We achieve the robust nonadiabatic holonomic two-qubit controlled gate in one step based on the ground-state blockade mechanism between two Rydberg atoms. By using the Rydberg-blockade effect and the Raman transition mechanism, we can produce the blockade effect of double occupation of the corresponding ground state, i.e., ground-state blockade, to encode the computational subspace into the ground state, thus effectively avoiding the spontaneous emission of the excited Rydberg state. On the other hand, the feature of geometric quantum computation independent of the evolutionary details makes the scheme robust to control errors. In this way, the controlled quantum gate constructed by our scheme not only greatly reduces the gate infidelity caused by spontaneous emission but is also robust to control errors.

We predict a new physical phenomenon, induced fission-like process and chain reaction of hadronic molecular states. As a molecular state, if induced by a $D$ meson, the $X(3872)$ can split into $D\bar{D}$ final state which is forbidden due to the spin-parity conservation. The breeding of the $D$ meson of the reaction, such as $D^0X(3872)\to D^0\bar{D}^0D^0$, makes the chain reaction of $X(3872)$ matter possible. We estimate the cross section of the $D$ meson induced fission-like process of $X(3872)$ into two $D$ mesons. With very small $D^0$ beam momentum of 1 eV, the total cross section reaches an order of 1000 b, and decreases rapidly with the increasing beam momentum. With the transition of $D^*$ meson in molecular states to a $D$ meson, the $X(3872)$ can release large energy, which is acquired by the final mesons. The momentum distributions of the final $D$ mesons are analyzed. In the laboratory frame, the spectator $D$ meson in molecular state concentrates in the low momentum area. The energy from the transition from $D^*$ to $D$ meson is mainly acquired by two scattered $D$ mesons. The results suggest that the $D$ meson environment will lead to the induced fission-like process and chain reaction of the $X(3827)$. Such a phenomenon can be extended to other hadronic molecular states.

Using a three-dimensional classical ensemble model, we investigate the dependence of relative frequency and relative initial phase for nonsequential double ionization (NSDI) of atoms driven by orthogonal two-color (OTC) fields. Our findings reveal that the NSDI probability is clearly dependent on the relative initial phase of OTC fields at different relative frequencies. The inversion analysis results indicate that adjusting the relative frequency of OTC fields helps control returning probability and flight time of the first electron. Furthermore, manipulating the relative frequency at the same relative initial phases can vary the revisit time of the recolliding electron, leading that the emission direction of Ar$^{2+}$ ions is explicitly dependent on the relative frequency.

We report on experimental measurements of the transition dipole moments (TDMs) between the intermediate state $5{s}5{p}\, ^3\!{P}_1$ and the triplet Rydberg series $5{s}n{s}\, ^3\!{S}_1$ in an ultracold strontium gas. Here $n$ is the principal quantum number ranging from 19 to 40. The transition $5{s}5{p}\, ^3\!{P}_1$–$5{s}n{s}\, ^3\!{S}_1$ is coupled via an ultraviolet (UV) beam, inducing Autler–Townes splitting of both states. Such a splitting of the intermediate state is spectroscopically measured by using absorption imaging on a narrow transition $5{s^2}\, ^1{S}_0$–$5{s}5{p}\, ^3\!{P}_1$ in an ultracold gas of strontium atoms. The power and size of the UV beam are carefully determined, with which the TDMs are extracted from the measured Autler–Townes splitting. The experimentally obtained TDMs are compared to the calculations based on a parametric core potential, on a Coulomb potential with quantum defect, and on the open-source library Alkali Ryderg calculator, finding good agreement with the former two models and significant deviation with the latter.

FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)

Electromagnetic metasurface with chaos patterned surface could bring rich interaction modes contributing to fully disordered random motions in deterministic systems, which preform uncertainty, irreducibility and unpredictability. We investigate the influence of the correlation function (CF) properties of surface random patterns on the wave absorption performance. The complicated correlation function provides a fully developed random state, broadening the absorption bandwidth significantly and is helpful for reaching higher absorption rate. With the increasing number of peaks in the correlation function, the absorption band at $-15$ dB reflectivity widens significantly, band at $-20$ dB reflectivity begins to emerge. As the first peak's distance from the original point in the CF is enlarged, the absorption trough is gradually formed and deepened to $-35$ dB level. The results give in-depth understanding of the relation between absorption behavior and controlling parameters including correlation, image information and foam spacer layer thickness. This high absorption absorber has great application potential in customizable radio communication compatibility device and anechoic testing chamber.

We reveal a special subset of non-degenerate Akhmediev breather (AB) solutions of Manakov equations that only exist in the focusing case. Based on exact solutions, we present the existence diagram of such excitations on the frequency-wavenumber plane. Conventional single-frequency modulation instability leads to simultaneous excitation of three ABs with two of them being non-degenerate.

CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES

Strontium titanate (SrTiO$_{3}$) is a typical perovskite-type ceramic oxide and studying its high-pressure phases are critical to understand the ferroelastic phase transition. SrTiO$_{3}$ also can be used as an important analog of davemaoite (CaSiO$_{3}$) to understand the compositional and velocity structure of the Earth's interior. However, the high-pressure studies on the cubic-to-tetragonal phase transition pressure and elastic properties remain unclear for SrTiO$_{3}$. Here, we investigate the phase transition and elasticity of single-crystal SrTiO$_{3}$ by Raman and Brillouin scattering combined with diamond anvil cell. The acoustic velocities of single-crystal SrTiO$_{3}$ and the independent elastic constants of cubic and tetragonal SrTiO$_{3}$ are determined up to 27.5 GPa at room temperature. This study indicates that $C_{11}$, $C_{12}$, and $C_{44}$ exhibit abnormal changes at 10.3 GPa, which is related to the cubic-to-tetragonal phase transition. Interestingly, a significant softening on shear modulus and a large anisotropy of shear wave splitting ($A_{\mathrm{S}}^{\mathrm{PO}}$) jump are observed at 10.3 GPa. Using obtained elastic constants, the coefficients of the Landau potential are calculated to understand the phase transition between cubic and tetragonal. The calculated coefficients of the Landau potential are, $\lambda_{2} = 3.12\times 10^{-2}$ GPa, $\lambda_{4} = -2.02 \times 10^{-2}$ GPa, $B^* = 1.34 \times 10^{-4}$ GPa and $B = 1.66\times 10^{-4}$ GPa. The elastic results have profound implications in understanding the structure of the Earth's interior and indicate that the presence of tetragonal Ti-bearing CaSiO$_{3}$ helps to explain the large $A_{\mathrm{S}}^{\mathrm{PO}}$ of the Earth's mid-mantle.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

A central research topic in condensed matter physics is the understanding of the evolution of various phases and phase transitions under different tuning parameters such as temperature, magnetic field and pressure. To explore the pressure-induced evolution of the magnetism and Fermi surface of the heavy fermion antiferromagnet YbPtBi, we performed tunnel diode oscillator based measurements under pressure at low temperatures in high magnetic fields. Our results reveal that the magnetic order strengthens and the Fermi surface shrinks as the pressure increases, which are consistent with typical observations for Yb-based heavy fermion compounds. In addition, an anomalous change in the quantum oscillation amplitudes is observed above 1.5 GPa, and determining the origin requires further study.

We utilize both analytical and numerical methods to study the superconducting transition temperature $T_{\rm c}$ near a fermionic quantum critical point (QCP) using a model constructed by Xu et al. [Phys. Rev. X7, 031059 (2017)] as an example. In this model, the bosonic critical fluctuation plays the role of pairing glue for the Cooper pairs, and we use a Bardeen–Cooper–Schrieffer-type mean-field theory to estimate $T_{\rm c}$. We further argue that the $T_{\rm c}$ computed from the BCS theory approximates a pseudogap temperature $T_{\rm PG}$, instead of the Berezinskii–Kosterlitz–Thouless transition temperature $T_{\rm KT}$, which is confirmed by our determinant quantum Monte Carlo simulation. Moreover, due to the fact that electron density of state starts to deplete at $T_{\rm PG}$, the critical scaling of the underlying QCP is also affected below $T_{\rm PG}$. Thus, when studying the critical behavior of fermionic QCPs, we need to monitor that the temperature is above $T_{\rm PG}$ instead of $T_{\rm KT}$. This was often ignored in previous studies.

We study ultra-high-mobility two-dimensional (2D) electron/hole systems with high precision capacitance measurement. It is found that the capacitance charge appears only at the fringe of the gate at high magnetic field when the 2D conductivity decreases significantly. At integer quantum Hall effects, the capacitance vanishes and forms a plateau at high temperatures $T\gtrsim 300$ mK, which surprisingly disappears at $T\lesssim 100$ mK. This anomalous behavior is likely a manifestation that dilute particles/vacancies in the top-most Landau level form Wigner crystals, which have finite compressibility and can host polarization current.

For conductors in the ballistic regime, electron-boundary scattering at the sample edge plays a dominant role in determining the transport performance, giving rise to many intriguing phenomena like low-field negative magnetoresistance effect. We systematically investigate the magneto-transport behaviors of BN-encapsulated graphene devices with narrow channel width $W$, wherein the bulk mean free path $L_{\rm mfp}$ can be very large and highly tunable. By comparing the magnetoresistance features and the amplitude of $L_{\rm mfp}$ in a large parameter space of temperature and carrier density, we reveal that the boundary-scattering-dominated negative magnetoresistance effect can still survive even when the ballistic ratio ($L_{\rm mfp}/W$) is as low as 0.15. This striking value is much smaller than the expected value for achieving (quasi-) ballistic transport regime ($L_{\rm mfp}/W \ge 1$), and can be attributed to the ultra-low specularity of the sample edge of our graphene devices. These findings enrich our understanding of the effects of boundary scattering on channel transport, which is of vital importance for future designs of two-dimensional electronic devices with limited lateral sizes.

We experimentally investigate second-harmonic transverse voltage response to ac electrical current for a magnetic nodal-line semimetal Fe$_3$GeTe$_2$ (FGT). For zero magnetic field, the observed second-harmonic voltage behaves as a square of the longitudinal current, as it should be expected for nonlinear Hall effect. The magnetic field behavior is found to be sophisticated: while the first-harmonic response shows the known anomalous Hall hysteresis in FGT, the second-harmonic Hall voltage is characterized by the pronounced high-field hysteresis and flat ($B$-independent) region with curves touching at low fields. The high-field hysteresis strongly depends on the magnetic field sweep rate, so it reflects some slow relaxation process. For the lowest rates, it is also accomplished by multiple crossing points. Similar shape of the second-harmonic hysteresis is known for skyrmion spin textures in nonlinear optics. Since skyrmions have been demonstrated for FGT by direct visualization techniques, we can connect the observed high-field relaxation with deformation of the skyrmion lattice. Thus, the second-harmonic Hall voltage response can be regarded as a tool to detect spin textures in transport experiments.

We predict that the square lattice layer formed by [Co$_2$N$_2$]$^{2-}$ diamond-like units can host high-temperature superconductivity. The layer appears in the stable ternary cobalt nitride, BaCo$_2$N$_2$. The electronic physics of the material stems from Co$_2$N$_2$ layers where the dimerized Co pairs form a square lattice. The low energy physics near Fermi energy can be described by an effective two-orbital model. Without considering interlayer couplings, the two orbitals are effectively decoupled. This electronic structure satisfies the “gene” character proposed for unconventional high-temperature superconductors. We predict that the leading superconducting pairing instability is driven from an extended $s$-wave ($s^\pm$) to a $d$-wave by hole doping, e.g., in Ba$_{1-x}$K$_x$Co$_2$N$_2$. This study provides a new platform to establish the superconducting mechanism of unconventional high-temperature superconductivity.

Hui-Can Mao, Yu-Feng Li, Qing-Yong Ren, Mi-Hai Chu, Helen E. Maynard-Casely, Franz Demmel, Devashibhai Adroja, Hai-Hu Wen, Yin-Guo Xiao, and Hui-Qian Luo

Chin. Phys. Lett. 2022, 39 (9):
097501
.
DOI: 10.1088/0256-307X/39/9/097501

We use neutron powder diffraction to investigate the magnetic and crystalline structure of Cr$_2$GaN. A magnetic phase transition is identified at $T \approx 170$ K, whereas no trace of structural transition is observed down to 6 K. Combining Rietveld refinement with irreducible representations, the spin configuration of Cr ions in Cr$_2$GaN is depicted as an incommensurate sinusoidal modulated structure characterized by a propagating vector $k=(0.365, 0.365, 0)$. Upon warming up to the paramagnetic state, the magnetic order parameter closely resembles to the temperature dependence of $c$-axis lattice parameter, suggesting strong magneto-elastic coupling in this compound. Therefore, Cr$_2$GaN provides a potential platform for exploration of magnetically tuned properties such as magnetoelectric, magnetostrictive and magnetocaloric effects, as well as their applications.

The switchability between the two ferroelectric (FE) states of an FE material makes FEs widely used in memories and other electronic devices. However, for conventional FEs, its FE switching only occurs between the two FE states whose spatial inversion symmetry is broken. The search for FE materials is therefore subject to certain limitations. We propose a new type of FEs whose FE states still contain spatial inversion centers. The change in polarization of this new type of FEs originates from electronic transfer between two centrosymmetric FE states under an external electric field. Taking BaBiO$_{3}$ as an example, we show that charge-ordering systems can be a typical representative of this new type of FEs. Moreover, unlike traditional ferroelectrics, the change in polarization in this new type of FEs is quantum in nature with the direction dependent on the specific FE transition path. Our work therefore not only extends the concept of FEs but may also open up a new way to find multiferroics.