay/amplification [64], ultra-high-resolution magnetometer with Planck constant accuracy [65], and detection of gravitational redshift [66] are all first-class observations.

【19, Deriving the fluid equation of a quantum many-body system by machine learning. 】
 We are living in the age of Artificial Intelligence (AI). AI is also playing an active role in the world of physics, where it is being used in various applications such as advanced measurement and material informatics. The paper proposes a method to symbolically obtain a table of fluid equations from quantum many-body data using machine learning, and successfully reproduced existing equations and discovered unknown equations. I enjoy the technique of extracting unknown laws from data, as it seems to provide us with new ideas.