Select Pressure-Induced Topological and Structural Phase Transitions in an Antiferromagnetic Topological Insulator Cuiying Pei, Yunyouyou Xia, Jiazhen Wu, Yi Zhao, Lingling Gao, Tianping Ying, Bo Gao, Nana Li, Wenge Yang, Dongzhou Zhang, Huiyang Gou, Yulin Chen, Hideo Hosono, Gang Li, Yanpeng Qi Chin. Phys. Lett.    2020, 37 (6): 066401 .   DOI: 10.1088/0256-307X/37/6/066401 Abstract   HTML   PDF (5392KB) Recently, natural van der Waals heterostructures of (MnBi$_{2}$Te$_{4}$)$_{m}$(Bi$_{2}$Te$_{3}$)$_{n}$ have been theoretically predicted and experimentally shown to host tunable magnetic properties and topologically nontrivial surface states. We systematically investigate both the structural and electronic responses of MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$ to external pressure. In addition to the suppression of antiferromagnetic order, MnBi$_{2}$Te$_{4}$ is found to undergo a metal–semiconductor–metal transition upon compression. The resistivity of MnBi$_{4}$Te$_{7}$ changes dramatically under high pressure and a non-monotonic evolution of $\rho (T)$ is observed. The nontrivial topology is proved to persist before the structural phase transition observed in the high-pressure regime. We find that the bulk and surface states respond differently to pressure, which is consistent with the non-monotonic change of the resistivity. Interestingly, a pressure-induced amorphous state is observed in MnBi$_{2}$Te$_{4}$, while two high-pressure phase transitions are revealed in MnBi$_{4}$Te$_{7}$. Our combined theoretical and experimental research establishes MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$ as highly tunable magnetic topological insulators, in which phase transitions and new ground states emerge upon compression.
 Select Resonant Quantum Search with Monitor Qubits Frank Wilczek, Hong-Ye Hu, Biao Wu Chin. Phys. Lett.    2020, 37 (5): 050304 .   DOI: 10.1088/0256-307X/37/5/050304 Abstract   HTML   PDF (582KB) We present an algorithm for the generalized search problem (searching $k$ marked items among $N$ items) based on a continuous Hamiltonian and exploiting resonance. This resonant algorithm has the same time complexity $O(\sqrt{N/k})$ as the Grover algorithm. A natural extension of the algorithm, incorporating auxiliary "monitor" qubits, can determine $k$ precisely, if it is unknown. The time complexity of our counting algorithm is $O(\sqrt{N})$, similar to the best quantum approximate counting algorithm, or better, given appropriate physical resources.
 Select Imaginary Time Crystal of Thermal Quantum Matter Zi Cai, Yizhen Huang, W. Vincent Liu Chin. Phys. Lett.    2020, 37 (5): 050503 .   DOI: 10.1088/0256-307X/37/5/050503 Abstract   HTML   PDF (2767KB) Temperature is a fundamental thermodynamic variable for matter. Physical observables are often found to either increase or decrease with it, or show a non-monotonic dependence with peaks signaling underlying phase transitions or anomalies. Statistical field theory has established connection between temperature and time: a quantum ensemble with inverse temperature $\beta$ is formally equivalent to a dynamic system evolving along an imaginary time from 0 to $i\beta$ in the space one dimension higher. Here we report that a gas of hard-core bosons interacting with a thermal bath manifests an unexpected temperature-periodic oscillation of its macroscopic observables, arising from the microscopic origin of space-time locked translational symmetry breaking and crystalline ordering. Such a temperature crystal, supported by quantum Monte Carlo simulation, generalizes the concept of purely spatial density-wave order to the imaginary time axis for Euclidean action.
 Select Consistent Scaling Exponents at the Deconfined Quantum-Critical Point Anders W. Sandvik, Bowen Zhao Chin. Phys. Lett.    2020, 37 (5): 057502 .   DOI: 10.1088/0256-307X/37/5/057502 Abstract   HTML   PDF (686KB) We report a quantum Monte Carlo study of the phase transition between antiferromagnetic and valence-bond solid ground states in the square-lattice $S=1/2$ $J$–$Q$ model. The critical correlation function of the $Q$ terms gives a scaling dimension corresponding to the value $\nu = 0.455 \pm 0.002$ of the correlation-length exponent. This value agrees with previous (less precise) results from conventional methods, e.g., finite-size scaling of the near-critical order parameters. We also study the $Q$-derivatives of the Binder cumulants of the order parameters for $L^2$ lattices with $L$ up to $448$. The slope grows as $L^{1/\nu}$ with a value of $\nu$ consistent with the scaling dimension of the $Q$ term. There are no indications of runaway flow to a first-order phase transition. The mutually consistent estimates of $\nu$ provide compelling support for a continuous deconfined quantum-critical point.
 Select Unusual Destruction and Enhancement of Superfluidity of Atomic Fermi Gases by Population Imbalance in a One-Dimensional Optical Lattice Qijin Chen, Jibiao Wang, Lin Sun, Yi Yu Chin. Phys. Lett.    2020, 37 (5): 053702 .   DOI: 10.1088/0256-307X/37/5/053702 Abstract   HTML   PDF (865KB) We study the superfluid behavior of a population imbalanced ultracold atomic Fermi gases with a short range attractive interaction in a one-dimensional (1D) optical lattice, using a pairing fluctuation theory. We show that, besides widespread pseudogap phenomena and intermediate temperature superfluidity, the superfluid phase is readily destroyed except in a limited region of the parameter space. We find a new mechanism for pair hopping, assisted by the excessive majority fermions, in the presence of continuum-lattice mixing, which leads to an unusual constant Bose-Einstein condensate (BEC) asymptote for $T_{\rm c}$ that is independent of pairing strength. In result, on the BEC side of unitarity, superfluidity, when it exists, may be strongly enhanced by population imbalance.
 Select Metal to Orthogonal Metal Transition Chuang Chen, Xiao Yan Xu, Yang Qi, Zi Yang Meng Chin. Phys. Lett.    2020, 37 (4): 047103 .   DOI: 10.1088/0256-307X/37/4/047103 Abstract   HTML   PDF (1184KB) Orthogonal metal is a new quantum metallic state that conducts electricity but acquires no Fermi surface (FS) or quasiparticles, and hence orthogonal to the established paradigm of Landau's Fermi-liquid (FL). Such a state may hold the key of understanding the perplexing experimental observations of quantum metals that are beyond FL, i.e., dubbed non-Fermi-liquid (nFL), ranging from the Cu- and Fe-based oxides, heavy fermion compounds to the recently discovered twisted graphene heterostructures. However, to fully understand such an exotic state of matter, at least theoretically, one would like to construct a lattice model and to solve it with unbiased quantum many-body machinery. Here we achieve this goal by designing a 2D lattice model comprised of fermionic and bosonic matter fields coupled with dynamic $\mathbb{Z}_2$ gauge fields, and obtain its exact properties with sign-free quantum Monte Carlo simulations. We find that as the bosonic matter fields become disordered, with the help of deconfinement of the $\mathbb{Z}_2$ gauge fields, the system reacts with changing its nature from the conventional normal metal with an FS to an orthogonal metal of nFL without FS and quasiparticles and yet still responds to magnetic probe like an FL. Such a quantum phase transition from a normal metal to an orthogonal metal, with its electronic and magnetic spectral properties revealed, is calling for the establishment of new paradigm of quantum metals and their transition with conventional ones.
 Select Discriminating High-Pressure Water Phases Using Rare-Event Determined Ionic Dynamical Properties Lin Zhuang, Qijun Ye, Ding Pan, Xin-Zheng Li Chin. Phys. Lett.    2020, 37 (4): 043101 .   DOI: 10.1088/0256-307X/37/4/043101 Abstract   HTML   PDF (20890KB) Recent discoveries of dynamic ice VII and superionic ice highlight the importance of ionic diffusions in discriminating high-pressure ($P$) water phases. The rare event nature and the chemical bond breaking associated with these diffusions, however, make extensive simulations of these processes unpractical to ab initio and inappropriate for force field based methods. Using a first-principles neural network potential, we performed a theoretical study of water at 5–70 GPa and 300–3000 K. Long-time dynamics of protons and oxygens were found indispensable in discriminating several subtle states of water, characterized by proton's and oxygen ion's diffusion coefficients and the distribution of proton's displacements. Within dynamic ice VII, two types of proton transfer mechanisms, i.e., translational and rotational transfers, were identified to discriminate this region further into dynamic ice VII T and dynamic ice VII R. The triple point between ice VII, superionic ice (SI), and liquid exists because the loosening of the bcc oxygen skeleton is prevented by the decrease of interatomic distances at high $P$'s. The melting of ice VII above $\sim$40 GPa can be understood as a process of two individual steps: the melting of protons and the retarded melting of oxygens, responsible for the forming of SI. The boundary of the dynamic ice VII and SI lies on the continuation line ice VII's melting curve at low $P$'s. Based on these, a detailed phase diagram is given, which may shed light on studies of water under $P$'s in a wide range of interdisciplinary sciences.
 Select A New Path to Improve High $\beta_{\rm p}$ Plasma Performance on EAST for Steady-State Tokamak Fusion Reactor Baonian Wan and the EAST team Chin. Phys. Lett.    2020, 37 (4): 045202 .   DOI: 10.1088/0256-307X/37/4/045202 Abstract   HTML   PDF (573KB) High $\beta_{\rm p}$ scenario is foreseen to be a promising candidate operational mode for steady-state tokamak fusion reactors. Dedicated experiments on EAST and data analysis find that density gradient $\nabla n$ is a control knob to improve energy confinement in high $\beta_{\rm p}$ plasmas at low toroidal rotation as projected for a fusion reactor. Different from previously known turbulent stabilization mechanisms such as ${\boldsymbol E} \times {\boldsymbol B}$ shear and Shafranov shift, high density gradient can enhance the Shafranov shift stabilizing effect significantly in high $\beta_{\rm p}$ regime, giving that a higher density gradient is readily accessible in future fusion reactors with lower collisionality. This new finding is of great importance for the next-step fusion development because it may open a new path towards even higher energy confinement in the high $\beta_{\rm p}$ scenario. It has been demonstrated in the recent EAST experiments, i.e., a fully non-inductive high $\beta_{\rm p}$ ($\sim$2) H-mode plasma ($H_{98y2}\ge 1.3$) has been obtained for a duration over 100 current diffusion times, which sets another new world record of long-pulse high-performance tokamak plasma operation with the normalized performance approaching the ITER and CFETR regimes.
 Select High-Pressure Ultrafast Dynamics in Sr$_{2}$IrO$_{4}$: Pressure-Induced Phonon Bottleneck Effect Yanling Wu, Xia Yin, Jiazila Hasaien, Yang Ding, Jimin Zhao Chin. Phys. Lett.    2020, 37 (4): 047801 .   DOI: 10.1088/0256-307X/37/4/047801 Abstract   HTML   PDF (4486KB) By integrating pump-probe ultrafast spectroscopy with diamond anvil cell (DAC) technique, we demonstrate a time-resolved ultrafast dynamics study on non-equilibrium quasiparticle (QP) states in Sr$_{2}$IrO$_{4}$ under high pressure. On-site in situ condition is realized, where both the sample and DAC have fixed position during the experiment. The QP dynamics exhibits a salient pressure-induced phonon bottleneck feature at 20 GPa, which corresponds to a gap shrinkage in the electronic structure. A structural transition is also observed at 32 GPa. In addition, the slowest relaxation component reveals possible heat diffusion or pressure-controlled local spin fluctuation associated with the gap shrinkage. Our work enables precise pressure dependence investigations of ultrafast dynamics, paving the way for reliable studies of high-pressure excited state physics.
 Select Pressure-Induced Metallization and Structural Phase Transition in the Quasi-One-Dimensional TlFeSe$_{2}$ Zi-Yi Liu, Qing-Xin Dong, Peng-Fei Shan, Yi-Yan Wang, Jian-Hong Dai, Rajesh Jana, Ke-Yu Chen, Jian-Ping Sun, Bo-Sen Wang, Xiao-Hui Yu, Guang-Tong Liu, Yoshiya Uwatoko, Yu Sui, Huai-Xin Yang, Gen-Fu Chen, Jin-Guang Cheng Chin. Phys. Lett.    2020, 37 (4): 047102 .   DOI: 10.1088/0256-307X/37/4/047102 Abstract   HTML   PDF (1268KB) We report a comprehensive high-pressure study on the monoclinic TlFeSe$_{2}$ single crystal, which is an antiferromagnetic insulator with quasi-one-dimensional crystal structure at ambient pressure. It is found that TlFeSe$_{2}$ undergoes a pressure-induced structural transformation from the monoclinic phase to an orthorhombic structure above $P_{\rm c} \approx 13$ GPa, accompanied with a large volume collapse of $\Delta V/V_{0}=8.3{\%}$. In the low-pressure monoclinic phase, the insulating state is easily metallized at pressures above 2 GPa; while possible superconductivity with $T_{\rm c}^{\rm onset} \sim 2$ K is found to emerge above 30 GPa in the high-pressure phase. Such a great tunability of TlFeSe$_{2}$ under pressure indicates that the ternary $A$FeSe$_{2}$ system ($A$ = Tl, K, Cs, Rb) should be taken as an important platform for explorations of interesting phenomena such as insulator-metal transition, dimensionality crossover, and superconductivity.