Abstract We reveal the critical effect of ultrafast quantum dephasing on the polarization of high harmonic generation from massless Dirac fermions in graphene. Under the elliptically polarized pump pulse, the elliptically polarized high harmonic generation is produced and exhibits a characteristic polarimetry of the polarization ellipse, which is found to depend on the decoherence time T2 for the dephasing in the multiphoton regime. T2 could then be determined to be a few femtoseconds directly from the experimentally observed polarimetry of high harmonics. This shows a sharp contrast with the semimetal regime of higher pump intensity, where the polarimetry is irrelevant to T2. An access to the dephasing dynamics would extend the prospect of high harmonic generation into the metrology of femtosecond dynamical process in the coherent quantum control.

We report the results of Raman spectroscopy on a self-intercalated van der Waals ferrimagnet Mn3Si2Te6. Using polarization-resolved Raman spectroscopy, we identify twelve phonon modes with Eg and A1g symmetries, in agreement with the first principle calculations. Below the ferrimagnetic transition temperature Tc∼ 78 K, we observed a significant deviation of the phonon frequency from the anharmonic model, accompanied by a strong line broadening, indicating a substantial spin-phonon coupling in Mn3Si2Te6. Among twelve phonon modes, this spin-phonon coupling is found to be strong mostly in the out-of-plane vibrational modes. These results highlight the dominant role of the interlayer superexchange interaction in determining the magnetic properties, reflecting the self-intercalated van der Waals structure of Mn3Si2Te6.

The interfacial carrier nonradiative recombination resulting from defects and energy barrier at buried interface hinders further enhancement of power conversion efficiency (PCE) and stability of perovskite solar cells. Herein, we report a simple and effective buried interface passivation strategy based on cation engineering through employing a new type of sulfonium salt ((2-carboxyethyl) dimethyl sulfonium chloride, CDSC) together with reference molecule (3-dimethylamino propionic acid hydrochloride, DPAH) to modify the interface between perovskite and electron transport layers. It is theoretically and experimentally revealed at the atomic scale that CDSC and DPAH chemically interact with both SnO2 and perovskite layer and accordingly well bridge both layers. Both modifiers can not only passivate the defects from the surface of perovskite and SnO2 films, but also reduce interfacial energy barrier via improving energy band alignment. CDSC are certified to be more effective in defect passivation and energy band modulation than DPAH, for the first time revealing that sulfonium cations are superior to commonly adopted ammonium cations. Finally, the DPAH and CDSC-modified devices achieve a PCE of 21.44% and 22.22%, respectively, far outperforming the control device (20.72%). The unsealed devices with CDSC maintain 92.5% of their initial efficiency after thermal aging for 1272 h.

New two-component pyrene probes based on oligo(2'-O-methylribonucleotides) for microRNA detection have been designed. They contain the (Py)A-modified adenine cluster (pentaadenosine fragment that contains 8-(1-ethynylpyrene)-deoxyriboadenosine in the center) and can form a three-way junction (3WJ) structure with a target RNA. We have chosen microRNA let-7a-3p as the RNA target because of the correlation of its concentration in cells with the appearance and progression of cancer. We have compared the thermal stability and fluorescence properties of the two-component probes based on oligo(2'-O-methylribonucleotides) that contain either deoxyriboadenosine (dAdA(Py)AdAdA) or the (2'-O-methylribo)adenosine (A(m)A(m Py)AA(m)A(m)) cluster with those of (dAdA(Py)AdAdA)-containing oligodeoxyribonucleotide. The changes in the fluorescence spectra of two-component probes after hybridization with the RNA target have been demonstrated. These probes can be used for designing a new microRNA detection system.

The effect of compression on the magnetic ground state of Sr_{2}IrO_{4} is studied with x-ray resonant techniques in the diamond anvil cell. The weak interlayer exchange coupling between square-planar 2D IrO_{2} layers is readily modified upon compression, with a crossover between magnetic structures around 7 GPa mimicking the effect of an applied magnetic field at ambient pressure. Higher pressures drive an order-disorder magnetic phase transition with no magnetic order detected above 17-20 GPa. The persistence of strong exchange interactions between J_{eff}=1/2 magnetic moments within the insulating IrO_{2} layers up to at least 35 GPa points to a highly frustrated magnetic state in compressed Sr_{2}IrO_{4}, opening the door for realization of novel quantum paramagnetic phases driven by extended 5d orbitals with entangled spin and orbital degrees of freedom.

The electric-current stabilized semimetallic state in the quasi-two-dimensional Mott insulator Ca_{2}RuO_{4} exhibits an exceptionally strong diamagnetism. Through a comprehensive study using neutron and x-ray diffraction, we show that this nonequilibrium phase assumes a crystal structure distinct from those of equilibrium metallic phases realized in the ruthenates by chemical doping, high pressure, and epitaxial strain, which in turn leads to a distinct electronic band structure. Dynamical mean field theory calculations based on the crystallographically refined atomic coordinates and realistic Coulomb repulsion parameters indicate a semimetallic state with partially gapped Fermi surface. Our neutron diffraction data show that the nonequilibrium behavior is homogeneous, with antiferromagnetic long-range order completely suppressed. These results provide a new basis for theoretical work on the origin of the unusual nonequilibrium diamagnetism in Ca_{2}RuO_{4}.

We present and analyze Raman spectra of the Mott insulator Ca$_2$RuO$_4$, whose quasi-two-dimensional antiferromagnetic order has been described as a condensate of low-lying spin-orbit excitons with angular momentum $J_{eff}=1$. In the $A_g$ polarization geometry, the amplitude (Higgs) mode of the spin-orbit condensate is directly probed in the scalar channel, thus avoiding infrared-singular magnon contributions. In the $B_{1g}$ geometry, we observe a single-magnon peak as well as two-magnon and two-Higgs excitations. Model calculations using exact diagonalization quantitatively agree with the observations. Together with recent neutron scattering data, our study provides strong evidence for excitonic magnetism in Ca$_2$RuO$_4$ and points out new perspectives for research on the Higgs mode in two dimensions.