A team led by Philipp Werner, professor of physics at the University of Fribourg and leader of NCCR MARVEL’s Phase 3 project Continued Support, Advanced Simulation Methods, has applied their advanced quantum simulation method to the investigation of the complex material 1T-TaS₂. The research, recently published in Physical Review Letters, helped resolve a conflict between earlier experimental and theoretical results, showing that the surface region of 1T-TaS₂ exhibits a nontrivial interplay between band insulating and Mott insulating behavior when the material is cooled to below 180 k.

1T-TaS₂ is a layered transition metal dichalcogenide that has been studied intensively for decades because of intriguing links between temperature dependent distortions in the lattice and phenomena linked to electronic correlations.

Upon cooling, the material undergoes a series of lattice rearrangements with a simultaneous redistribution of the electronic density, a phenomenon known as charge density wave (CDW) order. In the phase reached when the material is cooled to below 180 k, an in-plane periodic lattice distortion leads to the formation of star-of-David (SOD) clusters made of 13 tantalum atoms. Simultaneously, a strong increase in resistivity is observed. Additional interesting properties of the low temperature phase include a transition to a superconducting state under pressure as well as the possibility to switch this phase into long-lived metallic metastable phases by applying short pulses of laser or voltage, making the material potentially interesting for use in future memory devices.

The two tetraquarks, T^a_cs0 (2900)⁺⁺ and T^a_cs0 (2900)⁰, are observed in joint analysis of the B⁰→ D⁰D_s⁺π^– and B⁺→D^– D_s⁺ π⁺ decays. The new tetraquarks are observed with masses around 2.9 GeV in both the D_s⁺π⁺ and D_s⁺π^– mass spectra. The former corresponds to the first observation of a doubly charged open-charm tetraquark with minimal quark content csud and the latter is a neutral tetraquark composed of csud quarks. The D_s⁺π⁺ and D_s⁺π^– mass spectra in the top images above indicate that the sum of contributions from conventional resonances (particles) cannot explain experimental distribution around the mass of 2.9 GeV. On the other hand, the experimental distributions are well understood when the contributions of the two new teraquarks are included in the analysis as shown in the two bottom images above. The mass and the width are determined to be 2.908±0.011±0.02 GeV and 0.136±0.023±0.011 GeV, respectively. The quantum numbers are determined to be J^P=0⁺. In the language of particle physics the two tetraquarks are isospin partners.

In the conventional quark model, strongly interacting particles known as hadrons are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons. In their fundamental 1964 papers [1] and [2], in which they proposed the quark model, Murray Gell-Mann and George Zweig mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration. It took 50 years, however, for physicists to obtain unambiguous experimental evidence of the existence of these exotic hadrons. In April 2014 the LHCb collaboration published measurements that demonstrated that the Z(4430)^– particle, first observed by the Belle collaboration, is composed of four quarks (ccdu).