Can a landlord enter without permission in california?

Yes. But only in very limited situations.

In an emergency situation (fire, water pipe burst, etc.), they may enter without notice, as may the fire department or police department.

During an eviction, they may (not sure) be able to enter to show it, but usually only if you agreed to that, and they give you "reasonable prior notice."

Generally, the landlord may enter during normal business hours (e.g. 8AM-5PM, Monday-Friday) to show it to applicants, do agreed repairs or other allowable actions.

They must give 24 hours written (I believe) notice.

When tenants and landlords get along, a phone call is enough and hours are flexible.

Some cities have other housing laws that may apply.

There are also cases in which the landlord gives 24-hour notice and the tenant still refuses to let them in. They can be immediately notified of eviction, but the landlord has no legal right to enter those premises if it will result in a breach of the peace (e.g., getting punched in the face).